Pushrod tube seals

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Pushrod tube seals

Postby Pharisee » Mon Nov 02, 2020 1:02 pm

When I rebuilt my M18S over last winter, I followed the advice given by some on this forum and used BS210 o-rings (4 on each tube) instead of the prescribed solid rubber seal at the top of both pushrod tubes. What with Covid-19 and other stuff, I haven't put too many miles on the bike since the rebuild... around 6-700. After a brief run out this morning, I was somewhat surprised to find the top of the crankcase covered in oil. It was running down the outside of the exhaust pushrod tube. "Oh dear!" says I... or words to that effect.
An hour later, I lifted the head and both pushrod tubes just dropped out. They weren't being held by the seals at all. Then the seals dropped out. They were very hard, and very definitely not the same shape as they were when I installed them. Square section o-rings tend not to seal very well, it seems. My fault.... I had bought Nitrile o-rings. What a mistake-a to make-a. I have now ordered the Viton equivalents (higher temperature tolerance and better resistance to oil) and will re-assemble in due course.
So... If you're thinking of doing the same, make certain that you buy the Viton o-rings and not the cheaper Nitrile ones.

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Re: Pushrod tube seals

Postby SPRIDDLER » Mon Nov 02, 2020 1:58 pm

Pharisee wrote:My fault.... I had bought Nitrile o-rings. What a mistake-a to make-a.

No help to you but I've not had any leakage from my Nitrile 'O' rings, one set of which was fitted for about five years and the other nine years. (I didn't replace them because they were leaking but as a matter of course when the head was off).

Square section o-rings tend not to seal very well, it seems.

I assume you mean they were originally round section but had become square section over time although I'm surprised that when in position in the head they would deform to square section.
You did fit the top flat washer in the head and the lower dished one at the top (reduced radius) end of the p/rod didn't you :?
Last edited by SPRIDDLER on Mon Nov 02, 2020 4:56 pm, edited 2 times in total.
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Re: Pushrod tube seals

Postby Pharisee » Mon Nov 02, 2020 4:53 pm

SPRIDDLER wrote:No help to you but I've not had any leakage from my Nitrile 'O' rings, one set of which was fitted for about five years and the other nine years. (I didn't replace them because they were leaking but as a matter of course when the head was off).


Perhaps not all nitrile seals are created equal?


SPRIDDLER wrote:I assume you mean they were originally round section but had become square section over time although I'm surprised that when in position in the head they would deform to square section.
You did fit the top flat washer in the head and the lower dished one at the top (reduced radius) end of the p/rod didn't you :?


Yes, the two metal washer were fitted and the o-rings were circular in section when I installed them. I assume that they had deformed to a square(ish) section by being compressed between the two metal washers when the head was torqued down and being constrained between the bore in the head and the outside of the tube. Maybe 4 rings was one too many. I'll try to ascertain how much compression is needed when I put it back together. A bit like getting the right compression (0.035") on the seals at the bottom of the pushrod tubes on the Meriden Triumph twins.
Last edited by Pharisee on Mon Nov 02, 2020 4:58 pm, edited 1 time in total.

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Re: Pushrod tube seals

Postby SPRIDDLER » Mon Nov 02, 2020 4:56 pm

How very strange that they should flatten/become squared off.
The lower of the two in your hand seems to have a peculiar annular indentation/groove running around it - looking more like the usual type of oil seal having a wire spring within the circumference.
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Re: Pushrod tube seals

Postby Pharisee » Mon Nov 02, 2020 4:58 pm

SPRIDDLER wrote:How very strange that they should flatten/become squared off.
The lower of the two in your hand seems to have a peculiar annular indentation/groove running around it - looking more like the usual type of oil seal having a wire spring within the circumference.


That was the bottom seal of the four. The indentation was caused by the dished washer.

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