Wrong pistons, Need advise for correct ones please

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Wrong pistons, Need advise for correct ones please

Postby Sibs6 » Thu Jan 16, 2020 2:12 am

I had my cylinders on my 59 G12 honed and a new piston kit set for them. Thought all was good till I turned the motor over to set the distributor and the pistons bottomed out close to BDC.
After much research and measuring, I believe I have a set of 56/58 G11 pistons. ( GPM 726 and have 72 5 on the top).
I believe the pistons are +20 .
My cylinder bores are 72.56 and 72.54
Going by this info, am I right at +20? What brand would yas recommend and where would you suggest I purchase them from.
I seem to remember the original pistons saying 20 on top but that's 35+ years ago .
Thanks Sibs6

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Re: Wrong pistons, Need advise for correct ones please

Postby g5wqian » Thu Jan 16, 2020 6:52 am

hi SIbs ;

2.835" is standard , 2.855" is +20 , 2.875" is +40 , sorry i dont have metric equivalents off my head .

the g11 and g12 use same bore size but different stroke although there was one version i think around the time 1959 yours was made that had odd pistons .
first thing is check how many fins on your cylinder barrels , 7 is for a G11 , 8 is for a G12 , because you have a 59 model which may be an export g12 and i think in 1959 some were made using a 650 crank but G11 barrels and special pistons made to suit to alter the piston crown height/gudgeon pin accordingly .

theres 1/4" difference in the stroke between the g11 and g12 so i would think the difference could be seen easily if parts were assembled and the top of the bore checked when piston is at top dead .

if its a 650 and was built with the G11 barrels with the special pistons and you cant find the right pistons , then at worst you could get some G12 barrels and easy obtainable matching pistons and still have a 650 ..

the only thing im not sure of is the exhausts , the downtube diameter on G11 heads had 1 1/2" and G12 had 1 5/8" , and i dont know if G11 heads and downpipes on an "export" 650 would be slightly different for mounting purposes if you had the extra 1/4" height of barrell and not using G12 heads with the relevant downpipes , it may not be a problem though but which set up you have i dont know .

i am not conversant with the variants of barrel and head combinations for 650 models around this year but hopefully my message will prompt some thought and inspection on it which will answer your question .

ian

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Re: Wrong pistons, Need advise for correct ones please

Postby Sibs6 » Thu Jan 16, 2020 11:40 am

Thanks G5wqian,
The barrels have 8 fins. Motors definitely a G12.
I was looking up the article on G11 and G12 pistons and noted the 59 only, was shorter between the center of the Gudgeon pin and the bottom of the skirt. I assume with the longer stroke it was to clear the crank on BDC
I will take your specs above and compare them to what I have.
Where would be a good place to order new pistons?
Thanks Sibs6

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Re: Wrong pistons, Need advise for correct ones please

Postby JEAN-NOEL » Thu Jan 16, 2020 2:20 pm

Hello Al,
When you says you had a look up the article on G11 and G12 pistons, are you refering to the topics by Don Madden and Warren Wheeler published in the technical tips ?

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Re: Wrong pistons, Need advise for correct ones please

Postby Sibs6 » Thu Jan 16, 2020 5:46 pm

Yes that's the article I am referring to.
So I rechecked my bore and there both 2.856 so that would be +20.
Now to find and order new piston kits so I can get the motor back together.

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Re: Wrong pistons, Need advise for correct ones please

Postby ajscomboman » Thu Jan 16, 2020 6:23 pm

Pt No. 800030/2 59 only +20 pistons from the parts service.

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Re: Wrong pistons, Need advise for correct ones please

Postby Eamonn » Thu Jan 16, 2020 6:25 pm


Corners like a Lego man

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Re: Wrong pistons, Need advise for correct ones please

Postby ajscomboman » Thu Jan 16, 2020 8:33 pm


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Re: Wrong pistons, Need advise for correct ones please

Postby Sibs6 » Fri Jan 17, 2020 11:57 am

Thanks everyone for you help. really appreciate it. Hopefully now some of my hair can grow back.lol
I will be ordering the pistons today.

Last question, what about ring gap?, is the re a specific gap for +20 that I should have inside the inside so when they expend when hot they done seize up?
Thanks Al

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