Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: stoutgoose on May 07, 2020, 09:35:55 AM
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First start since rebuild and within a a couple of minutes running oil starts dribbling from the head above the gasket line from between cylinders 2 and 3. A small pool had formed within a few seconds.
Scanning the forum, I found a old post below that mentions using the domed nuts and copper washers on studs 5-8. During the rebuild I put in ARP studs and the nuts that came with them which were for the F2 yet didn't have domed ones. Also, I don't recall any of the washers being copper. Irritatingly, there was no mention in the manual about either copper washers or the import of the domed nuts.
My question is this; how can so much oil come from this area, and has anyone else experienced this? I'll be removing the engine to rectify this but am worried it may be something else as well.
Old thread:
"hya there serge, to help you understand, look at your f3 head, there are only four oil return holes...
Each "pair" of valves share one hole, whereas on the k series heads,there is a return oil hole next to EVERY valve.....
Be VERY VERY carefull that you do TWO THINGS RIGHT. Down at the crankcases BEFORE you put the barrels on, make sure you have installed the four tiny "o" rings on the studs that do not have the knock pins, I believe from memory these are labelled 5,6,7and 8 in the tightening sequence, but NOT IN EVERY MANUAL!! ( I have two manuals that show different tightening sequences!)
The purpose of these "O" rings on the studs is two-fold, the stop oil rising up the way AND believe it or not (BUT ITS ABSOLUTELY TRUE!!) they stop water and weather from entering down the way so to speak, this can be proved by looking at your cylinder head in a moment!
IF, you take a screwdriver or other probing device (but not your personal tool so to speak) and poke it down "those" (5,6,7and8) stud HOLes ON YOUR CYLINDER HEAD, AND LOOK through the fins, "OH MY GOD" YOU MAY WELL EXCLAIM, "why can i see my screwdriver?"
You will then be in the enviable position of being in the f2/3 "know" This is because "those" holes are not fully enclosed like in the k series, and should you forget OR neglect to replace those copper washers and domed nuts with new copper washers OR put the domed nuts on the wrong studs, you WILL get oil pissing out between the fins when the motor is started.........MORE THAN ONE person has experienced this! ( including me!) and I now have a mission to stop other people from the pain this causes!!
If ever you get the chance to see UNDERNEATH the barrells of a k and an f2/3 side by side, DO IT!
If ever you get the chance to see a k series AND f2/3 series head side by side DO IT!
Only close inspection reveals the differences and its well worth the looking and then the knowing!
kindest of regards,
kaceyf2"
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Sorry to hear that, there’s nothing more disheartening than getting an engine back in and everything fastened up, only to realise that it’s going to have to come out again. That’s when I go and get drunk instead.
I’m not familiar with the F2 head, but I assume they still have the large rubber bungs under the cam holders. My CB750 leaked badly from there, but I had to change them anyway when it ate it’s crankshaft. The originals had turned to stone with age and didn’t seal anymore.
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Below is a copy from official workshop manual for your engine.
[attachimg=1]
Shows location of special capped nuts and sealing washers to prevent oil leaks.
For sealing they mean the copper type. If you're unfamiliar with copper annealing, just heat to red on a gas ring and quench in water to soften the material and they'll seal properly.
Hope it helps your rebuild.
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This could have caught me out, I bought an F2 as a project and did not realise the difference as I had only ever worked on K engines, luckily I spotted it on this or the US site.
Cheers
Dennis
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Not trying to be silly but it is possible to fit the rocker gasket wrong and it leaks
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Many thanks for this. My 'original' Honda workshop manual and the Haynes manual both omit this information. I'm just amazed at the volume of oil that was discharged, although the location from where the oil is coming (upper fins between cylinders 1 and 2 and 3 and 4 suggest this is the issue). Pucks were replaced and sealed in with Honda Bond. Rocker gasket is good, and the cam tower studs were never removed. I want to make sure this is the only cause of the issue before the engine goes back in again.
Thanks all for your replies.
Below is a copy from official workshop manual for your engine.
(Attachment Link)
Shows location of special capped nuts and sealing washers to prevent oil leaks.
For sealing they mean the copper type. If you're unfamiliar with copper annealing, just heat to red on a gas ring and quench in water to soften the material and they'll seal properly.
Hope it helps your rebuild.
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It would have been a good idea whilst the cam towers were removed to give the oilways a good clean (apologies if you did it). This is where the obstructions end up and loss of oil will destroy the tower first then the cam itself. It blocks just below the cam journal. For what it takes whilst the cam cover is off again I would take the cam out and do the clean and replace the oil feed o-rings to the towers. It becomes a huge job after the engine goes back in. (Like I said, apologies if you have done it already just stressing the point)
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Oil leaks in this area are caused by:
1, rocker cover gasket on up side down.
2, the cam tower studs were not sealed.
3, the bolts in the cam towers are too long and cracked the bottom of the hole. Or the bottom of the holes were already cracked.
4, pucks not sealed.
5, seal no good on the barrel dowels.
6, dome nuts and new sealing washers not used.
