Hitchcocks Forum

Hi guys. I have now received my new 535cc piston and steel conrod for my 1996 Classic from our hosts and am ready to place in my refurbished alloy barrel. The Accralite piston paperwork says I need to bore to 5.3-to-5.7 thou clearance. Various reports of seizing due to incorrect bore clearance have been noted, and in all cases this minimum is stated to be 6thou. I know this is minimally different from that stated by Accralite, but the costs of getting it right later are now too high (value of the rand has more than halved over the past 12 months so that, for example, my new conrod landed costs more than the cost of a new chinese scooter).
So question 1 is “what is the correct clearance?”

The new conrod may be a different weight than the OEM alloy one (very likely) and the piston+rings etc are certainly heavier than the OEM 500cc one (OEM=513g and 535cc=541g).
So question 2 is “if the OEM crank+piston was balanced to ~65%, should I aim for a similar balance factor now”?
Cheers, ChrisD

Hi Chris ive got my 1994 bored to 535 clearence was as stated by accralite crank needs to be rebalanced mine is 64% works well got to run it in carefully but no big stick and crank needs to be trued between centres close as you can get it. Before I upped the carby to 32mm mikuni I was running a 135 main jet and 27.5 pilot with pod filter, you will find the extra 35cc makes a big difference on hills I now run a 19 t gearbox sprocket I think it better to run slightly rich for a bit. Oh make sure you lap the oil pumps in . Happy fiddleing.

Hi Chris. Question 1 - Ask Hitchcocks - I assume the piston came from them? Question 2 - Don't worry about it, you wont be able to detect any difference, just make sure the crank is trued as well as possible. Cheers, Paul.

If the measurements with your piston are from Accralite through Hitchcocks I can’t imagine that they are incorrect. But what is as important is a very careful period of running in. It seems that seizures with these pistons almost always occur in the early miles – the result of going too fast too hard too soon.

Or in some cases due to the fact these bikes run very lean to meet emission regs and people don't even consider the fact that the carb jetting will need adjusting.

Thanks for the answers, guys. I’ve been running my original 535 (alloy barrel) for ~2000km and it was going pretty well (32mm Mk2, genuine 72mph at half-throttle is pretty good).

The question about bore clearance was because I (and indeed several others reporting here a few years ago) kept seizing the 500cc forged piston because the necessary clearance stated by Accralite was wrong, so I want to be sure.

BW: I am replacing the OEM alloy conrod with a new steel one. We are sure there must be a weight difference, so I probably need to balance it. Besides which the OEM setup was always rough over 3500rpm, so maybe even then it was incorrectly balanced. I agree, though, that a few percent either way is probably not relevant.
From the collected wisdom here, does anyone have any suggestions as to how exactly many holes to drill in the flywheels (and where) if we need to?

Hi Chris, what I meant was you needn't worry about changing the balance factor when you go from an alloy to a steel conrod and / or change to a forged piston - it really wont make any noticeable difference, I speak from experience here rather than theory. The overall mass of a Bullet crank is so great, you are talking about a drop in the ocean here. Perhaps if you had a 12000 rpm 125 you would have reason to worry about it. DO make sure you get the crank trued to an absolute minimum of runout at the mainshafts, though. Cheers, Paul.