Seizure on a cast barrel

[Strummer10]

Not sure how he's running it. Should know more tomorrow night if we get the barrel off and see what's what. Whats 4 thou in mm's i'll check. Do most agree I can just clean out barrel and rub down piston and carry on.

[B-Race Tuning]

^agree100% on 4 thou on cast cylinders.
Cleaning up piston should be no problem at all. Make sure ALL traces of aluminium in the bore removed. Also make sure rings are free and clean of aluminium, and the lands are clean with tiny chamfer.

[camel]

0.004"...4 thou=0.1mm...a feeler guage between the tightest point of the piston is a crude method but is usually ok to determine clearance

[HxPaul]

4 thou for me on cast barrels.

yes just take the ally out of the bore with emery paper and take the high spots off the piston.

Is that with a cast piston or forged piston,or both.I was always led to believe that 3.5 thou is the optimal piston to bore clearance,in fact the Grandturismo GT 185 and 200 has a clearance of 0.08mm to 0.09mm - 3.1 to 3.5 thou.

[Toddy]

4 thou for me on cast barrels.

yes just take the ally out of the bore with emery paper and take the high spots off the piston.

Is that with a cast piston or forged piston,or both.I was always led to believe that 3.5 thou is the optimal piston to bore clearance,in fact the Grandturismo GT 185 and 200 has a clearance of 0.08mm to 0.09mm - 3.1 to 3.5 thou.

I think it May be Edens Preference Paul

[YAMLAM]

my rapido 225 with Avanti piston was done by Harry Barlow, he said .004" clearance.

[eden]

if you wana nip up the first time you give it some throttle for a while go for 3.5 if you don't go for 4

[Strummer10]

Eden,

Is that 1.0 mm's in conversion ?

[Pepot]

Maybe I'm wrong, but isn't the tolerance determined by the quality of the piston? Whats enough for a piston might be too tight for another...

[Rich_T]

4 thou for me on cast barrels.

yes just take the ally out of the bore with emery paper and take the high spots off the piston.

Is that with a cast piston or forged piston,or both.I was always led to believe that 3.5 thou is the optimal piston to bore clearance,in fact the Grandturismo GT 185 and 200 has a clearance of 0.08mm to 0.09mm - 3.1 to 3.5 thou.

I think it May be Edens Preference Paul

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.