Anyone seen cam chain destruction like this?

flynlo

New Member
Long story short: I bought a '77 CB750F, mostly original albeit some UNI pod filters. Typical blemishes given the age, condition and backstory of previous ownership. Factory original cracked tires and rusty drive chain and a few shade tree mechanical band-aids. When the gent I bought it from said it ran "pretty rough" I wasn't too concerned given the rough looking state of the carbs, gas leaking from the pinch welds in the tank and oil dripping from the pan. I'm building it into a café so tearing the bike down to the frame is pretty much step one for me anyway. Half the bikes I've built I didn't hear run beforehand.

Before I pulled the engine I drained the oil and oh boy was it sparkly. Metal dust and metal shavings (image 1)

The cam chain is the culprit. Looks like it was doing some self-clearing.

You can see in the images where the chain was making contact with the engine case/cylinder/cylinder heads.
The tensioner and guide wheels are badly worn and chipped. The cam guide is also badly worn, with the metal of one of the guide pins exposed, causing damage to its seat in the cylinder. The seat on the top engine case is also damaged. It's imprinted and cracked.

My guess is improper adjustment of the cam chain tensioner. (The tension didn't seem out of ordinary when I was pulling the camshaft and top end off). The engine still had all the original Philips head bolts and they looked new, so safe to assume I'm the first person since assembly to be inside this engine.

Anyone else seen something like this before?

I'm now wondering if the engine isn't worth saving. I doubt replacing the guide, chain, tensioner /guide wheel will be enough. The gouges in the guide pin seats on the engine case and cylinder looks like it'll cause the chain to wonder, slip or otherwise lead to accelerated wear, or worse, cam chain lockup/snapping.

I have a '76 CB750K in the shed that's waiting to be built so it's not a total loss if I have to put this on the back burner.
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Luugo86

'73 CB350, '78 XS650 Cafe Killer
Looks like there would be a lot of play in those guide seats. Some of the Jedi Masters here will know if it is worth saving or not, I hope it is, would be a shame to waste a motor
 

irk miller

You've been mostly-dead all day.
DTT BOTM WINNER
Replace the tensioner and cam chain, do a proper rebuild and roll. I wouldn't be worrying about that cam chain wear in the case unless it's completely worn through.
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
+1,

Honda twins of the era always look worse than that with long deep grooves. The rollers are destroyed and the chain has been flapping in the breeze like old style laundry on the washing line.

New chain, new tensioner or replacement rubber wheels, a good flush and she should be good to go as long as the rest of it isn't also worn out and neglected. Expect to need a valve job and new guide oil seals at least.
 

flynlo

New Member
Appreciate the replies. I’ll put some work into the engine and give it a fair shake, see how it’ll run. Truth be told, I had already ordered replacements for the worn cam chain bits before I noticed the damaged seats in the cylinders and upper case half. The rest of the engine is in great shape so should be a quick rebuild.

I pulled the valves today and all 4 exhaust valves have the slightest amount of play in the guides compared to the intake valves which have no play. Looks like I’ll be replacing those as well.
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
If you plan on replacing valve guides, you must use a suitable drift to remove them and do it with the head straight out of the oven. Typically Honda guides are pre-sized and don't need to be finished reamed, but when you fit them, the head should be heated and the guides cooled to avoid distortion and galling.
 

cardinal

Been Around the Block
I picked up a cratered CB450 with a more extreme version of that problem. One of the chain guides had broken off and so the chain was rubbing on the inside until it eventually wore a hole right through between the two pistons. I've been (slowly) getting it back together. I ended up replacing the piston cylinder and getting new valves and pistons and rings and a complete overhaul.
 

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