Fractured Cam Chain Guide

javaman

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Lexington, MA
I recently began restoring a 1973 CB450K6. I disassembled the engine in the fall of 1978 after having heard some alarming engine knocks. I shut it down and started to take the engine apart. I am trying to finish what I started. I've been through the crankcase, the transmission, and the crankshaft and discovered no obvious problems. Today I started disassembling the cylinder head, where I discovered that the rear cam chain guide had failed. The bracket is broken and one edge of the wheel appears to have broken off from the whole. The front guide appears to be intact. The cam chain tensioner shows no obvious problems although the plastic wheel shows moderate wear. I'd appreciate your thoughts on likely cause, places to look for or rule out related damage, and any other recommendations for how to proceed.rear cam guide.jpg
 
Yuck that does look bad. How many miles on the engine? I would replace the cam chain and any of the rollers that appear worn or damaged and do a careful inspection of the rest of the head to make sure the valves and cams are in good shape.
 
It’s very likely that your cam chain got loose enough for the cam to skip a tooth.

Are there any grooves in the cam chain tunnel on the head? Have you inspected all the valves and followers? It’s not unusual for valves to get bent in those cases.
 
It's awesome that you are coming back to this after almost 48 years. Most of the time it's a new owner picking up where the previous owner left off, but the times where an original owner sees new life in their own machine are the best.

Are you aware of CMSNL? It's a company based out of the Netherlands that has a big collection of NOS and aftermarket parts for a variety of motorcycles. Here is a link to the fiches for your model. They seem to keep some stock in Florida and move things as needed. I've made a few orders and have been thoroughly impressed. David Silver Spares USA is another helpful source for parts as well as Partzilla and eBay.

I'm wondering why your roller doesn't resemble the one shown in the fiche, which appears to be more of a sprocket than a pure roller. We're looking at part #12, right?

1628.png

CMSNL has a few NOS rear roller units at over $200 and there are some on eBay at various price levels. Here's a cheapish one. It's always possible that a member could have a spare at a reasonable price, too. There is a parts wanted section for such inquiries.

With the motor apart it's a good opportunity to replace anything questionable, if better replacements can be had within budget. You may as well split the cases and replace all seals while you're in there, too.

I hereby nominate this endeavor for a project thread — a completely unofficial and utterly friendly suggestion. Mainly, I don't want to miss out on the eventual return to roadworthiness.
 
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stl360+450 said:
I'm wondering why your roller doesn't resemble the one shown in the fiche, which appears to be more of a sprocket than a pure roller. We're looking at part #12, right?
Click to expand...
That guide sprocket completely disintegrated, the outer sprocket teeth are all gone so that explains the ugly noise he heard long ago. I can't say I've ever seen one that bad. I'd be replacing the cam chain (TwoWheelsFrank used to be the best price on an OEM DID chain but maybe not now with tariffs and shipping) and closely inspecting all the other guides/rollers and the tensioner assembly too. The chain must have been pretty loose for a while to help create that situation, though it could have just been a fluke defective guise but I've never seen one of those disintegrate like that ever. This is what it really looks like

1784290205082.png
 
RobMan said:
Yuck that does look bad. How many miles on the engine? I would replace the cam chain and any of the rollers that appear worn or damaged and do a careful inspection of the rest of the head to make sure the valves and cams are in good shape.
Click to expand...
13041. I'm tearing down and cleaning the top of the engine today.
 
EzPete said:
It’s very likely that your cam chain got loose enough for the cam to skip a tooth.

Are there any grooves in the cam chain tunnel on the head? Have you inspected all the valves and followers? It’s not unusual for valves to get bent in those cases.
Click to expand...
that makes sense. So far, I don't see any grooves on the head, but I do see groves on both the lower edge of the cylinder and the upper surface of the crankcase.cam chain wear of cylinder bottom.jpgupper crankcase cam chain wear.jpg
 
ancientdad said:
That guide sprocket completely disintegrated, the outer sprocket teeth are all gone so that explains the ugly noise he heard long ago. I can't say I've ever seen one that bad. I'd be replacing the cam chain (TwoWheelsFrank used to be the best price on an OEM DID chain but maybe not now with tariffs and shipping) and closely inspecting all the other guides/rollers and the tensioner assembly too. The chain must have been pretty loose for a while to help create that situation, though it could have just been a fluke defective guise but I've never seen one of those disintegrate like that ever. This is what it really looks like

View attachment 59477
stl360+450 said:
It's awesome that you are coming back to this after almost 48 years. Most of the time it's a new owner picking up where the previous owner left off, but the times where an original owner sees new life in their own machine are the best.

Are you aware of CMSNL? It's a company based out of the Netherlands that has a big collection of NOS and aftermarket parts for a variety of motorcycles. Here is a link to the fiches for your model. They seem to keep some stock in Florida and move things as needed. I've made a few orders and have been thoroughly impressed. David Silver Spares USA is another helpful source for parts as well as Partzilla and eBay.

I'm wondering why your roller doesn't resemble the one shown in the fiche, which appears to be more of a sprocket than a pure roller. We're looking at part #12, right?

View attachment 59471

CMSNL has a few NOS rear roller units at over $200 and there are some on eBay at various price levels. Here's a cheapish one. It's always possible that a member could have a spare at a reasonable price, too. There is a parts wanted section for such inquiries.

With the motor apart it's a good opportunity to replace anything questionable, if better replacements can be had within budget. You may as well split the cases and replace all seals while you're in there, too.

I hereby nominate this endeavor for a project thread — a completely unofficial and utterly friendly suggestion. Mainly, I don't want to miss out on the eventual return to roadworthiness.
Click to expand...
yes, i found cmsnl, and thanks for calling attention to how bad the roller had been destroyed
Click to expand...
 
thanks for explaining what likely happened, and for a pointer to a supplier. There is some damage to the other rollers, and some grooves where the chain was contacting the crankcase and the passageway in the cylinder
 
EzPete said:
You're going to want to go deeper and make sure the sprocket on the crank wasn't damaged when the chain failed as well.
Click to expand...
thanks, I've got everything torn apart and mostly cleaned up. I will check the full cam chain path, and will be changing the chain.
 
javaman said:
thanks for explaining what likely happened, and for a pointer to a supplier. There is some damage to the other rollers, and some grooves where the chain was contacting the crankcase and the passageway in the cylinder
Click to expand...
Seeing wear grooves on both sides of that cylinder tunnel tells me the cam chain was pretty damn loose at one point, and for a while. That likely contributed to the guide sprocket blowing apart.
 
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