Crankcase clacking when hot - NOT kickstart

[GG]

Hello all, I've had my 1966 Superhawk cb77 since 1978, I know it well, but am stumped. When hot there is a metalic clacking - quite loud - that is not regular. It is definitely from inside the crankcase - not the head, and when I put my "ears" on the kickstart shaft it does not appear to come from it. The noise is mostly heard at idle. Motor does not vibrate under load (not a piston rod problem, I presume), and gets worse as the motor heats and the oil thins. All gears work well, just the noise. Can any of the gears sort of creep over and the synchronizers clack? I'm out of ideas, and not excited about tearing the whole thing apart not knowing what I'm looking for!!!

Thanks!!
GG

[deuce_7]

Had a similar experience with 1967 CL77 Scrambler. Same type of noise.

One of the 6mm knock bolts (90081-253-000) that secure the cam chain guide roller pin had totally backed out, causing the chain guide assembly to swivel slightly on the remaining knock bolt, binding / twisting the chain and guide pin enough to cause the clacking noise. The motor would still run.

Eventually tore down the engine for a top end rebuild, which is when I discovered the missing knock bolt had dropped to the bottom of the crankcase. The engine was reassembled with new cam chain, roller, guide pin components and knock bolts, and the noise didn't reappear.

Certainly similar symptoms, although there are certainly other possibilities.

[GG]

Thank you so much for the reply! It gives me something to go on.

I have a question - can the fix be done without separating the lower case - just by removing the top end?

Again, thank you!

GG

[deuce_7]

The cam chain guide and pin assembly bolts to top of upper crankcase between the middle cylinder head studs. Splitting cases not required if cam chain is to be reused.

Believe the cases must be separated to install a new cam chain.

Other noise sources I had considered were piston slap (usually lessens as engine warms up), wrist pins or bottom end rod bearings. Hard to isolate unless engine is torn down and inspection and measurements taken.

[GG]

Thanks Deuce,

I'm almost certain it isn't piston slap, nor rod / wrist pin. When I had the head off I checked things out (not with a calipers, but ...) and everything looks very good. I only took the head off because the head gasket (original) had blown. Cylinders still had hone marks! (I did hone it again and install new standard rings). The motor still looks like brand new (I baby her!). I'm thinking that something has come loose and "jiggles" only at idle - that's the only time I hear it. But it is a pretty sharp metallic sound, kind of like the chain shearing off to one side, like the condition you describe. I took the chain tensioner out, but it doesn't give me enough room to see the bolts. Grrr. Might have to go with the head off again, drat it. I assembled it beautifully with no leaks.

All advice appreciated! Thanks again,

GG

[Tim Allman]

You say that the sound comes from low down in the engine so I don't think that the cam chain would be the most likely cause. The first place I would look is at the primary drive chain. It's low in the engine, would do the same thing if loose and it's easy to get at, being behind the left side cover.

If this were an earlier engine I would feel more certain as they came without primary drive chain tensioners. However, pulling the side cover is a lot easier than servicing the can chain system. Look for chain marks on the inside of the cover and at the condition of the tensioner.

[GG]

Hi Tim,

I'd already thought of that, but that's not it (I was HOPING it was). Mine is a 1966 with the tensioner, and I pulled the cover and ran the bike for just a tiny bit, and there is no noise there, nor does it flop very much. At idle it DID make the noise, but coming from somewhere inside. The sound seems to be tied to the crankshaft, which would make me suspect something with the cam chain, or? And it is low, not up top. It is not bearing noise, it is a definite "clack", but pretty loud. Scary.

Thanks for the tip! I'm contemplating what to do.

GG