LIGHTWEIGHT G2 PISTONS

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
dodger
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LIGHTWEIGHT G2 PISTONS

Post by dodger »

Pistons2.jpg
I have been looking for a 8:1 high compression piston for my G2 project. I was sold one by Roy Bellet but I am unsure if it is the 8:1 CSR piston or the 10.5:1 scrambler engine piston.
To me it looks a little tall compared with the standard piston in the picture to give just an increase from 7.8:1 up to 8:1 for the CSR, looks more like a hike up to 10.5:1 for the scrambler, though it could possibly be a 1965 on CSR piston which was upped to 9.5:1.
Can anyone with certainty identify the piston on the right as to wheather it is an early CSR piston, a later CSR piston or a Scrambler piston? There are no identity numbers or letters on the piston.

Laurie
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John Jarrett
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Re: LIGHTWEIGHT G2 PISTONS

Post by John Jarrett »

To positively identify can you post a picture of the crown of the piston?
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ajscomboman
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Re: LIGHTWEIGHT G2 PISTONS

Post by ajscomboman »

It's a CS piston 10.5 - 1 without a shadow of a doubt. I know that because the other guy who wanted to buy it before you got to Roy's stand specialises in CS lightweights and required it for a project.
dodger
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Re: LIGHTWEIGHT G2 PISTONS

Post by dodger »

Piston crowns.jpg
Thanks Rob, I suspected as much, shame I've parted with £30.00 for something I can't use.
I've posted a picture of the piston crowns so that John Jarret can also confirm what you have said, thanks John in anticipation.

I'll have to post a wanted Ad. to see if anyone has a CSR piston going begging.

I wonder why the club doesn't stock them, there seems to be quite a few Lightweight CSRs in the club.

Laurie
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G15 Roy
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Re: LIGHTWEIGHT G2 PISTONS

Post by G15 Roy »

Being such a good chap and don't want to spoil my reputation 100 % refund just bring it back next Saturday.
Roy
dodger
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Re: LIGHTWEIGHT G2 PISTONS

Post by dodger »

Thanks Roy, will do, what a good chap you are.

Laurie
John Jarrett
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Re: LIGHTWEIGHT G2 PISTONS

Post by John Jarrett »

Thanks for the picture of the crown of the piston. Rob, it may be a CS piston but it is definitely not a 10.5:1 piston. I have attached a picture of the crown of a genuine BHB CS 10.5:1 piston which has a much higher crown.
IMG_1335a.jpg
Kind regards,
John
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Janet
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Re: LIGHTWEIGHT G2 PISTONS

Post by Janet »

dodger wrote:I'll have to post a wanted Ad. to see if anyone has a CSR piston going begging.

I wonder why the club doesn't stock them, there seems to be quite a few Lightweight CSRs in the club.
I don't know for certain why not but I suspect that one very good reason is that there were 4 different standard piston manufactured for 250 lightweights. The early CSR used the same as the G2/M14, the 1965/66 CSR had a different one and the CS had two in different years. Add to that the different oversize pistons that later owners require and you'd need a stock of at least 4 sizes for each one, so 16 pistons and rings to cover every requirement.

The spares department stock pistons for the basic G2/M14. I don't know whether these could be used in the CS or later CSR, with presumably a reduction in performance. My 64 CSR has the same barrel and head as the 65/66 CSR but uses the same piston as the G2 so I can get one if I need to. These days with a small old bike, 75mph is fast enough for me. I never was a daredevil thrill seeker.
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dodger
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Re: LIGHTWEIGHT G2 PISTONS

Post by dodger »

Now I don't really know what I've got, John Jarre'ts picture of the piston crown certainly looks to be a taller example than mine as the flat band across the top is narrower than mine, perhaps it is a 9:1 later CSR item after all.

Anyone else got any ideas?

Thanks John for the input.

Laurie
John Jarrett
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Re: LIGHTWEIGHT G2 PISTONS

Post by John Jarrett »

Going through my scrap second hand pistons, the "match" by eye to the shape of the crown on your piston is 043646. That piston is from a 1965/66 CSR. From the promotional material the stated compression ratio in 1965 was 9.5:1.
Kind regards,
John
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