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Discussion Starter #1
Evening gents,

So I tried to attach a video of the actual leak on my engine, I don't know if that worked.....

I did a full tear down and rebuild of a 1970 cb350 twin engine. Starts up and runs fine, just need to work through the mikuni carb tuning....fun

I do have a pin hole leak between the cam case and head, right exhaust valve side, where the engine stud would run.

Very odd to me, since there is so much surface area in that particular area.

All mating surfaces were decked prior to reassembly.

I did not spray my gaskets with permatex or any other gasket sealant.

Of course it only leaks when running, I verified all my head bolt torques are to spec, and thought even maybe excess oil in the engine could have caused it.

Before I pull the engine, are there any other possible causes? Shortcuts to seal this without pulling the entire engine and breaking it down to the head?

Just throwing it out for the ones much wiser with these engines then myself.

Thanks in advance for the help.
https://cloud.tapatalk.com/s/5f6151dc21b4c/20200820_125831.mp4

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I would agree with you that the location of the leak isn't in the usual places.
For my engine builds I follow the advice of a known racer/builder who specs the stud threads be oiled then torqued to 16.5 foot pounds. Out of 10 rebuilds I have not had leaks or weepage. I also have never had any nut or stud strip.
I only check for flatness at the top of the cylinders and bottom of the head.
 

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Awesome, that's at least one more thing to try, I'll up the head bolt torque to 16.5 to see if that helps it. I followed the service manual and set them to 14.5.

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There is an oil flow check bolt, 6mm thread 10mm socket, in right front side of cam case near your leak.
That bolt is removed with engine running to verify oil flow to top end. If loose or missing sealing washer, leakage can occur. Bolt is forward and above right side cam bearing case, near tach cable exit. HondaJohn
 

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On my engine the oil check bolt is left of cam case, exhaust side. Is there another on on the tach side? I did delete my tach cable, I don't think that would have anything to do with it, but open to the consideration

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I don't have a CB350 in my shop now, to look at, and Im getting old (70+) so I may have steered you wrong, but I thought you may have had a loose oil check bolt, the oil travels up the studs to feed the cam, rockers ect. Posting a photo is a help, I tried to see your video but was not able to. Show us a photo? HondaJohn
 

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Discussion Starter #7
Thanks, I appreciate all the help I can get. I know the guys on here know a ton about these engines. I've been trying to think of anything that I may have missed.

Knowing that oil travels up the studs helps me understand how the oil is even getting into this spot to begin with.

I've attached two photos, one of the tach side that I'm dealing with, and a closer photo with where the oil leak is, circled in yellow.

It is a very small bubbling leak, from one little spot, only when the engine oil gets up to temperature.

Still going to try increasing the head bolt torque to 16.5, and hopefully it will squeeze that hole shut.

Thanks again for the help


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Well there IS a gasket between the cam case and cylinder head, and the oil comes up the long cylinder studs at front outside corners. Was the gasket replaced?, that gasket is hard to get removed, it gets very baked, and the cam case aluminum is soft and easy to damage. The head and cam case can be removed as a unit if you "break" the chain and use a replacement master link for a CB450 (peen over the pins) or a clip type for a CB/CL 77. The "Honda" way is to dis assemble the cam end bearing covers, rocker arms, and cam sprocket to slide the cam out with the chain intact, back in the day, when a seized piston or leaky head or base gasket needed repair, if the cam case gasket was not leaking we cut the chain to pull the head as a complete assembly, and not disturb the cam case, end caps, rockers ect. and use a CB450 master link, still available, the OE chain was DID, master link available at 4 into1, and other places too. looks to be that you may need cam case gasket replacement, try to be sure where leak is, and if you're going there, get head and base gaskets, don't try to reuse them or you may need to go back in again. Good photos, nice carbs, what size?, My CB350 had CB450 CV carbs, with Yoshimura "slipper" type pistons, and cam, ran great, but was easy to seize a piston, they were so lightweight they were unstable. 2 rings an inch wide in front and rear, no sides, like some of the latest dirt bikes, that was the 1970s, piston and materials tech is much improved. Also if you want to find the leak faster, clamp shut the breather hose, start the engine and run at hi idle, the crankcase will build pressure soon (a few mins.) and force oil out where it can, don't over pressurize or may push out an oil seal,( 5-8 mins). To check if you got pressure in engine, stop engine, unclamp hose and listen for pressure release, woosh. HondaJohn
 

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My mistake, good call on the front/rear studs mix up for the oil delivery. You are correct the oil passage goes across the upper crankcase behind the cylinders. Have not disassembled a CB350 in many years, had not seen any photos. Now seeing his photo is looks like the cam case gasket may be the problem, HondaJohn
 
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