Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: sim2011 on July 10, 2010, 11:41:55 PM
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Nightmare !
Just finished my restoration, started engine again for the second time only to notice oil oozing out of the LH side head gasket joint, I cant believe it , it had new gaskets and new O rings the correct Honda ones and a pattern head gasket from Dave Silver.
so its engine out again GRR , no doubt on strip down I will find where its leaking , I'll probably get the barrels and head skimmed to be sure , this time round I've opted for the genuine Honda head gasket at
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Hi si , i have had this problem on a kawasaki 550 i was so dissapointed but at least i could strip the top end in situ, where as with the honda it is an engine out job, am am currently building two 750 SOHC engines and i am really worried this could happen to me, most people you talk too say just use honda gaskets and put them in dry, a few people grease the gaskets, and in some cases a thin smear of blue hylomar just help it seal, i used blue hylomar on one of my gpz's and it was fine but i am in two minds wether to use it on my honda engines, they do a hylomar spray on gasket dressing , that seems to be the perfect choice a thinly sprayed non setting gasket dressing, or copper gasket spray
Pete
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Are you sure its from the gasket itself and not running down from either one of the seals under the cam bearings or from a stud hold holding same? the studs need good threads and sealing compound to stop leaks and the seals need a sealant around the edge
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Never had one leak, but I only ever use genuine Honda head gaskets, the rest of the gaskets I use tend to be pattern but not on the head.
Cheers
Den
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Hi Everyone
Thanks
I did set the seals under the cam bearings in non setting sealant and caulked the edges the disc seals were new genuine Honda.
The Oil is definetly coming out of the head gasket.
Just waiting for a helping hand to lift it out and then I will report my findings
Si
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Are the head bolts stretched? What value are you torquing up to? I fitted some high tensile bolts (Ape I think) and these allow a bit more torque. May be worth thinking about
Cheers
Seamus
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Hi
Had a chat with pete about this at the ace cafe 70's day last month. I've only ever had total sucess with this by using blue hylomar very sparingly (you never want ANY loose sealant to get into oil ways) worked on them quite a lot in 70's and 80's and always used this method. Clean surfaces completely, no old gasket trace at all, use isopropanol alcolhol for final clean, was easier to coat gasket both sides rather than casting, standard studs / torque + pattern etc. Friend has an original 750 F2 and is the most particular person that I know and I must of helped him lift that engine out about six times and put it back again as he insisted on doing exactly as the manual said i.e. no sealant, eventually persuaded him to go with hylomar and it didn't leak. These were all Honda original gaskets (one of the pull out's was to do valve guides as well as leak).
As you know it's a bit of a pain on these as it's motor out each time, all the notes about other leak points are also relevant.
I believe that if you use stronger studs you have the "potential" to distort main bearing area as Honda must have designed it to accept the loadings it required.
I would take a guess that the surfaces were absolutely degreased on factory assembly so this may be a reason why they can work dry with original gaskets so worth considering
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Very original gaskets had some sort of a "Sticky" coating on them arround the chain tunnel and oil feed areas
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Thanks Brian
I'll wait and see what Honda send me, The engine will be out tonight so I should be able to post where its leaked
Thanks
Simon
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As Bryan said the originals had a coating that looked shiny and had a feel that was rubbery/tacky to the touch around critical areas, I guess with a really clinical degrease this should work and I can never recall a factory built motor leaking in normal circumstances.
I'd be VERY careful about taking a skim if you can avoid it. This changes (albeit mildly) cam timing, compression, valve to piston clearance etc and may require special base gasket to regain correct dimensions.
In my experience these are very stable castings and unless they've been butchered then normally remain stable and flat. I'd have a careful check initially with a good quality steel rule to see if you can spot any thing other than flat and consider from there.
They are not easy to skim as you have steel liners in ally for the barrels so getting a clean cut takes someone who knows what they are doing, plus the head i think has to sit up on spacers to mount flat (avoiding protruding valve guides) this is quite different to car components and may present you with some problems and potentially scrap the castings.
