Reboring and new pistons process

peterg

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My pistons to cylinders gaps is way out of spec so I need to rebore and buy new cylinders.

Cylinders are stamped 010 (75.010)
Pistons are stamped 959 (74.959)
Gap 0.051

Actual measurements from my mechanic
Cylinder 75.0316
Piston 74.570
Gap 0.460

Cylinder 75.0316
Piston 74.676
Gap 0.355

I’m eyeing pistons from Cruzinimage.com and have sourced a local shop for reboring (Gord Bush Performance in Toronto ON - no affiliation).

Let’s say I go 2nd (0.05mm) oversized for the pistons.

1. Are 2nd oversized pistons made for this bike made to a nominal size (74.45 mm) which will provide the in spec gap in a cylinder honed to the actual size 75.50 mm?
2. Can the shop do the rebore or should I wait to get the pistons first to be on the safe side? Probably a strong “yes” to that question.
 
When I rebuilt my XL250 engine, the machinist gave me a tip about when I installed my new piston in my now overbored jug. He said that when you go to put in the new piston, only oil (or assembly lube) the skirt of the piston, do NOT put anything on the rings. Also, oil the jug and wipe the excess so there's just a thin layer of oil. The purpose is to set the rings to the piston. I wish I had known this when I rebuilt the KM100 engine because I oiled the rings like they did back in the old days. I've observed the piston and jug of both, and the KM100 is already scoring. but I had it sent out to be nikasiled when I built it, so I'll fix it when it doesn't want to run anymore, nikasil isn't cheap.

Also, buy cast pistons instead of forged Pistons. Cast pistons have a tighter tolerance and will expand quicker than a forged piston with a looser tolerance. Forged pistons are for high rpms for a long amount of time, not so much for daily riding. I have forged in both of the engines I built because I didn't know any better then.
 
FWIW, I bought a set of the .50 OS pistons from Cruzin Image. They came with everything needed to instal. Rings, keepers, wrist pins and of course the pistons. Everything looked pristeen. Took them and the cylinders to a machine shop that had the lastest and greatest new equipment. The cylinders were bored .002 over the pistons. Everything buttoned up and they sealled right up as the miles built up.
 
FWIW, I bought a set of the .50 OS pistons from Cruzin Image. They came with everything needed to instal. Rings, keepers, wrist pins and of course the pistons. Everything looked pristeen. Took them and the cylinders to a machine shop that had the lastest and greatest new equipment. The cylinders were bored .002 over the pistons. Everything buttoned up and they sealled right up as the miles built up.
I ordered that set last night!

Jim. I’ve been studying your excellent XS650 Top End Buildup thread so I’m ready to put this thing back together.
 
Well…I ordered the pistons on December 3, they were shipped from Tokyo on December 6, arrived in Toronto on December 8, dropped off at my apartment door step at 10 pm and stolen sometime over night. Vendor service and courier speed was way to good to end well. Looks like I’ll order another set.
 
Well…I ordered the pistons on December 3, they were shipped from Tokyo on December 6, arrived in Toronto on December 8, dropped off at my apartment door step at 10 pm and stolen sometime over night. Vendor service and courier speed was way to good to end well. Looks like I’ll order another set.
Well that sucks!
 
Well…I ordered the pistons on December 3, they were shipped from Tokyo on December 6, arrived in Toronto on December 8, dropped off at my apartment door step at 10 pm and stolen sometime over night. Vendor service and courier speed was way to good to end well. Looks like I’ll order another set.
Well I don't understand people who steal random packages. Guy has no idea what Pistons they're for.
 
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