G12 pistons

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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G12 pistons

Postby vrossi46 » Tue Oct 03, 2017 9:48 am

Morning all. I have just removed the head and barrels from my 1959 standard G12 and have noticed that the nos. on the pistons - 025601/1 - are not the same as the parts catalogue. Basically I am trying to find out if the compression ratio is correct.
There is a manufacturers stamp (?) BHB within a circle, with a small number 59 by the side, and also a capitol 'M' on the piston crown.
Does anyone have any info. please.
Cheers.
Dave.

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Re: G12 pistons

Postby dave16mct » Tue Oct 03, 2017 3:57 pm

What shape are the piston tops? 59 G12 pistons should be shaped like a 'parson's hat', so they have a round lump in the middle with a land around. They were very hard to get at one time (only made for one year) so some resorted to reshaping later pistons to suit. They are available from the club now.
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Re: G12 pistons

Postby vrossi46 » Tue Oct 03, 2017 5:33 pm

Hi Dave. The pistons indeed have a resemblance to a 'parsons hat'. I take it from your description that the compression ratio would be the correct 7.5 to 1.
Thanks again.
Dave.

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Re: G12 pistons

Postby robcurrie » Tue Oct 03, 2017 5:38 pm

There are two articles in the "Technical Articles" section on the Jampot site under heading "Twin Pistons" which have some sizes and part numbers - but not yours unfortunately. Don Madden has published additional information and it is in the photo section somewhere, it will take me a short while to find it.
My 1962 CSR also has BSB pistons and they are stamped 62 so I guess the 59 stamped on your pistons means they were manufactured in 1959.The part number for my pistons was also not listed and comparing the crown height to known pistons, I guess mine are around 10.75 : 1, my CSR was fitted with the factory Speed Kit.

Rob C

Edit
Additional piston info from Don:
http://www.jampot.com/pic_display.asp?id=2560
Last edited by robcurrie on Tue Oct 03, 2017 5:55 pm, edited 1 time in total.

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Re: G12 pistons

Postby dave16mct » Tue Oct 03, 2017 5:51 pm

Found it Rob,
in Christian's archives :

http://archives.jampot.dk/technical/Pis ... istons.pdf

Dave

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Re: G12 pistons

Postby robcurrie » Tue Oct 03, 2017 5:59 pm

Nice find Dave!

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Re: G12 pistons

Postby vrossi46 » Tue Oct 03, 2017 6:10 pm

Great stuff guys. Thanks very much.
Cheers.

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Re: G12 pistons

Postby g5wqian » Sat Oct 07, 2017 4:10 pm

hi Dave ;
one thing to look at which may have been missed out is that you have a 1959 model and i think during that year only , they would have built some 650 engines using 7 fin barrels and special pistons with a different pin height .

i think most were exported to the usa but many might have also stayed here or have come back over the years .

so you might also check your cylinder barrels to see if you have 7 or 8 fin types .

the 7 fin was mostly used on the 600cc engine with shorter stroke but it seems that in 59 an amount of bikes were built using a 7 fin barrel and different piston to run with a 650 crank .

if its any use my 56 ajs model 30 has somewhere between 135-145psi compression pressure and is not a high compression csr version , and uses +40 GPM pistons .

one word of warning , if you are buying any headgaskets from the club spares and are thinking of the solid copper ones , then make sure you are sold actual 1.5mm thick gaskets and not the 0.93mm that were stocked and were made in the wrong thickness , but still sold as 1.5mm thick .
it caught me and a couple of others out and we were sold 0.93mm thick gaskets .

i work with non ferrous materials every day and buy them from various suppliers and it is a certainty that whoever made the copper gaskets used 1mm thick plate and not 1.5mm .

the problem with the thin material is that the cylinder spigot will push itself into the cylinder head and there will be no proper seal from the head gasket if used .

however you will be fine with the clubs composite head gaskets as they are 1.7mm thick .

cheers .
ian

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Re: G12 pistons

Postby BillC2 » Mon Oct 16, 2017 2:07 am

Interesting. I am just starting on a 1958 G12 which has the 7 fin cylinder. I assume from the comments that these pistons are made of unobtainium. I read somewhere they moved the pin up for the added stroke and cut the bottom of the pistons to clear the crank untill they went to the taller cylinder. I also saw some where that other than the crank and pistons it is a G11. True?

Thanks
Bill

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Re: G12 pistons

Postby ajscomboman » Mon Oct 16, 2017 3:59 pm

BillC2 wrote:Interesting. I am just starting on a 1958 G12 which has the 7 fin cylinder. I assume from the comments that these pistons are made of unobtainium. I read somewhere they moved the pin up for the added stroke and cut the bottom of the pistons to clear the crank untill they went to the taller cylinder. I also saw some where that other than the crank and pistons it is a G11. True?

Thanks
Bill


Quite correct, for a brief period in 58 there was a short run of 650cc motors built using the new longer stroke crank and special one off pistons which did indeed have the pin moved closer to the crown and the skirt machined off. The rest of the motor, ie heads, barrels, manifold etc were all 600cc components. As far as I'm aware all were shipped to the USA and due to the short piston they were very prone to piston slap! As far as availability of the pistons I've seen many sets advertised on the US ebay website and know of a few sets here in the UK.

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