I'd say the hylomar route was much less risky. Our reasoning behind this was that they leaked at the point they do because of a non-adhearance of the gasket to the surface (you can usually lift the gasket straight of fwith no visible residue and see the oil path during strip down) so if you can get it to adhere with on spec gasket / degrease then great.
When you take them apart from original assembly those sticky areas usually showed sign of the gasket tearing with a thin part of the surface remaining on the casting, confirming to me that Honda required them to be physically bonded hence the gasket treatment.
It'll be interesting to see what you find, it's just fiddly wiggling the motor out without scratching any part of the frame!
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This is a genuine 750F2 head gasket
(http://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/20091125_12.jpg)
, & as BryanJ has said, there are areas of the gasket that have there 'sticky' patch next to the cam chain tunnel & around the oil feed holes. I always use a thin smear of copaslip on my graphite headgaskets, & have never had a problem. But if I did have a problem on the top end, I had done this mod
(http://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/TimsCB750F3-4004.jpg)
to the frame so that the engine was more accessible.
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Well I've carefully taked it apart,
The head gasket is soaking with oil all over , the bolts were correctly torqued and the cam rubber discs hadnt leaked.
To me it looks like the Honda O rings arent thick enough ??
Mine measure 2.36mm
To me it looks like it has leaked throught these .
Nightmare !!
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Oooooh! looks like nothing is sealing that should. Haven't got anything available to measure to help with o-rings currently.
You could try taking off gasket and leaving o-rings in place then slide head back on and feeler gauge the gap to see if they are going to be compressed or not to give you an estimate. They'd obviously have to exceed the gasket thickness to do anything.
Not worth wasting a 65 quid gasket until you can get some sort of answer on the o's i guess. have you got the old o-rings? They are usually compressed when they come out so i'd estimate at .5mm plus gasket thickness to be effective.
What is the counterbore depth where the o-rings go?
BTW yoshi823 I like that frame mod of your ex F2 very neat piece of work.
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I may be wrong but did'nt the K3 onwards have tube like seals in place of the earlier O rings?
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Right
Took the follwing measurements
David Silver supplied Pattern Head gasket 1.16mm
Honda Genuine O rings 2.36mm CMS Parts fiche states as 2.5mm but mine have been torqued down
Cylinder Counterbore depth .93mm
That leaves just .27mm to seal
I will post again when the genuine head gasket and new o rings arrive.
Thanks
Simon
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Hi Seamus
You are both wright and wrong , the eraly K3 had this set and the later ones Dowels with rubber seals, my bike is a very early K3 produced December 1972 for Jan 73 launch.
My engine has never been apart and I took plenty of photos when stripping it down.
Thanks
Simon
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Typical Honda, a new model definition, but still using the older stuff in it. I have the same with a K2 that seems to have a lot of K1 parts fitted.
It was worth a thought, although I had not realised that your motor had never been apart.
Be interested in the final diagnosis though.
Good luck
Seamus
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That "O" ring is part number 91309-035-000 and is 11 x 2.5
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Hi
The Genuine Honda Head Gasket turned up Pt No 12251 - 300 - 307
This measures 1.15 (pattern measured 1.16) so it aint that
The new O rings measure 2.44 the new old ones measured 2.36 after being torqued down.
The only thing I can assume is that the Genuine Honda gasket might squash down more when its been torqued down ?
The New Head gasket does not have any sealant on it at all ( unlike the F2 one )
This leaves with a bit of a headache
Do I use Silicone sparingly on the O rings ?
Do I use sealant around the cam chain and oil feed holes on the Graphite gasket ?
If So what does anyone recomend ?
Answers on a post card to mr confused
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I have made the decision to use blue hylomar on my head gasket to help seal it when i rebuild my top end and i am going to seal the o rings, rubber pucks and anything else with threebond 1194.
Before you reassemble your engine place your head on the barrel dry with no gaskets but with all the dowels and check that there is nothing preventing the two from fitting together properly, its got to worth a try as something is not right to leak so much oil out
good luck
Pete
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I've Never used silicone on these engines, I think it's really good stuff in the right application but have taken too many things apart to find silicone bits all over the place and have a huge fear of getting any of it into small oil ways.
As Pete says, it may be good to check if the head is going clear down onto the barrels with no gasket first to see if you can spot anything.
