1963 G80CS O-rings at barrel base

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1963 G80CS O-rings at barrel base

Postby jack clegg » Fri Aug 31, 2018 12:36 pm

Dear Folks,

Has anyone else found problems with the two thick O-rings which fit in the lower pushrod recesses in the cylinder barrel? The ones I just removed had been cut on both ID & OD by the edges of the recesses in the barrel. When I reassembled with new ones, the barrel did not appear to seat fully home at the base. I could get a 3 thou gauge in the gap (at 40 foot pounds). Also the pushrod clearance had increased. I lifted the barrel again & found the new ones deformed at their circumference. I ended up slicing away the outer circumference of the O-rings & refitting, but am not really happy with this solution. When I first got the bike I found these O-rings were missing. I'm wondering if these O/S O-rings were the reason why.
I have not yet had the opportunity of consulting with Steve at AMCCS, maybe Monday if he don't see this post. I cannot believe they are the correct size for the application. You should see the mess these old O-rings are in.

Regards Jack
John Marshall

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Re: 1963 G80CS O-rings at barrel base

Postby jack clegg » Thu Aug 15, 2019 10:24 am

Despite the overwhelming response to this thread, I have more for those interested. Took the barrel off again this week. The modified (reduced circumference) O-rings I last fitted, were still being chopped up by the barrel. After measuring these O-rings (5mm thick 29mm OD) & their recesses in the bottom of the barrel, I concluded that 4mm thick 29mm OD O-rings were the correct type. Got some today & they fit so much better. Despite being 1mm thinner, they still stand proud of the barrel base & I reckon will solve the problem.

What size does the club supply I wonder?

Am I the only person to have found these O-rings to be a problem?

TTFN Jack
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Re: 1963 G80CS O-rings at barrel base

Postby Rob Harknett » Thu Aug 15, 2019 12:11 pm

The only time I have had a problem is when I failed to make sure the head was clean & all steel washers and rubbers correctly fitted. However, I have never used rerpro parts yet except push rod tubes. Which at least confirmed the repro tubes I got hold of were OK.
Here's some dimensions of rubbers to consider. 1939 /1955 I have not checked before or beyond that time period, which used.
Top rubber . Old pt. no. 38-G4-E368 / new part no. 016372
Rubber Dia 4 mm i/s dia 21 mm o/s dia 29 mm
Bottom rubber. Old pt. no. STD691 / new pt. no. 000691
Rubber thickness 4 mm I/s dia 18 mm o/s dia 26 mm height 13.5 mm
Steel washer thickness as stated in parts list

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Re: 1963 G80CS O-rings at barrel base

Postby jack clegg » Thu Aug 15, 2019 3:24 pm

Cheers Rob. Just want to remind you this is an alloy 1963 barrel, no pushrod tubes or steel washers. Interestingly, the top rubber (016372) dims you quote are exactly what I have now fitted to the barrel base. What is the part number of the G80CS lower O-rings one wonders? I would bet money on them being the same.
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Re: 1963 G80CS O-rings at barrel base

Postby ajsph » Thu Aug 15, 2019 3:44 pm

On my 18CS I use this size of o rings Top ID 19mm Thickness 5mm Bottom ID 23mm thickness 3mm
regards Poul

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Re: 1963 G80CS O-rings at barrel base

Postby jack clegg » Fri Aug 16, 2019 10:07 am

Gor blimey Poul, that's odd. I can't see how a 3mm o-ring would seal the bottom. The depth of the lower o-ring seat is about 3.8mm. Are you sure of this?
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Re: 1963 G80CS O-rings at barrel base

Postby Rob Harknett » Fri Aug 16, 2019 10:31 am

jack clegg wrote:Cheers Rob. Just want to remind you this is an alloy 1963 barrel, no pushrod tubes or steel washers. Interestingly, the top rubber (016372) dims you quote are exactly what I have now fitted to the barrel base. What is the part number of the G80CS lower O-rings one wonders? I would bet money on them being the same.

I did not realise your engine never had push rod tubes. Parts list and sales cats seem to suggest it did, but not studied it thoroughly.
016372 has been used for many years. See old part no. 38-G4 etc. Introduced in 1938 on the Matchless G4. I seem to recall that's when push rod tube dia. got larger

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Re: 1963 G80CS O-rings at barrel base

Postby ajscomboman » Fri Aug 16, 2019 3:26 pm

Rob Harknett wrote:
jack clegg wrote:Cheers Rob. Just want to remind you this is an alloy 1963 barrel, no pushrod tubes or steel washers. Interestingly, the top rubber (016372) dims you quote are exactly what I have now fitted to the barrel base. What is the part number of the G80CS lower O-rings one wonders? I would bet money on them being the same.

I did not realise your engine never had push rod tubes. Parts list and sales cats seem to suggest it did, but not studied it thoroughly.
016372 has been used for many years. See old part no. 38-G4 etc. Introduced in 1938 on the Matchless G4. I seem to recall that's when push rod tube dia. got larger


It's a shortstroke motor Rob, none of them had pushrod tubes as they were cast tunnels in the ali barrel and in later tourer motors into the iron barrels.

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Re: 1963 G80CS O-rings at barrel base

Postby ajsph » Fri Aug 16, 2019 4:39 pm

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.
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Re: 1963 G80CS O-rings at barrel base

Postby jack clegg » Sat Aug 17, 2019 10:46 am

Where did you get your O-rings from Poul? From a standard gasket set or did you buy them individually? I could see a 3mm o-ring fitting the top recesses but no way the lower recess. The top ones on mine are 2mm thick but work fine. It is these 5mm lower ones which are the problem. 5mm is too big & as the barrel is torqued down & the o-ring compresses, it has no room to deform & spreads out into the gasket mating face area on the outer edge & into the pushrod tunnel area on the inner edge. As the final torque is applied the barrel cuts into the O-rings on both outer & inner edges. Whilst it appears to seal OK, when dismantling you find the o-rings are very badly cut by the edges of the o-ring recess in the lower barrel.
John Marshall

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