measuring piston cylinder wall clearance PTW

measuring piston cylinder wall clearance

I'll stick a drill bit in one side and measure the bit for fun but must take to machinist with good gizmo's to get the number to go by. I face this twice, first to get Trixie smoke habit cured with off the shelf Taiwan pistons and then on Peel so not to seize when the wick turned up. Gotta bone up on cast vs forge piston expansion clearances too and ring gap fudging. IIRC Cmbat's with ordinary regular pistons like .0045"-ish. IIRC Peel's foraged gets .0065" PTW.

[video]http://www.youtube.com/watch?v=iAdQPqnmnTo[/video]

In the high perf 2-stroke world, the cast piston is king. You're right, a forged piston will expand more and must be set up with greater clearance to avoid seizure. Rattly loose... I can tell a snowmobile with Wiseco's from 50' away when it starts up cold.

Ok, I've read that a cast piston is harder more rigid than forged piston so cast is better to use, right up the point it shatters, then a forged piston fitted more loosely to take more loads for mo power, especially with power adders on top. Interestingly the racer forums I've read say the looser forged pistons used less oil than the tighter cast ones. Then the material and type of cylinder the pistons run in must/should be compensated for too. Oh I was so hoping Trixie would be no brainer, and may yet be if just a gasket re-torque smoke issue. I got away with just a clean up hone and new rings on my old mower, so why not my old cycle?

Hobot,

I use one of these and a micrometer.



http://www.victornet.com/subdepartments ... d=byn3AKDV

I have a more traditional bore guage too, but it's such a pain to set up and these have proved to be accurate enough, I use them for bore measurement when fitting hardened valve seat rings too.

Cheap and Easy....

I don't like using a micrometer like in the you tube video, a micrometer is better.

Mike

Yep sir an internal bore guage set is the cat meow for this but I don't own any and don't want too for as little use as I'll get compared to buying new Cdo parts, so off to a machinist with guages as I've done a few times prior. I have recovered my ole mower and got a couple acres of lawn knocked down below knee high and only lost- replaced one blade/shaft assemblely doing so. I found it mowing again but too late to reuse its parts.

Roger that....!

My bloody lawnmower leaves parts all over the yard every time I break it out...!

Right but all the parts my Norton drop off are quality British products.

measuring piston wall clearance by ring gap

Ok the plot thickened and Wes was right about the bore being a bit over .040" so ring gap too wide and not pressing ring to bore to seal well, therefore blow by creating the rest of the oil seal smoking issues. He described his past decades idenical finding with over worn bore and ring gaps. Normal top ring gap is 0.010 to 0.012" but Wes found *0.025"* up and down the bore. We did find some bore scoring visually that can barely be felt by fingerprint or finger nail strokes. Pistons sides look like new yet. So do I get a .060 over ring or a Total Seal set a bit over size or just .040" over size?

One funny suspense moment was as we lifted barrel a nut and washers still on cylinder base fell off for tinkle sound as same time i saw wet sump oil level splash but also the crank/rods move as pistons came free. Like surgeons we counted parts, even found an extra mystery split washer but relieved no Easter Egg hunting in the case sump. Trixie was pasture ridden last Sunday, this Sunday most her oil was all in her bottom. Carbonized black abrasive fluid to be rid of by taking out some brush that's an eye sore. A night time task.