I doubt if the gasket itself squashes to any degree, not at the torque used with flat surfaces. The design arrangement is almost perfect in that the head and the barrels are compressed buy the studs so you don't get any local distortion as you can with a stud screwed directly into the block (as on many cars).
Also check that the barrels have been seated completely before you place the head on as any binding here can the show at the head gasket when compression is applied during start-up.
I presume your torque pattern is as per book?
The suggestion for using blue hylomar is just the smallest amount spread as thin as poss both sides but to cover the whole area minus barrel rings as they should compress. The aim being to have ZERO loss of hylomar once clamped up.
I've never used increased torque as the studs are designed to be pulled into their elastic tolerance BUT NOt past it into stretch.
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Just a quick question
Are there any dowels between the head and barrels at all on your engine?
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Hi
Yes 2 sets of 2
2 at the bottom of the barrells and 2 between barrels and head and they are the same height,
I have checked that these dont interfere with anything
I still think its got something to do with gasket and o ring thickness
Thanks
Simon
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It seems there is something odd happening, It's always a concern if you can't identify what is really wrong and keep taking pot shots at it as you don't know if you've really fixed the problem.
Could be focusing on the wrong angle for a reason why, may be worth describing the logic to see if anyone can spot a prob in it.
If the cam cradle plugs are not leaking (they go into clear air and not the gasket anyway, don't they) the only scource of pressurized oil is from the O-rings protecting oil supply to the cam area, so the oil must be coming from them as a primary fault, right or wrong?
The head gasket is primarily there to seal the barrel to combustion chamber area and the rest is to take up any minor variation in the machined faces, if it was the original intention to stop pressured oil it would probably have had something like the barrel rings in copper around oil ways. Back to the O-rings?
Why don't they seal? If the O-rings were too big (total volume) they would hit the capacity of the counterbore and possibly prevent the head properly compressing the barrel rings, so leaking compression?
When the O-rings have been in a long time they look square is section showing that they've been squashed and seal against the casting.
Soooo the problem appears to be that the rings are not sealing against top / bottom faces, Why? (sounds like a corporate problem statement!! bit of blue sky, out of the box thinking there ;D)
What affects that:-
Dimension, is it correct?
Material, some rings are noticeably harder that other (pattern? some applications would have a different material spec to this one, think it's called Sure rating) and would affect the physical sealing to the faces.
Clamping, assuming this is correct, but are you sure of your torque wrench accuracy. I've always used a small scale one for this 0-40lbs as it gives good seperation of scale and more accurate actuation at light loads. Have you got any genuine new Honda O-rings to use?
Hope this is of some use, any comments welcome.
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Firstly oil leaking from anywhere tends to appear as if its comming from the head gasket area
Secondly only reason for a new gasket and "O" rings to leak immediately is damage to or not flat condition of barrel/head joint surface.
There are several known causes of oil in the head gasket area.
(1) Most obvious is the oil feed O rings in the gasket but this is only after time
(2) The rubber seals under the cam bearings not sealing due to damage or lack of clamping between bearings and head
(3) oil leaking down the threads of the bearing retaining studs/bolts as the holes go right through, this is more common if the threads in the head have been repaired with helicoils as oil has two threads to work down.----Normally this can be curred by using Hylomar on the threads JUST NOT GREAT GOBS OF IT TO BLOCK OILWAYS. A better form of thread repair in this area is a "Keensert" fitted with a sealing compound on its outer thread
(4) oilways to cam blocked causing overpressure at the joint although the cam normally seizes before the O ring blows.
Lets assume (always dangerous) that the engine did not leak like the Torrey Canyon before stripdown,
the most common cause would then be (2) then (3) or damage during stripdown.
The other assumption here is that the head nuts run free and all washers are in place.
I hope this helps and you sort the problem, as to gaskets back in the 70's we rarely used genuine sets due to cost and mostly Noboru with not a problem to my knowledge
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I seem to remember that the 750F2 cylinder head had a modified part to the oil drain at the front of the head, so that not only would oil drain down via the cam chain tunnel but also via the head studs. I think that there were two holes per side, as in down the stud holes between 1 & 2 cylinders and 3 & 4 cylinders. On the F2 head there was only the one drain hole. What i'm getting at is, maybe it's not the oil supply to the head, but the drain of the oil back to the crankcase, that's causing the problem.