7, wrong size 0'rings used on the barrel to head on, the two oil feeds.
8, head studs too tight and has cracked the oil ways in the head.
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Wow, head back off to check that lot.
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As BryanJ says, its very easy to get the rocker box cover gasket on upside down.
I did this and had oil dribbling out the front of the engine after a re-build but was able to change it with the engine still in the frame.
See You Tube at:
https://www.youtube.com/watch?v=2orch75W2uI
If that link doesn't work, search for CB750 VALVE COVER GASKET CHANGE
Might save you having to pull the motor out again.
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Thanks all. I performed a complete rebuild on the engine, so it was completely stripped, chemically cleaned, the head skimmed and block decked, new guides and seats ground, repainted/powder-coated and reassembled using new seals and gaskets.
I made sure all oilways and journals were cleaned and used a MLS head gasket to assist with eradicating the oil leaks associated with the seal height issues.
The rocker gasket is on correctly and there are no leaks (yet) from the head gasket. The pucks were installed using Hondabond. I ground the four head dowels to compensate for the decking and skim and used a larger base gasket to ensure pison clearances were maintained.
I know for sure that at least one of the copper washers is on the wrong stud because I can see it on the one exposed studs at the front of the head. I also did not use domed nuts on studs 5-8 because the manual I posess omitted this crucial information.
The oil seepage is coming from the areas surrounding suspect studs 5-8, but the thing that confounds me is the amount of oil that came out; as per the description quote in my initial post, the oil dribbled out quickly whilst the engine was running. It's alarming how much oil is leaking down the threads of studs-5-8 and past the (non-sealing) washers.
Thanks for all of your comments.
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That is a very confused way to build a engine. Deck a engine that is already a high compression engine is asking for trouble :o
This is a common problem that I come across when building customers engines. If the bolts in the towers are in the wrong place it punches out the bottoms. There are two lengths of bolts on the towers, get them wrong and you have holes were, there was not before.
[attachimg=1]
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Thanks that’s a useful tip; I’ll check the bolt lengths when I get the engine out again.
Out of interest, what’s the issue with decking the block and using a thicker base gasket to compensate? What’s the alternative?
(When I say decked, .006" was taken off to smooth the surface and remove some dinks in order to make a perfect surface for a MLS gasket).
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You said, deck. That is a skim ;)
Not very easy to skim the barrels on the top as, you would have remove the liners and skim the top with a alloy cutter in the head skimmer then, you would need to replace the liners and use a cast cutter on the liners or do the liners on a milling machine.
Seems to be a lot of expense to fit a MLS gasket when, you could of just fitted a Honda one or Vesrah ;)
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I used HT Howard in Slough who use equipment that detects the metal type and adjusts the cut accordingly; in this case it accounted for the alloy head and steel liners without the need to remove the liners and the associated work you mention. The cut is excellent.
The original oval fire-ring gasket for the F2 is obsolete from Honda; do Vesrah make the oval type? Anyhow, the cost of the skim was reasonable and with the reduced leak potential promised by the MLS gasket I thought it a good idea, but time will tell.
Thanks for the information.
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Anyone care to explain what an MLS gasket is for those of us who haven’t a clue?
Sent from my iPhone using Tapatalk
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Anyone care to explain what an MLS gasket is for those of us who haven’t a clue?
Sent from my iPhone using Tapatalk
Multi layer is a number of steel plated gasket with a sealer between the plates. Some have rivets to keep the plates together and are not suitable for some heads unless, you start machining recesses out for the rivets to sit in.
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I used HT Howard in Slough who use equipment that detects the metal type and adjusts the cut accordingly; in this case it accounted for the alloy head and steel liners without the need to remove the liners and the associated work you mention. The cut is excellent.
The original oval fire-ring gasket for the F2 is obsolete from Honda; do Vesrah make the oval type? Anyhow, the cost of the skim was reasonable and with the reduced leak potential promised by the MLS gasket I thought it a good idea, but time will tell.
Thanks for the information.
Honda 410 head gaskets are ready available ;)
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Update to close the thread. I removed the engine and stripped the valve gear to take a look at the studs. I located the copper sealing washers which weren't in the correct place (my manual made no mention of sealing washers and cap nuts). I removed the Kibblewhite nuts on studs 5-8, annealed the sealing washers and put them on studs 5-8 along with the correct cap nuts, and reassembled the bike.
Problem solved. The engine is now leak-free. It is rather surprising just how much oil can find its way down a stud thread and under a standard steel washer.
I'm now off to burn my 'Honda' manual and the Haynes version, and locate a real one. Many thanks for all your replies.
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Good news you've sorted the problem.
The Honda manual I posted picture from (it's not mine but belongs to an F2 owner) has a part # 6141000
Just searched with "Honda 6141000" and quite a few come up for sale. It is their genuine manual for the last of sohc 750 it seems, pretty comprehensive for specs etc too.
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I found a PDF copy of that manual on the net a while back.....it's a handy document to have.... took a while to print off too
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Many thanks.