What could cause a valve guide to crack at the bootom

Kool_Biker

Kool_Biker

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After 6K + great riding kilometers on my 1972 750 Commando, I noticed the bike - gradually - beginning to smoke after a cold start up. The smoke became intense but only lasted for about a minute, after which the Commando rode and felt healthy.

So upon inspection of the head, I noticed at least two valve guides had hairline cracks at the combustion side.

What could possibly cause this?
 
Misread.

Guides cracking is an odd one. The exhaust one could get cracked if carbon buildup on the valve does not get removed as the valve gets pulled into the guide and so the guide cracks.
 
Last edited:
Kool_Biker said:
After 6K + great riding kilometers on my 1972 750 Commando, I noticed the bike - gradually - beginning to smoke after a cold start up. The smoke became intense but only lasted for about a minute, after which the Commando rode and felt healthy.

So upon inspection of the head, I noticed at least two valve guides had hairline cracks at the combustion side.

What could possibly cause this?
Click to expand...
Weird. The only things that come to mind is what @kommando said or that the guide (or maybe valve) is worn and wobbling as it opens and closes. Also, not sure that's the cause of the smoke - could be intake seals and/or parking the bike on the side stand. If the guides are the problem, I would expect it not to clear up. Does it smoke from both exhausts?

I don't find them hard to change but you will get a LOT of advice on that, and I have been very lucky to be able to lap in the new valves without cutting the seats.

If you were in the US I would tell you if not comfortable doing it, talk to Jim Comstock. If you do it yourself do not pull them in or out cold - heat is required and proper pulling tools which I'm not sure are available. I bang them out and in but only with a closely sized drift which I had made. 5/16" and 8mm drifts will work, but both are slightly small, and the guide may be slightly mushroomed at the driving end requiring a little reaming especially is not cast iron.
 
kommando said:
Misread.

Guides cracking is an odd one. The exhaust one could get cracked if carbon buildup on the valve does not get removed as the valve gets pulled into the guide and so the guide cracks.
Click to expand...
It is odd. My (limited) experience is a coked-up exhaust valve that gets stiff to move in the guide fails to close completely, causing very noticeable running symptoms.
 
Kool_Biker said:
After 6K + great riding kilometers on my 1972 750 Commando, I noticed the bike - gradually - beginning to smoke after a cold start up. The smoke became intense but only lasted for about a minute, after which the Commando rode and felt healthy.

So upon inspection of the head, I noticed at least two valve guides had hairline cracks at the combustion side.

What could possibly cause this?
Click to expand...
It's a long time since I've looked at commando valve guides and I'm away from my workshop to check
But are your guides slightly counter bored inside?
The counterbore shrouds the valve stem a little and stops the build up of carbon on the valve stem that can make a valve stick
Maybe yours have cracked instead of the valve sticking?
Where did the guides come from? What are they made of?
 
Thank you all for your prompt responses. Yes, the head was pretty cocked up (my fault with earlier carb settings) and there are slight if visible wear marks on the valve stems - hence possible 'wobble'. Both sides were smoking, started at about the same time, and yes, the smoke completely cleared after a minute or two. Not sure what the guides were made of. In any case the head will soon be going to a competent shop in the UK. They might discover more ...
 
Kool_Biker said:
Thank you all for your prompt responses. Yes, the head was pretty cocked up (my fault with earlier carb settings) and there are slight if visible wear marks on the valve stems - hence possible 'wobble'. Both sides were smoking, started at about the same time, and yes, the smoke completely cleared after a minute or two. Not sure what the guides were made of. In any case the head will soon be going to a competent shop in the UK. They might discover more ...
Click to expand...
Do the guides look bronze in colour or steel? Do you know who supplied them?
What competent shop are you planning to use?
 
The valves are steel, and there are no tel tale signs of a piston strike. They were supplied and fitted by Norvil (UK) which undertook the original resto of my head 20+ years ago. As for a competent shop in the UK, they again come to mind, although I would like to have offers from others shops, so please - please propose. Many thanks
 
Kool_Biker said:
The valves are steel, and there are no tel tale signs of a piston strike. They were supplied and fitted by Norvil (UK) which undertook the original resto of my head 20+ years ago. As for a competent shop in the UK, they again come to mind, although I would like to have offers from others shops, so please - please propose. Many thanks
Click to expand...
Norvil would be the very last place I'd ever use
i very much doubt those guides were made of the correct material and that's where your problem comes from
 
kommando said:
Steer well clear of Norvil, if anything goes wrong again they will pin the blame on you.


or


Both have long waiting lists for good reason.
Click to expand...
Bit late regarding Norman, he told me earlier this year that his plan was to retire in November, and, it's November.

For a job like this I would certainly suggest Pat Seager, Seager Engineering as above.

Pete Lovell is has probably done work for Norvil in the past, but I understand direct dealing with him is a different and more positive experience!

Not sure who else!
 
Many years ago, when I was racing my short stroke Triton 500, It went off-tune, and I could not find the reason. When I took the head off the motor, there was a small mark on the piston and a small dent in the exhaust valve. Until I found that, I was tuning to try and regain the loss of performance. I had replaced the valve guides. I had wanted bronze, but could only get cast iron guides. One of the guides had a small triangular piece missing out of the end of it, and that had gone through the valve seats.
 

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