I also seem to remember, & it has been a few years now, that there were some recesses in the top of the barrel that had places for dowels & the tall sleeve rubber seals, but the engine came without them. I filled these recesses with dowels & their relevant sleeve seals, & never had a problem.
This is the fiche for the 750K3...
http://www.cmsnl.com/honda-cb750-four-k3-us_model481/partslist/E++05.html (http://www.cmsnl.com/honda-cb750-four-k3-us_model481/partslist/E++05.html)
and this is the one for the F2...
http://www.cmsnl.com/honda-cb750f2-supersport-england_model14689/partslist/E03.html (http://www.cmsnl.com/honda-cb750f2-supersport-england_model14689/partslist/E03.html)
Parts 2 & 3 arn't shown on the K3 fiche, & it was these parts that I added extras to on my F2. There's also a seal at the base of the cylinder stud that used to compress with the barrel being bolted down...part 22 on this fiche...
http://www.cmsnl.com/honda-cb750f2-supersport-england_model14689/partslist/E14.html (http://www.cmsnl.com/honda-cb750f2-supersport-england_model14689/partslist/E14.html)
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Assuming that all discussed is mechanically ok and as you've stated that cam discs were not leaking but definitely coming from gasket joint, then I'd be looking at the pressure side for the cause.
I was considering the blocked cam feed bit that Bryan has posed, but would have thought that the relief valve on the pump would stop that and guessing that it was ok before stripping.
About the function of the O-rings, I'd say that to work properly that they would have to have "traction" on the mating face to the casting so that when subjected to internal pressure that the ring could not slip over the surface to be deformed by the pressure into leaking oil. If they are assembled with ANY lubricant (from workshop handling or release agent from manufacture) I'd guess at a lowered pressure capability.
With that statement, I'd say that through using Hylomar on the gasket that I may have been inadvertantly catching a leak with what is a secondary gasket from the point of design intent.
If it were mine I would build it now by degreasing completely all the mating surfaces plus O-rings, hands / fingers etc and coating both sides of the head gasket with Blue Hylomar (anybody would think I owned the company) let it dry, then assembling otherwise dry and torqueing to spec.
Then hopefully enjoy some sunshine on it.
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Can you clarify for me exactly where the oil was comming from as if it was the end at LH side that would mean if the "O" ring leaked the oil found its way from the middle stud to the end without leaking out anywhere else which although not impossible is rather improbable
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Hi Brian
The whole head gasket was dripping with oil almost like that both o rings were not sealing i have checked that the head mates with the cylinder with both dowels in and everything is fine.
Look at the pic below
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a skewed in dowell or a bit of muck under one will cause a gap for oil to gush out and blue Hylomar is not a silicone type of sealant, i,l be useing either the blue or golden to seal c/cases once ready to build up, blue never let me down in over 40 odd years in engine work..
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With oil all over it does suggest that either the head or block is not flat or that they are not being clamped together enough by being held too far apart. I know that there are several different lengths of dowel pin, perhaps they have been mixed up? Try fitting the head to barrel with no gasket---it should allow the mating surfaces to touch ensuring enough squish of gasket and O ring
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Update
I've taken the head and barrells of to SEP to be checked out so I'll let you know what they find.
I managed to find the original Honda head gasket and its mighty different to what Honda supply today See pics , the 1973 one also had gasket sealant around the cam chain tunnel the 2010 one doesn't !! The 2010 one is also bereft of those copper sealing rings .
Also I have noticed that on the 73 version all other holes (not the copper Ones) have the same sealant applied as the can chain tunell
Any thoughts on this will be welcomed.
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Hi again. To me that would confirm that you need a sealant at key points if not all (I've always covered total area) on any gaskets pattern or otherwise.
These are my reasons:- Honda clearly designed (original gasket proves it) to have some type of sealing additional to the gasket material around the oil ways.
