AJS & Matchless Owners Club

I don't think you been given this link which may help you to identify your '650' piston.

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

Thanks for that, I just found another set of drawings on the web as well, really useful info, it would appear that the ones I require have a top of piston to pin centre of 1.66". Perfect, looks like I have the wrong set. Have a great Easter.
Max

Today I machined the replacement piston and for reference it weighs in at 395g against the original of 360g, inc. pin and clips, I doubt that it will make any significant difference to the balance of the engine, although I have not done a dynamic test as the crank is now in the cases so too late. Anyway pictures of the finished item here for those that are interested.

ajscomboman wrote: ↑Fri Feb 26, 2021 8:07 pm

Mighty Max wrote:The '59 650 piston that fits the G5, do the over bores match i.e is a +20 G5 the same as the +20 650 piston? I gather that the skirt has to be shortened by about 1/4" presumably the balance will be about the same once modded? I would imagine the carb might require a small re-jet as well.
Max

Yep they size up the same, the rear skirt of the piston needs to be shortened to clear the flywheels as the crank and head are off centre. Gives higher compression but the jetting should remain standard, I don't recall having to change mine.

I notice the normal G5 flat top piston has valve cutaways, without measuring, is this going to be a problem as the new pistons don't have them? Don't need any valve / piston collisions.

You should be fine, the shoulder of the piston is slightly lower than the standard piston but the dome helps to compensate and up the comp.

Been running this new higher comp piston for a while now, certainly goes well however there is a certain amount of piston slap when I back off the throttle slightly at about 40/50mph, does anyone else experience this, maybe all lightweights do it, 40 years since I had one on the road before. I thought it was the rotor nut loose but that has remained tight, that I do remember as being a pita..

They are prone to slap and to compensate I have seen Wellworthy piston with a longer front thrust face and deep scallop to clear the flywheels. I only removed the rear portion of the 59 piston to clear them. Still had a very small amount of slap when cold but nothing of concern when warmed up.

This is what I've used - A G12 piston - was a little while ago and it still hasn't run. Needed a little fettling on the valve pockets I think but it fits!

P.S - Just found the post I did :- viewtopic.php?f=16&t=24107&p=206527&hil ... on#p206527

If you want a standard piston these guys still supply them.
https://www.fwthornton.co.uk/15151-std- ... l-8-295352
£145.00
Yes the Hi comp pistons do slap a bit plus I managed to nip one up the other day, not sure why but needs investigation, was not going quickly but maybe the timing needs a bit of compensation, I was running std timing and super unleaded non E.