CB750 K3 Oil leak

[sim2011]

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

[UK Pete]

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
   

[Bryanj]

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

[Spitfire]

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

[sim2011]

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

[Seamus]

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

[K2-K6]

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

[Bryanj]

Very original gaskets had some sort of a "Sticky" coating on them arround the chain tunnel and oil feed areas

[sim2011]

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

[K2-K6]

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!

[Yoshi823]

This is a genuine 750F2 head gasket

, & 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

to the frame so that the engine was more accessible.

[sim2011]

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 !!

[K2-K6]

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.

[Seamus]

I may be wrong but did'nt the K3 onwards have tube like seals in place of the earlier O rings?

[sim2011]

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