Polini 208 cylinder to piston clearance for a rebore?

I had a Polini 208 cylinder rebored ( Asso Vertex 2 ring piston)  and was advised by Taffspeed to get 0.09mm bore clearance, did that and got the bike dyno'd. Ran it very gently ( max speed 45 miles per hour ) for 450 miles and got a small seize for my efforts , so I then ran it very very gently for another 250 miles ( thats 700 miles total) and got another seize at 55mph, then ran it at breaking in pace for another 400 miles ( thats 1100 miles in total  [6] ) and figured it must be fully ready to go now so opened the throttle and let rip , did 5 or 6 miles at 70 - 75 mph , and then was cruising at 62 +/- mph when it seized really hard with no warning. It took all my riding experience to stay on , and after a little sideways speedway stunts I managed to stop without hitting any of the other traffic -I was really lucky. [8o|]

18 degree timing, fully synthetic pre mix , 24mm carb with 3mm spacer and DRT float chamber conversion , B8es plug with .55 gap, Sitoplus exhaust , 125 main jet ( set by Taffspeed at the Dyno run and told me it was still rich).

The piston has seize marks all the way round below the rings, and the plug is a very nice light chocolate brown colour. Piston crown is undamaged.

I think this is purely down to the rebore being too tight, I have honed the barrel and even after this there is still only 0.23mm piston clearance.


Any suggestions as to what the correct rebore / reuse clearance should be as I have no wish to try that trick again ?

the right piston to bore clearance should be 0.004 INCH NOT mm. 0.004 inch is 0.1016mm.


And a 125 main jet sounds lean lean lean.

0,10-0,12 mm

1,2-1,4 mm squish

10,5:1 comp.

Junk the fuking SI 24/24

The piston clearance to the cylinder should .12/3 not 0.09  that is why it is seizing

Thats what I thought mate ! I was running a 128 main when I took it to Taffspeed for a Dynojet tune, Ian down jetted me to 125 and then told me it was still too rich and I could go to 122 after 500 miles ( I didn't do that thank God !)

I will go back to 128 when I put the top end back on.

Hello peterlengland,


Where is the point where the clearance has to be adjusted to 0,12mm? 




if its of any use to you as a guide, heres the tolerances for a standard px200

In my piaggio factory manual it states that for a standard px200 the clearance is 0.215mm  with a tolerance of  +/- 0.025mm on the cylinder and 0.015mm on the piston

these measurements are taken from 30mm down from the top of the cylinder and 5mm down from the bottom of the lower piston ring. (remember that a piston is usually, slighty tapered) 

also the piston ring gap should be (again on a standard px200) 0.25mm -0.4mm with 2.0mm being the max usable gap.

 i just had a rebore in a standard px200 cast iron cylinder i had knocking around for a new piaggio/asso piston (1st oversize).

i had very similar problems when i tried to run it in, it seized twice in the first 150km

it was the tightest fitting piston/barrel ive ever fitted. Afterwards I checked the ring gap (should have done that to start with) and it was only 1.5mm could of had something to with it. the piston clearance was around 0.19mm according to my measurements

it turns out the tuning shop i took it too thought that 0.215mm was too much and made it less. before it was working perfectly and im careful with the revs etc when running in. i ended up re fitting the old piston and barrel and no more seizing, (although it could do with new piston rings) 


125 mainjet?


I'm runing 130 for my Polini 208 and Sito+. -and it is perfect.


125 sounds a bit low for Polini...

maybe! don,t forget you have the 24/24 carb modified so you will be coming down on the main, to high a main will slow the scoot plus waste petrol, last week i was on a 125 main, dyno,ed scoot at scoot/erotica and dan said come down to 122 and this is with the supply hole drilled, mild standard barrel tune, jl pipe...

I think the big difference is in the barrels Gadge mate, a tweaked standard barrel is not going to need the same as a Worb 5 ported Polini pot . . . . . . . .I should have stuck to common sense over science in the first place. I paid for the Dyno runs and ended up making the bike less reliable. [:P]