AJS & Matchless Owners Club

I have a 1967 250 CSR which I have just bought. Engine seized on first trip out. +20 bore and piston fitted recently according to paperwork.
The bore seems untouched, the piston has some heat marks, and very slight grazing in one area. Club piston.
I intend to have a piston/ cylinder gap of .006 to allow for expansion of the club piston in a steel cylinder barrel.
I'm going to order +20 piston and rings, and circlips. Do I need a gudgeon pin, or anything else?

Hi and Welcome

Not wishing to teach you to suck eggs but if this is a new piston and rings in a rebored barrel are you sure you weren't over driving them?

If there's no damage to the barrel or rings and only slight scuffing to the piston I'd be tempted to clean up the scuff with fine wet and dry, rebuild the engine and take things a lot easier for a good while.

Regards Mick

Hi Mick - nope 25mph on Scarborough seafront. Hmmm - that might work, but the piston was very tight after seizing. Maybe hone 2 thou?

My first trip to Oliver's Mount was on a 1966 AJS 14CSR! Personally, I'd agree with Mick D. Again, you probably know this, but it's less about pure speed than keeping stress off the engine, neither labouring it nor over revving it. Steep rises and falls getting into Scarborough.

Good luck with it.

Johnny B

I'm with MickD on this and I'd have the bore honed to give the piston whatever clearance you feel is right. With an unknown piston I use a rough general rule of thumb of bore diameter in inches multiplied by 1.5 plus half a thou for luck (e.g. bore dia 3" x 1.5 = 4.5 + 0.5 = 5 thou clearance) but I've only applied it to 350 and 500 singles. Not knowing your piston I'd be reasonably happy with 6 thou. if that's what it is now.

As far as scuffing etc on the piston I work on the basis that the piston is only there as something to connect the bang to the conrod and to hold the rings to prevent the bang from getting past the piston. (Simplistic naivete is my specialist subject).

Later...........
Just had a thought that since you believe it's recently been bored and has a new piston you should check the piston ring gaps as they may be too little. Offhand I think it should be about 12 thou but it's late and I'm old.

Yeah - I agree with the replies. Going to clean up the piston and take it, along with the barrel to my local engineering firm and get a 6 thou gap. Then gently gently!

I agree with SPRIDDLER : 5 thou, instead of 6, for your 69 mm piston.