SOHC.co.uk Forums > CB750

excessive pressure in cam cover possibly causing leaks

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Brocky2:
Hello,
I have done a major rebuild of my CB550K3 1978. I finally have it running fairly well but I have oil leaking out of my cam end covers which I know is not unusual. I have replaced the rubber O rings on the cam end covers and the aluminium and copper washers on the cam end cover mounting bracket bolts but it still leaks from there somewhere. However, it also seems to be bubbling out of the bolts holding the large cam cover onto the cylinder head, the ones inside the rubber gasket.  The bubbling bolts are not the bolts holding the cam end cover in place. If you blow into the breather you can push air in but then it stops going in. Should you be able to keep blowing air through? It's really annoying as this is my last fix. Any suggestions or help greatly appreciated.

McCabe-Thiele (Ted):
On the positive side you know there is oil reaching the camshaft.
If you remove the vent pipe from the top vent cover can you feel any air venting out?

Update: Did you fit the oil restrictors & O rings on the block?

K2-K6:
Put the breather into a glass jar of engine oil when running to see if it's sucking or blowing, that'll at least let you see if there's pressure or not.

Johnny4428:
Also don’t want to state obvious but did you place the sealing washers underneath the end cover brackets?
(between brackets and cylinder head)

Oddjob:
IIRC there are 6 bolts inside the oring gasket and 6 outside. The 6 inside are supposed to be fitted with sealing washers so they don't leak, in theory that is, never seen one leak myself. Best type of washer is alloy and then copper IMO.

As for the end cover leakage. Check the threads in the head where the bolt goes, I've seen more than a few blown out and then the bolt fails to pull hard enough into the head to crush the washers and thus seal the bolt hole. This is normally caused by testing for oil coming up after a rebuild, this then flows into the bolt hole as it's right next to the oil passage oil, the bolt is then inserted and it tries to compress the oil at the bottom of the hole and of course can't. The bottom threads then get blown out by the pressure and you find the bolt will tighten up and then spin when enough pressure is applied. Any oil getting down those holes should be sucked/blown out before the bolt is fitted or you risk this happening.

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