SOHC.co.uk Forums > CB750
CB750 K3 Oil leak
Yoshi823:
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
and this is the one for the F2...
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
K2-K6:
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.
Bryanj:
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
sim2011:
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
rigwit:
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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