What top end?

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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robcurrie
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Re: What top end?

Post by robcurrie »

There were higher compression pistons available for the model 31 back in the day, here is a link to my first post on Jampot which has some information:

http://www.jampot.com/forum/viewtopic.p ... 046#p88046

Here is the picture Don refers to in the above post:

http://www.jampot.com/pic_display.asp?id=2560

There is also a technical article on the Jampot site about twin pistons:

http://www.jampot.com/article_read.asp?id=23

I will see if I have any photos of the pistons in my CSR showing protrusion, which I think is the whole chamfer protruding above the spigot.

The Club has commissioned new crankshafts which should be available in a few months time.

Rob C
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Re: What top end?

Post by trophy »

Thanks Rob. The info included in your post is impressive, relevant and very welcome, many thanks. It never ceases to amaze me how much knowledge members have tucked away, and also in the club archives. Thanks Rob and Don. As regards the clubs new cranks I doubt I will be able to stretch that far so will keep my fingers crossed someone might have a pre-used one available.
Ride safe all.
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robcurrie
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Re: What top end?

Post by robcurrie »

Here are photos of my pistons. I am guessing that the stamping E62 on the top (not visible in photo) is the date code for May 1962, which ties up to the bikes age, so they are probably originals fitted as part of the Speed Kit, although they were machined for non-standard rings at some later stage.
BHB part no. 028446 High comp piston top.jpg
BHB part no. 028446 High comp piston side.jpg
And I measured 32.5mm down the centreline of the plughole to the piston. (The piston isn't square to the plughole).
I will add two more photos to a new post showing the shape of the piston face presented to the plughole.
20150215 32_5 to piston.jpg
Rob C
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robcurrie
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Re: What top end?

Post by robcurrie »

I put plasticine on the plug and screwed it into the head, this is the shape where it met the piston:
20150215 before.jpg
20150215 after.jpg
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saltbox alf
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Re: What top end?

Post by saltbox alf »

The rings look very much like 'Cords' the waffle shape oil control was designed to flex to the shape of a worn bore and reduce oil bypass. I remember fitting these on loads of old jaloppy's I had and have to say they were quite effective at the job. that is to say I could see behind me, instead of a smoke screen :lol:
Allons-y, amis de Matchless et AJS
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Re: What top end?

Post by trophy »

Many thanks everyone for the info this thread has generated, really interesting and studying it all just shows how very easy it is to get confused with what piston fits what cylinder and what year :? Phew, talk about a minefield! Anyway, thanks to Rob's posting and the club's archives I have confirmed my Hepolite piston stamped 15762 +.020 is correct for my 8 fin cylinder and is 7.5:1 cr. Also the info shows that the deck height of the piston should bring the piston top edge (not the crown) level with the top of the cylinder spigot at TDC. On my bike it is 3 and a bit mm below confirming I have the 600 crank fitted.
Having said all that I took it for run yesterday, primarily to do a chop test to prove carburation main jet setting, and it ran well, better than its ever run, despite the lower compression. (which suits my age and 11 weeks old new hip) So if it proves difficult to source a 650 crank its not the end of the world.
Ride safe all.
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Re: What top end?

Post by mdt-son »

robcurrie wrote:Here are photos of my pistons. I am guessing that the stamping E62 on the top (not visible in photo) is the date code for May 1962, which ties up to the bikes age, so they are probably originals fitted as part of the Speed Kit, although they were machined for non-standard rings at some later stage.

Rob C
Hi Rob,

Looking into piston options for my 1964/65 G12CSR, I really would like to achieve the 9:1 CR mentioned in an earlier tread, or maybe a compromise between 9:1 and the speedkit's CR of 10.25:1. I am a little confused though. Looking at your photos, the piston top is much raised compared with the 026325 pistons (CR 8.5:1) I am used to. I really believe 028446 will yield a CR of 10.25:1, so what was the p/n of the 1965-66 CSR pistons giving a ratio of 9:1? I would expect a 030xxx or 031xxx part number. Unfortunately, the supplemental spares list for 1965 is not available at this time.

Second question - the pistons are handed, aren't they? Pockets doesn't appear to be located on a straight centric line, yet the factory lists the piston pair under one part number. Does anyone have details of pocket locations for l/h and r/h pistons?

Third question, has anyone had CSR pistons custom made out of forged blanks and if so, are manufacturing records on file with a certain company?


TIA,

Knut Sonsteby
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robcurrie
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Re: What top end?

Post by robcurrie »

Knut, I can't help with questions 1 and 3, but I can tell you that both pistons had the same part number, so they are not handed. I marked them so they went back in their original bores.

Perhaps Don will pop in and answer your other questions.

There is a document (I think in Christian's Archive) with details of all the twin pistons.

Rob C
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Re: What top end?

Post by robcurrie »

Just to add some more information to this topic - I had the heads off to re-aneal the gaskets and measured the combustion chamber volume to calculate the CR for my pistons, it turns out that they are 10.5:1. This explains why I can keep up with the modern classics at our local club!

Rob C
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Re: What top end?

Post by g5wqian »

i have a 56 model 30 600cc , uses standard 7.5-1 type domed pistons and compression pressure after 10 kicks is 135-140psi on each cylinder with 1.7mm thick composite head gaskets , if this is of any interest for comparison.
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