I'll be honest... I hate math and think they should teach how to find these answers in school with apps. Sorry, just need to get my piston ordered. So, just getting my permission slip from the enduros first before I hand my machinist a piston. He says 30 thousandths is what I need.. but yamaha is in .25 increments of mm. .030in= 0.7620000mm Is this correct? I should be getting a 4th over 71mm?

He might be referring to aftermarket suppliers such as Wiseco.

You are correct when you say that Yamaha oversizes are in increments of 0.25mm.

Some folks recommend not buying the oversize piston until the re-bore has been carried out - sometimes a 0.25mm re-bore has not been enough to clean the wear up and 0.5mm was needed. Your machinist also needs to be familiar with re-boring Yamaha cylinders.

Please wait for others to confirm, or otherwise, my points above.

If your machinist hasn't yet seen the cylinder how does he know you need a 30 thou, or did the piston have 0.50 on it & he saw that so he feels sure it'll be safe at 30 thou or close to yamaha 3rd over 0.75. Take the cylinder in for him to measure might be best incase it's beyond going one yamaha step. I always thought of 0.25mm as 10 thou but looks like its a gnats whisker more from your calculations--but is close.

1 inch = 25.4 mm

Move decimal point two places to the left on both sides...

0.010 inch = 0.254 mm

Multiply both sides by 3 because you want 0.030 inch

0.030 inch = 0.762 mm

Factory Yamaha pistons come in 0.25 mm increments.

1 mm = 0.03937 inch so 4th over would be 0.039 oversize

3rd over would be 0.75 times 0.03937 or 0.0295 inch... just a half thou under 0.030 inch.

I'd ask your machinist if third over was close enough... no need to waste a bore. Enduronut has a large stock of Yamaha pistons and rings at good prices.

Well said. Thank you.