valve hitting piston

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vincentvv
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valve hitting piston

Post by vincentvv »

Hello everyone,

My friend Maxim has a problem on his g80cs from 61. It's backfiring sometimes while driving (read loud bangs wich wakes up the village). So we openend the cilinder and besides it needed a good cleaning we noticed that the exhaust valve is slightly touching the piston. What could be the problem? Camshafts not in correct position, incorrect valve? Anybody ever saw something like this?

Image

Image

Kind regards,

Vincent!
Not fired is always a misser!
39speedtwin
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Re: valve hitting piston

Post by 39speedtwin »

It may require a thicker gasket or spacer between the barrel and crankcase.
ajsph
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Re: valve hitting piston

Post by ajsph »

it looks like a piston from 56 to 59
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Poul
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vincentvv
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Re: valve hitting piston

Post by vincentvv »

Is there a difference in height between a piston for a 56-59 and one for a 61?
Not fired is always a misser!
SPRIDDLER
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Re: valve hitting piston

Post by SPRIDDLER »

Has the head been skimmed?
When I enlarge the photo 300% it seems to have unusually rather rough annular rings in the finished surface. There's a nick or cut at 6 o'clock in the mating edge as well which makes me think it could be the remains of damage so the head was skimmed to get rid of the damage. :?

I would have thought that if the valve was hitting the piston due to the stem being too long the valve itself would be damaged in a one-off event and prevent th'engine running properly.
'1608' is probably on the money with his sticky valve suggestion.
Last edited by SPRIDDLER on Sun Aug 20, 2017 6:09 pm, edited 3 times in total.
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1608
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Re: valve hitting piston

Post by 1608 »

If there is only light damage and it has been coming on over time then it may be a sticking exh valve. You say it needs a clean, then maybe burnt oil residue has built up on the valve stem causing it to stick. Light damage seems to indicate that the valve is kissing the piston and being forced back onto its seat. Probably the cause of the back-fire too.
Let us know the result please.
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vincentvv
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Re: valve hitting piston

Post by vincentvv »

No Dont think the head has been skimmed, could be the exhaust valve sticking. Good thinking... I will try to replace the valves and let him try again and see if it runs witouth bangers...
Not fired is always a misser!
ajsph
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Re: valve hitting piston

Post by ajsph »

the piston crown is different, the same is the head.
Poul
g80csp11
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Re: valve hitting piston

Post by g80csp11 »

Even a high compression piston should not hit valve /piston

1) I'd advise double checking the valve timing is correct
2) is piston correct way round as cut outs are different sizes (difficult to get wrong as valves are different sizes )

you can do a dummy trial fit with some 'plasticine' on the piston and bar over to see how much clearance you have

a sticking valve is a possibility , but I would have though it would have seized that valve by now if that was the case

is it the black CS that he had in Poland , must admit it sounded like timing with banging

compression height ( centre of pin to top of piston on the G80CS short stroke engines are:

1956/59 compression height 2.297" ( compression ratio 8.7:1) piston length 3.703" part N 023123
1956/59 compression height 2.422 ( high compression 10:1) piston length 3.828"
1960 onwards compression height 2.453" ( compression ratio 8.8:1) piston length 4.053" part no 026665
1960 onward compression height 2.453" ( compression ratio 12:1) piston length 4.106" - flat top

not sure if this helps , but at least confirms piston dimension , most are from catalogue except HC 1960 onwards that I measured my piston
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PGGuse
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Re: valve hitting piston

Post by PGGuse »

isn't it the inlet valve?
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