Author Topic: How many rings in an M20 piston  (Read 627 times)

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DEAN SOUTHALL

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How many rings in an M20 piston
« on: 25 April, 2017, 09:35:39 »
I've got a piston but needed rings. I can only find the usual three ring sets yet the piston I have has 4 ring grooves. The top (extra) one is thinner than the other, non oil ring. Having just got a NOS BSA replacement piston this also has 4 grooves but came with rings fiited, but only three. the top groove has no ring. Perhaps the top groove is not a ring groove but serves some other purpose. I've not come across this before so can anybody explain what's going on?

TTJOHN

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Re: How many rings in an M20 piston
« Reply #1 on: 25 April, 2017, 10:05:33 »
All M20s have four grooves in the piston, the top groove is left blank & does not need a ring in it, I think this was a thing by BSAs when they developed the 500 & 600 side valve left this as a sort of oil clearing groove, just make sure you have the rings the correct way up.

Regards TTJohn

Mike Farmer

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Re: How many rings in an M20 piston
« Reply #2 on: 25 April, 2017, 10:59:22 »
Greetings.

Now that's quite strange because at the mo I have a brand new M20 piston in my hand with four grooves. Three above the gudgeon pin and one below.

The top two are normal compression rings(of identical size) the third is an oil control ring and the one below the pin is also an oil control ring.

It is definitely M20 because I have fitted it and it runs perfectly in the barrel. I've actually changed to M21 spec so piston never used in anger, thus brand new.

So what have I got. Its not NOS so I guess that its after market. I cant even remember where I bought it.

Regards

MIke

DEAN SOUTHALL

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Re: How many rings in an M20 piston
« Reply #3 on: 25 April, 2017, 12:44:38 »
TTJohn. thanks for your informative and reassuring reply. Off tomorrow to get the barrel bored to fit the STD piston!

Mike. I have seen the pistons you describe, with a ring towards the bottom of the piston, on the original epuip BSA V-twin trike (but I'm running on B33 pistons as they are a damned sight lighter!). So perhaps its a prewar thing.

Mike Farmer

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Re: How many rings in an M20 piston
« Reply #4 on: 26 April, 2017, 09:42:55 »
 :)
Hi.
I've never really thought about the Three Wheels before so I have no idea. If I was interested where would I find one and at what sorta cost. I had a quick scan on E but nothing about them there. Take care

Mike

DEAN SOUTHALL

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Re: How many rings in an M20 piston
« Reply #5 on: 26 April, 2017, 16:54:37 »
Mike
The BSA Front Wheel Drive Club has info on BSA cars including trikes.
http://www.bsafwdc.org/
Complete Trikes are fetching a lot of money these days but of you fabcy a serious project I know of a dismantled one that is pretty much complete. It is a four cylinder engine model though.

JulianS

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Re: How many rings in an M20 piston
« Reply #6 on: 26 April, 2017, 17:57:09 »
From 1934

Mike Farmer

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Re: How many rings in an M20 piston
« Reply #7 on: 26 April, 2017, 19:19:08 »
 :)
Hi Dean and Julian.
Thanks for that. What sort of condition and price are we talking about. I don't understand the significance of it being 4 cylinder. Where is it.

If you would prefer to do so I am quite happy to take a personal message. But here is fine if its ok with everyone else.

General question: - whats the spares situation???

Mike 8)

JulianS

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Re: How many rings in an M20 piston
« Reply #8 on: 26 April, 2017, 20:08:21 »
The V twin and 4 cylinder models.

Mike Farmer

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Re: How many rings in an M20 piston
« Reply #9 on: 27 April, 2017, 09:32:18 »
 :).Hi

Thanks for that. Really interesting at least now I have some idea of what is being said.

Mike

DEAN SOUTHALL

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Re: How many rings in an M20 piston
« Reply #10 on: 27 April, 2017, 19:45:13 »
Mike
the one I know of is near Chesterfield. If you are seriously interested I'll try and get details and PM you.

The club has a reasonable selection of used spares and has had quite a few parts re-made

They tend to be pretty resilient, as do their owners, judging by this story...

http://www.bsafwdc.org/magazine-articles-republished/aa-men/

Mike Farmer

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Re: How many rings in an M20 piston
« Reply #11 on: 28 April, 2017, 07:09:59 »
Hi  Dean..

Sent you a PM.

Mike