I don't believe that sufficient torque can be exerted to do it totally dry with no additional sealant of some sort (copper rings, factory applied sticky-bits).
If you don't have that original spec of gasket OR something superior to it you'll have to use a substitute.
You can't increase the torque without stepping outside the original design spec, anybody that knows anything about the engineering design process will tell you that looking at this design (layed down in 1967 ish) was an absolute masterpiece then and still is an example of how to make a first off reliable engine from scratch; just one example is the stud design that is waisted (thinner) in the centre than at the threads to give equal torque load over the whole length and cross-section of each stud!!! also how the bottom threads are buried down into the casing so that the load is pulled from further within the material again to spread load indicates how thorough the design team went to work.
To me, the risk of using Blue Hylomar is only to avoid ANY loose sealant floating around as it survives in oil. The rest is all upside, as previously posted (by rigwit?) this stuff is seriously good.
With a nice new tube you can get a really thin spread coating onto the gasket, it even looks and feels like the original coating when it's dry.
Be interesting to see what your barrel / head inspection shows, let us know.
Maybe a bit too enthusiastic design appreciation there, maybe should be filed under "soapbox" but they are really good...............really.
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I dont ever remember seeing a Honda gasket with copper rings in it
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Never seen copper rings either, but have only worked on CB750's up to K2 and F1/F2.
Cheers
Den
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Just my 2 cents worth , i've never had to use sealant apart from under the cam tower pucks & the crankcase halves, just made sure all surfaces are nice and clean, cheers Mick.
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All
Thanks for the input , I have just been looking at the parts manual fiches and the K2 US head gasket clearly shows rings of some sort around the holes so these will be the copper ones in my pic , my guess is that they used this on mine as its a Dec1972 build date.
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I have a parts book so early its on thick individual pages dated 1974 and yes it does look like there is something but there was/is only one part number for the gasket from the CB750 to the K4 (thats as far as the book goes)
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Just been playing with DS website and a K2 top gasket kit is discontinued but the 750A top set exists at
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To me, the risk of using Blue Hylomar is only to avoid ANY loose sealant floating around as it survives in oil. The rest is all upside, as previously posted (by rigwit?) this stuff is seriously good.
blue hylomar is actually for pipe threads, but is a great gasket sealer for uneven parts it is non setting in a sense, but does harden up a bit - like half way from blue tack to Plasticine. Golden hylomar is non setting but isn't as resilient as the blue for imperfection filling and red is totally setting only ever used red on waterpumps and diff casings. another tip is I always coat non metallic gaskets with a layer of grease both sides helps with instillation sealing and doesn't make the gasket dry out and go brittle so if ever have to take cover off its a doddle.............
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Hi Simon,
I have a K4 (same colour) not changed the top head set, but I did overhaul the carbs last year and had a few problems with setting up (purchased a standard CB750 carb kit,120 main jet)
Anyhow after a bit of reseach I found that the USA immision laws changed in 1973 and the carb main jet was replaced with a 105 unit (K3,4,5) which runs hotter & leaner...I think..??.. Honda also modified the head gasket to compensate for the extra heat, hence the copper rings on the origonal gasket.
Hope the information helps, and please let us know if the new gasket has copper rings
Alan
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To all
Quick update, I've just collected the barrells & Head from SEP and they have skimmed both.
The head was warped and the barrels had a slight dip in the middle.
So now I have
New Honda gaskets
Flat metal surfaces.
I am going to use sealant just havent decided yet on Blue Hylomar or clear silicone.
I'll keep everyone posted .
The new Honda head gasket does not have copper rings
Thanks for everyones input.
Si
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How much out were the head and barrels? Please keep us posted with reference the rebuils and subsequent oiltight engine.
Good luck
Seamus
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It'll be interesting to see how you get on, did they say how much they skimmed from each surface?
Hopefully you'll soon have it oil-tight and able to enjoy a good run out, thanks for keeping us informed it's alway's good to see the full story and eventual progress.
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Didn't put enough detail in last post but the question about how big a skim has an impact on cam chain tensioner also as the total shift is doubled (2 runs of chain, up and down) so the tensioner will reach max tolerance earlier in the chain wear cycle.
