1963 G80CS O-rings at barrel base

Information relating to the Matchless G85 500cc Heavyweight, AJS 7R, Matchless G45 and Matchless G50
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Re: 1963 G80CS O-rings at barrel base

Postby ajsph » Sat Aug 17, 2019 4:22 pm

Yes I buy them individually, try with 4.5mm and an outer diameter there fit the outer diameter in your barrel, and I use viton o rings
Poul

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Re: 1963 G80CS O-rings at barrel base

Postby raffles » Mon Aug 19, 2019 6:00 pm

There has been a problem with these "O" rings I have found that the O/D is correct the thickness is wrong
I have reduced the thickness by rubbing them on wet and dry using a circular motion until the correct thickness
is achieved about 8 to 10 thou above the groove.
Tony

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Re: 1963 G80CS O-rings at barrel base

Postby jack clegg » Tue Aug 20, 2019 10:19 am

Aha! I finally flushed out another discerning G80CS owner. Well Tony, you are bang on. 29m OD is correct & the thickness is the problem. I too attempted to reduce their dimensions, but it was far simpler just to buy the 4mm thick type. Easily found on Ebay & delivered in two days.
The real problem here is that these 5mm thick type will work perfectly well & appear to pass under the radar of most CS owners. It is however only on disassembly that one finds evidence of the crush-cut damage & an inkling that something is not quite right.
I appeal to all short-stroke alloy barrelled single owners to heed this small issue & report to their spares suppliers that 5mm is too thick.
John Marshall

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Re: 1963 G80CS O-rings at barrel base

Postby Rob Harknett » Sat Sep 07, 2019 11:50 am

Just a thought about sealing push rod tubes. Circa 1929 Wolverhampton push rod tubes were in two parts sprung loaded. At the time, rockers etc. were greased. No oil to the top end. The lower end of the sprung loaded tube just had a flat that sat on a felt washer. I guess this was good enough to seal against, what? Just muck from outside getting in. At the top end the tube was just a slide in fit. Felt washers continued to seal against grease in wheels for many years, also lasted many years. Why was grease dropped in favour of oil, to lubricate rockers etc. But continued in use on other bearings etc. I guess it was done so we got rubber seals and steel washers to use to prevent oil leaks. To give us something to talk about in trying to make them seal.

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Re: 1963 G80CS O-rings at barrel base

Postby ajscomboman » Sat Sep 07, 2019 1:16 pm

This thread has nothing to do with pushrod tubes Rob! There are no tubes on a shortstroke motor merely pushrod tunnels cast in the barrel. Please don't confuse and muddle this thread.

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Re: 1963 G80CS O-rings at barrel base

Postby Rob Harknett » Sat Sep 07, 2019 1:46 pm

ajscomboman wrote:This thread has nothing to do with pushrod tubes Rob! There are no tubes on a shortstroke motor merely pushrod tunnels cast in the barrel. Please don't confuse and muddle this thread.

Apologies Rob, I searched push rod tubes and this was the first/latest topic that showed up. Delete all if you wish.

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Re: 1963 G80CS O-rings at barrel base

Postby Stephen R » Tue Sep 10, 2019 5:33 pm

Mine has a '59 18CS (G80CS) motor with the same problems.
I have bodged it by putting two smaller section orings in each tunnel.
Seems to be okay.
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Re: 1963 G80CS O-rings at barrel base

Postby mdt-son » Sun Feb 23, 2020 5:22 pm

ajsph wrote:I am sure that I used a 3mm O Ring on my bike, but when your seat is 3.8 mm you have to use a 4.5mm O ring, an O ring has to be compressed 10 to 20%, and when the oil pressure is from the inside it has to fit the outside of the seat.
Poul


Since rubber is incompressible measured by volume, compression in one direction results in extension in the other two directions, the effect of which is a bulging of the free sides.
Thus, fitting a 5mm thick O-ring in a groove of 3.8mm may work thickness-wise (it will need a LOT of compression force though) but will ultimately result in the O-ring bulging inwards, causing the joint to leak.
Less is more! I suggest using an O-ring of 4.2mm thickness and 0.2 mm clearance on the outer diameter.

-Knut

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