Scooterotica

as someone who regularly measures to tenths of thous .0.0001" ..technically the difference between 0.0035" and 0.004"...is 0.0005"...5 tenths of a thou....in other words f### all.

On a standard engine then using a cast iron barrel taking account the comments, is there a better type make of piston over another to lessen seize risk ?

Oddly enough both 3.5 and 4 thou are both true. The target clearance for the GT bore is 3.5 thou. However, this is the production honed bore straight out of the shop. Once it is fitted and running for a few hours the bore will wear in to a more stable plateau of 4 thou where it will stay for a long time.

The clearance and reliability goes hand in hand with consistent piston selection. For example GT kits always use cast pistons from one source of supply, this means the characteristics of that piston can be reliably predicted. If you chop and change makes of piston, grades or forged and cast then this is where you break from the program and problems tend to be more prevalent.[/quote]


If Edens preference is to have his GT cylinder bored and honed to 4 thou,then once running for a few hours the bore will wear to 4.5 thou.I'm confused !

I run a forged piston
also its my belief that the bigger the bore the bigger the tolerance required.

eden wrote:......also its my belief that the bigger the bore the bigger the tolerance required.

I'd run with that theory, based on circumference and surface area.

Don't forget diameter!

eden wrote:I run a forged piston
also its my belief that the bigger the bore the bigger the tolerance required.

Absolutely, I have some Wiseco pistons in the garage and the information slip advice is 6 thou. The bottom line is that reliability is gained from experience, and experience is developed from lengthy study and consistent parts. If you change the variables to something of an unknown quantity then you can expect that everything may well not work as planned.

Agreed on the last post...................I like the fun of a steel barrel and the quicker pick up over nicasil, but ultimately they are 'likely' to seize and therefore the 'fail safe...fall back position' is go nicasil and be done with it..

All two strokes have the inherent probability of heat seize regardless of cylinder material. The more you push the duty cycle the higher the probability. Fundamentally, reliable iron kits are a proven reality and they are just as powerful as their alloy counterparts.

Hopefully strip it this weekend to discover what's up. cheers for help to date, will update.