Be interesting to see how you get on as I've never worked on one with a skim to my knowledge.
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Hi
According to Mark Paris's book a skim of .010" (0.25mm) is fine on the K3 as they had 1mm more deck height than the K0-K2 engines , I cant see that .5mm will make any diference to cam chain tension.
Any way just reasemmbled the engine this morning, I will recheck head bolt torque later on today.
I opted for Blue Hylor for both the base and head gaskets after checking their site its perfectly ok for High temp.
Just a thin smear was used both sides.
Next report will be later this week when I get a helping hand to put the lump back in.
Thanks for everyones input.
Simon
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Simon I did check that cylinder as i said and the deck height is approx 1mm with the counterbore being 0.9mm Cant remember how that ties up with yours
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Hope you've got it with that, it's nice when they run well and you know you can get a reliable run out on it.
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Fingers crossed, hope all turns out well this time
pete
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Here's a slightly different angle on it which may explain some of the experience old-v-new gaskets etc.
This era or production has covered the removal of asbestos from head gaskets so any original specs that used it would have to have been re-formulated. Commonly they went from asbestos to graphite and one of the characteristics that changed was it was more common for liquids to creep through the new ones, making it common practice to have a sealant where none was needed prior to this.
Graphite type gaskets also need retorqueing on some arrangements also and this is never part of the plan for SOHC 750 motors due to the need to remove from frame to work on. I'm guessing this but it may be why they are more succesful with sealant with the types available now both Honda and pattern.
There is a piece written in current "Classice Bike" along these lines, not specfically dealing with this application but if you overlay the info contained with our experience and the timeline involved I think you can form a more thorough resoning as to the use of sealant or not.
I guess this impacts many of the bikes on here.
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Moment of truth is looming !! Engine back in and should be able to fire it up tonight, I'll keep you all posted.
Si (Nearly running K3)
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Well just done 10 miles and all seems well on the leak front :) I need to do a longer run just to be sure , the front end feels a bit stiff new seals and oil , so I will monitor that as well.
Many thanks to everyones input , I think I can conclude that using some form of sealant is a must on the head gasket.
Thanks
Simon K3 (No Leaks)
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Seems like optimistic news so far. It's a good thread for anyone else looking in and working in the same area as I guess there are some relevant changes to consider since they were originally produced.
Thanks for considering our opinions of value, hopefully you get to enjoy it now.
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Nice one Simon, you done well to turn around the problem so quick and fix it , if that were me i probably would have let it sit there for a while and got all depressed about it, your rewards are deserved.
Pete
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Nice one Simon, you done well to turn around the problem so quick and fix it , if that were me i probably would have let it sit there for a while and got all depressed about it, your rewards are deserved.
Pete
Hehe +1
Are you noticing any difference in performance due to the skimming ?
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I didnt ride it before the skimmimg, I also have one less tooth on the front sprocket therefore its not a top end bike , more fun I hope.
Pete
I did get depressed for about 30 mins then thought lets do it ! it only takes about 1 hour to get the lump out then about 1 hour to strip the top end down, then ages to clean all the base gasket off properly.
I've done a total resto on this one in my spare time in less than 3 months so it can be done ( even with 1 wife & 3 kids)
I'll keep everyone posted with its progress, next is My Suzuki GT550 which I have already stripped down , Crank is at SEP in Kegworth who also skimmed my head and barrels (proper Mc engineering company)
Thanks again to everyone I hope this will help others.
Si
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SEP are one of the best out there, i used them to re-sleeve a barrel and some other work, they are top notch
Pete
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SEP sounds like a good recommendation? I've no knowledge of them but it seems from what they have done for you that they can do the work to a level you need.
My reasons for being careful about skimming barrels is that they are hard to do correctly without some experienced engineering behind them, I've seen a few that have not been successful so If you can get this level of service it makes it much more of an option.
The cam chain comment I made is I believe relevant as if you consider when you adjust the drive chain you only make a small move in length but the slack changes out of all proportion to how far you've just moved the spindle. I'm not clever enough to do the calculation but it must have something to do with sprocket diameter, I don't think it's a straight no ratio effect though.