AJS & Matchless Owners Club

Expat wrote:Wouldn't the incorrect piston height change the crank angle and therefore the valve timing?

Hi

Crank angle is the position of the crank pin with reference to the crank shaft centre line, (axis) - changing a piston can have no affect on this.

The valve timing may, however, require adjusting to cater for efficient charge transfer when a different piston is fitted.

Regards Mick

My incorrect use of terminology

Was thinking that if piston is incorrect and ignition timing is set from piston crown, then crank may be rotating further or not far enough when spark occurs thus valves would not be in correct time.

Shall I get me coat?

I dont have a good picture of the pistons i have fitted at the moment but they are quite flat with big valve cut outs . The 1955 pistons I have seen , on the internet , look to be at least 1/2 inch higher with the dome . Not sure what that would do to the valve timing .

I've run 500s with pistons from bog-standard 7-ish to one to around 10:1 in way overbored barrels, and not had to play with the valve timing (or not thought I needed to and therefore haven't, should I say!), so I think you'll probably be fine. That is using standard camshafts.
Playing with the valve timing is fraught with possibilities and not a fun place to go without having proven data to guide you, in my simple world anyway! One of the good things about the twins, compared with some of the singles, is that it is all marked out very clearly and 'you can't go wrong squire' (truly!)

All 500cc pistons have a domed top until 1960 when they changed heads, barrells and pistons. Pistons then were flat topped to suit the heads. My 1960 model 20 should have these but they are unavailable, so i've been running with the earler domed head pistons for years. I don't think they made oversizes for the 1960-on either.

Got the right barrels and have bored them out today to fit 67mm pistons , 40 thou over , pistons from the club so all good to go . I'll report back when I get them fitted , will be a week or two , think there will be a big improvement as the rings were way out of spec at about 60 to 70 thou gap . No wonder it was burning oil .

well its up and running . It actually fired first kick but didnt start till 3rd kick . I replaced the oil and it took a good 45 seconds to get back to the tank but i do have a filter fitted as well . little bit of smoke but is obviously needing run in . I need to get the tank chromed and painted but i may go for one of the indian offers . The one i bought for my bsa was very good .