Author Topic: Tip of the day series- Piston clearances  (Read 115 times)

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Tip of the day series- Piston clearances
« on: March 20, 2020, 08:20:00 AM »
Tip of the day...

Most piston and ring manufacturing company's use OEM standards to make parts fit.
If Yamaha states the bore is 64.00, the piston and ring parts are made to fit in that size of bore.
When bored oversized lets say 65.50mm the piston clearance is built into the piston sizing, so a cast Pro-X may have .0018 clearance, but a
Wiseco forged piston will measure .003 clearance in the same bore. A forged piston will need more piston the wall clearance to expand.
This way Piston Ring tolerances can be followed! IMPORTANT!
For every .001in bore size larger, the ring end gaps will get bigger times PI, which is .00314

I have found through 35 years of building RD aircooled engines I do set the piston to wall clearance for the job, type of piston, what bore the piston is and what brand piston is.  OEM Yamaha, Pro-X(Banshee), Wiseco, and Wossner are all different piston clearances.

Example....If I bore a stock RD 350/400 1st over @ 64.25mm with a Pro-X banshee piston and the bike is stock I'll set piston to wall @ .0019 for today's fuel. (never follow stock spec's unless your going to use canned race fuel)
If the RD is a hotrod, cut heads, aftermarket heads with pipes I'll be .0022
If the same mods but at 65.75mm bore I'll set clearances to .0025 for the thicker pistons that will need more running clearance.

Wiseco and Wosser pistons will follow suit, but using greater clearances due to being forged pistons.
No need for a Wiseco/Wossner forged piston in a stock bike unless a race bike like a Historic Production Class that will have the snot ran out out it for everything it will give you...
stock and update porting engines builds Cast Pro-X are fine.
Most of my stg 1 builds it's optional on cast or forged pistons, with stg 2 and up to full race porting, I only use forged pistons and even do
piston coatings to strengthen them of to be more fool proof ( like opp's I missed the jetting this morning and didn't hurt it in practice)

In Guillermo's setup @ 66.25mm bore, using uncoated pistons, it's perfectly happy at .0035 with a good warm up before a taxing run.

RD machine work, boring, porting, cranks and engine building.

Chuck 'SUPERTUNE' Quenzler III
Team Scream Racing LLC
1920 Sherwood St. STE A
Clearwater, FL. 33765