Newby with G80S piston questions

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Newby with G80S piston questions

Postby AndyO » Fri Oct 30, 2020 9:24 am

Hi everyone,
Finally got my hands on a 1954 G80S
Got it running after 30 years under a sheet! Lots of smoke and oil condensing in silencer. Head off, barrels off.
Oil seal ring broken, lots of play in exhaust valve/guide. Piston is marked 15thou o/s
Looking at manual it states 2 oversizes available, +20 and +40 so first mystery! Can I get rings for +15thou?
Manual states valve guide should have circlip. Nothing on mine. After much heat I drifted old exhaust valve guide out. On the spares website I can only see valve guides with groove for circlip. Ok to fit this?
I’ll have more questions about the 89N carb another time!
Any advice on my piston issues gratefully received!
Andy
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Re: Newby with G80S piston questions

Postby Pharisee » Fri Oct 30, 2020 10:00 am

I've not heard of a +15 oversize before but I'm relatively new to AMC machines myself. Someone much wiser than I will help with that, I'm sure.
I changed the valve guides on my '54 G3/LS. The old ones had no circlip, the new ones did. It wasn't a problem. Put the circlip on the guide and drift it in until the circlip is resting against the head.
You may need to change the steel valve spring plates that sit over the guide. You will need a pair that are relieved to sit over the circlip. The one on the left is for the circlip, the one on the right isn't.
John

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Re: Newby with G80S piston questions

Postby ajscomboman » Fri Oct 30, 2020 10:10 am

Pharisee wrote:I've not heard of a +15 oversize before but I'm relatively new to AMC machines myself. Someone much wiser than I will help with that, I'm sure.
I changed the valve guides on my '54 G3/LS. The old ones had no circlip, the new ones did. It wasn't a problem. Put the circlip on the guide and drift it in until the circlip is resting against the head.
You may need to change the steel valve spring plates that sit over the guide. You will need a pair that are relieved to sit over the circlip. The one on the left is for the circlip, the one on the right isn't.
John


Blimey John I hope you didn't drift the guide in that way! The correct way is to heat the head drift the guide up remove the clip and then drift the guide out into the combustion chamber. The insertion is a reversal of that procedure, insert the new guide from the combustion chamber drift in until the circlip can be fitted then drift back down till the circlip is held captive.

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Re: Newby with G80S piston questions

Postby AndyO » Fri Oct 30, 2020 12:09 pm

I drifted my exhaust guide out from the combustion chamber straight out. It was VERY tight even with lots of heat. My old Bonneville guides almost dropped out with heat! When I get a new one I’ll have to machine a snug fitted drift with a shoulder, unless anyone has any other suggestions for improvised drift tools.
Andy

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Re: Newby with G80S piston questions

Postby thornebt » Fri Oct 30, 2020 4:26 pm

+30 pistons / rings are also available but I think they are more difficult to find. It's another option though if you can locate one.

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Re: Newby with G80S piston questions

Postby g80csp11 » Fri Oct 30, 2020 5:46 pm

you can get any size ring from cox and turner , they even grind if necessary to fit non standard groove width

https://www.coxandturner.co.uk/piston-rings

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Re: Newby with G80S piston questions

Postby Andy51 » Fri Oct 30, 2020 6:10 pm

Hi Andy, you are correct that the factory oversizes were +20 and +40 thou, but over the last 70 odd years several companies have made pistons for our bikes. My 18S came with a +30 (in good nick, I still have it, happy to sell, PM me) which I replaced with an 8.5 to 1 +40 JP one as part of a rebuild. I also have a UK-made +60 version picked up as a spare at Kempton, so I wouldn't be surprised if someone had made a +15 one! Cheers, Andy

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Re: Newby with G80S piston questions

Postby Pharisee » Fri Oct 30, 2020 7:14 pm

ajscomboman wrote:
Pharisee wrote:I've not heard of a +15 oversize before but I'm relatively new to AMC machines myself. Someone much wiser than I will help with that, I'm sure.
I changed the valve guides on my '54 G3/LS. The old ones had no circlip, the new ones did. It wasn't a problem. Put the circlip on the guide and drift it in until the circlip is resting against the head.
You may need to change the steel valve spring plates that sit over the guide. You will need a pair that are relieved to sit over the circlip. The one on the left is for the circlip, the one on the right isn't.
John


Blimey John I hope you didn't drift the guide in that way! The correct way is to heat the head drift the guide up remove the clip and then drift the guide out into the combustion chamber. The insertion is a reversal of that procedure, insert the new guide from the combustion chamber drift in until the circlip can be fitted then drift back down till the circlip is held captive.


The old guides didn't have circlips, so they were drifted straight into the combustion chamber. As said above, the new guides were fitted with their circlips and pushed into the head from the rockerbox. The head had been heated in the oven to 200°C and the guides had been in the freezer. They pressed in with very little effort and I'm certain that neither the head nor the guides suffered any damage. The valve seats were subsequently re-cut and new valves lapped in.
John

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Re: Newby with G80S piston questions

Postby 56G80S » Sat Oct 31, 2020 8:15 pm

Maybe not damage done but Rob is right and I personally suffered from a motor vehicle machine shop ignoring my instructions in this regard; long story.

I usually do the job myself but didn't on this occasion and it resulted in my non at attendance at an IJR a few years ago when after a couple of thousand miles the exhaust valve came so loose that once it was eventually running at my cousins house in Basildon (en route to Harwich) I turned round and went home without stopping. Filled up with petrol at the furthest pump from the kiosk after tweaking the throttle stop screw to keep running, went and paid, ride away.

Johnny B

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Re: Newby with G80S piston questions

Postby Pharisee » Sat Oct 31, 2020 11:05 pm

Fair enough... I take your point and bow to the forum's collective experience and knowledge, but they were new, cylindrical, valve guides going into a pre-heated, vapour cleaned cylinder head. Physically, it was easier to push them in from the rockerbox side with the circlip fitted... What's the potential problem with that?
John

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