Motoscrubs.com

Hello!

Fitting a high compression arias piston on my 250 twin plug, I was wondering if you guys have input on cylinder/piston clearance as the arias info seems confusing:
https://cdn.shopify.com/s/files/1/0177/3836/files/motorcycle-installation.pdf?14556130666235922189

4032 alloy:

0.001 inch for race
0.002 inch for road

Usually clearances tend to be looser for race or am I wrong?

Never mind, Talked to them directly :
4032 with gapless rings is 0.002” total clearance (crazy tight)

If the alloy of the aluminum is high silicone content (i.e. low coefficient of expansion) then .002" will work fine. Could probably be tighter. Honda has been running pistons at this clearance for many years. I have an XR628 with a bore size close to 100mm and the piston clearance is .0018". Works great. Clean, high quality oil is important but that's the case with anything. I once installed a Poweroll big bore piston kit in a Honda CT70 that had a recommended clearance of zero. My machinist asked "how am I supposed to bore the cylinder to zero?". I said "measure the piston and bore the cylinder to that measurement". The engine ran fine.

Usually recommended to use a very fine hone for final sizing of the bore.

Matt

Hi Greg
I did get your email, but have been busy..

I would run at least 3 thou to 4 thou on a race bike regardless. Trouble is piston manufacturers generally have no real life experience of specific machines, so generalised figures are only really a guide. I doubt a Ducati cylinder is stable enough to take a 2 thou clearance. Chances are if your cylinder muff is still standard size it will be greater than 2 thou out of round to start with. Assuming the liner is round, the interference and expansion is unlikely to be even in use.

My pistons manufactured by Omega and they recommended 2.5 thou. I have never run less than 3 thou with these and more often I will run 4 thou in my race engines. Also I may mention that 5 thou runs fine and I have seen wrongly bored and worn bores running fine at 7 thou..... Needless to say hundreds of pistons later no real issues. Even at 5 thou plus there's no real audible piston slap either.

As I say to my customers, no prizes for a seizure. If I was you I would run 3 thou. I hope the Arias pistons have improved over the years, as in the UK there was a few batches about in the 90's that let go. I think it was a material issue, but that was a long time ago.

Regards Nigel

Hello Greg

Further to my comments about clearances, I had misunderstood that the use of the Arias h/c piston was in a road bike. Thinking and looking into it further, I have done a few expansion calculations and the difference between the two materials by my calculations is roughly around 1 thou from cold to working temperature at the skirt. Therefore if you were to run the 2 thou as suggested by Arias, at running temperature the clearance of a 2618 alloy piston with a cold clearance of 3 thou would be similar.
Hmm, think Arias using 4032 alloy is possibly trying to solve a problem that doesn't need solving, I'm thinking you may get away with the 2 thou recommended for road but 3 thou equally should work fine. I also think the tight 1 thou cold clearance for race use is to counteract an excess clearance at running temperature. Personally I wouldn't specify a piston made of 4032 for race use, it makes little sense for an old Ducati Single.
One other thing that seems vague about the Arias spec sheet, is that it doesn't seem to have any reference to the bore diameter. Surely a 50mm piston will require a different clearance to a 125mm piston!

I have amended the above as I miscalculated the expansion difference between the alloys. Didn't seem correct and has been playing on my mind! I am guessing the skirt temperature at roughly 150 degrees C when engine is at full temperature. Either way my answers still stand, but with my previous calculated figures I was wondering the suitability of the 4032 alloy at all. Sorry for any confusion.

Regards Nigel

Post above amended due to calculation error!

Hi Nigel,

Thanks for your answer!! , going to try the value they gave, ill be running twin plug with reduced advance too, engins should stay cool, it’s a bike I put a few hundred miles per year on anyway, skirt is molybdenum coated and crown ceramic

Okay Greg, maybe let me know how you get on out of interest.
Nigel