DOHC CB750 Oil Leak

[joem4790]

I recently got my bike (79 CB750) back together and noticed an oil leak coming from cylinder jug. I knew it had a leak before the rebuild but I assumed one of the o-rings that fit around the studs was worn or not sitting correctly. Upon further inspection it looks like the leak is coming directly from this mark/ abrasion along the cam chain guide area.

I replaced the front cam chain guide when I rebuilt the top end because it was broken, which was likely why the bike had been sitting for so long. It had definitely been rattling around in there for a while too because the inside was scarred. Is it possible that the guide has slowly worn through? Has anyone ever seen this before?

It seems to be coming from vertical mark in the picture.

 

[joem4790]

And my picture posted crooked...

I'll see if I can get some better ones this evening.

 

[Jimbonaut]

Is that a vertical line a weld or some kind of repair on the jug? Hard to see. When you replaced the cam chain tensioner, did you check that the tensioner bolt had an o-ring on it? If it's missing that would cause a leak.

 

[joem4790]

Now that you say that it kind of does look like a weld. I'm assuming that line isn't normally there? It's not on the opposite side. I've pressed on it with a screwdriver and it seems solid.

I replaced the o ring on the tensioner bolt too so I don't think it's that. You can see the oil start around that line during idle.

 

[Jimbonaut]

Nipped out to the garage and took a photo of the same spot on my engine -

Definitely looks like some kind of repair on your jug. Odd spot for it, wonder what caused that. Could a cam chain break and go through the cylinder? No idea. I'm no expert, but if that spot is leaking and it's been patched up with something iffy, then I've heard that DevCon is excellent for making repairs on engines. Holds up to the heat and all that, cures to metal strength. Something like this -

https://www.amazon.ca/Devcon-10610-Aluminum-Putty-lb/dp/B016ME6TTI

The stuff comes in all kinds of different metals, but guess you'll need aluminium for your job. The stuff works - you may want to check out other DevCon products to be sure.

 

[joem4790]

Thanks man! I wire brushed some of the paint off for a better look. It makes no sense to me what happened here.

I'll have to give that Devcon a try. Hopefully that'll keep me from having to pull the engine again. If all else fails it looks like I can pick up a used jug on eBay.

 

[Jimbonaut]

That looks like a fairly significant fix - guess the cylinder must have cracked somehow. Guys here will be able to give you some good advice on how to fix it - DevCon is great but not exactly cheap, and there may be other cheaper solutions. Good luck man