G80 Piston

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Dave T_LAPSED
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G80 Piston

Post by Dave T_LAPSED »

Seasons Greetings to everyone! Hope you had a Cool Yule.
As a Christmas treat to my latest aquisition, G80S 1959, I took off the head to cure the flood of oil which poured from the pushrod tubes!
I know, I know I shouldn't have touched it .... But I did!
The bore is at 83.5mm and smooth and only a tiny bit polished/wornin half way up. The piston, however, is a GPM seemingly new apart from the two little points fore and aft just under the oil control ring where it has "nipped up" at some time.
Thus I could reuse it but would prefer to replace it. I have seen a +040 piston on ebay but it is advertised as pre1955 can anyone tell me the difference between that and a post '55, is it just the compression?
Thanks in advance for any clarifications, Dave.
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Rob Harknett
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Re: G80 Piston

Post by Rob Harknett »

Check the year and model of your engine / bike, there was no 1959 G80S. G80S became known as G80 1958 onwards. G80S and as known as G80 1958 onwards was 82.5 x 85.5. G80CS of this time period was 86 x 82.5. You can check 59 , 55 and 54 parts lists here www.archives.jampot.dk to compare how pistons differed during the era + CR's with or without compression plate. Illustrations will show how different hi & low cr look, also bore and stroke clues, to decide if the e bay piston is correct for your bike, to your own satisfaction, having compared to what engine you have, parts lists and the piston on e bay. Check your complete engine number 59/G80 CS will be 3444 to 3845, 59/ G80 133119 onwards, if lower its an earlier G80S ( G80CS is of cause a comp machine so should also have a comp frame number with C suffix, not a road model number prefixed with an A. )
Last edited by Rob Harknett on Sat Dec 29, 2018 2:49 pm, edited 1 time in total.
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spookefoote1956
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Re: G80 Piston

Post by spookefoote1956 »

Got mine from Feked. JP piston. Works fine. £139.95.
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Duncan
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Re: G80 Piston

Post by Duncan »

Hi Dave

This link should give you some more information on the pistons and compression ratios: http://archives.jampot.dk/technical/Pis ... x-1962.pdf

You may find that not all oversize pistons from different manafactures are exactly the same dimensions, I changed a 350 one for the same oversize and had to hone the barrel to get sufficient clearance.

Edit:

I have just remembered that some modern 350 pistons have slightly larger gudgeon pins but I have no experience of modern 500 ones perhaps someone can comment, if it is different it is not a show stopper but you may have to re-bush the small end.
Dave T_LAPSED
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Re: G80 Piston

Post by Dave T_LAPSED »

Thanks for the replies!
My bike is a G80, don't know where I got the S from but it's definitely a '59 G80, I even have the club dating certificate, What a plonker!
If I can't find an old piston to fit I will go for a JP as they seem to be all that's available currently and ,yes, that might entail honing the bore. I know I shouldn't have opened the can of worms but the oil seals on the pushrod tubes came out like bakelite! One was split down it's length, I'll be replacing them with a stack of viton o-rings filled with high temp silicone.

Thanks again, Dave.
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Rob Harknett
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Re: G80 Piston

Post by Rob Harknett »

Most pistons on e bay are G80CS..NOS less common for the most common G80 /G80S bikes. New JP's may prove tight, but at less if tight, the bore could be opened a bit to fit.
Richard Mills
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Re: G80 Piston

Post by Richard Mills »

I don't think that it's a good idea to use silicone, excess may get into the oilways and block them.
Dave T_LAPSED
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Re: G80 Piston

Post by Dave T_LAPSED »

Hmm Silicone, I'll wait til it's cured before I tighten it up and I can remove any excess from the top of the tubes before I fit the rockers, good point, thanks, I'd hate to do all the work and then spoil the ship for a ha'porth of tar!
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Rob Harknett
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Re: G80 Piston

Post by Rob Harknett »

Dave T wrote:Hmm Silicone, I'll wait til it's cured before I tighten it up and I can remove any excess from the top of the tubes before I fit the rockers, good point, thanks, I'd hate to do all the work and then spoil the ship for a ha'porth of tar!
Will that work OK e.g will it compress and seal as you tighten the head down. Or flatten out the bottom of your push rod tubes. The thin cup washers in the head may get gunged up and stuck in the head. Be a job cleaning up next time. I prefer to use the correct rubbers etc.
Dave T_LAPSED
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Re: G80 Piston

Post by Dave T_LAPSED »

Thanks Rob but I don't trust modern rubber at such temperatures in the head, if it doesn't work I'll use the rubbers from the gasket set then!
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