Tip of the day series - Measuring pistons for boring cylinders

[SUPERTUNE]

One of the issues I see quite often when rebuilding customer engines is the piston to wall clearances are a lot of the time too excessive.
Even on somewhat freshly done cylinders without a lot of miles.

I have found that pistons that have a break-in coating adds to the problem and proper measuring equipment.
Because these coatings wear off fairly quick (about 800-1200 miles) and adds clearance, as the coatings are about .0008-.0012 than the actual piston will measure.
Use some medium green Scothbrite to rub off this soft coating to get the the piston measurement location and usually is at the bottom of the piston skirt as shown in the pics.











I even use a Mitutoyo micrometer...same as Yamaha uses other than mine is inches measurement!





Here's a measurement of Dave T's low mileage cylinders (1800)  .0049 was the reason his topend was noisy.



So a trip to the Rottler boring bar that I bought brand new in 1981 when I was 21 yo! (Yes... it's my left arm)
I can tell you exactly how bad the previous machinist did work, as I can see how crooked bore a lot of them are.
Mine still cuts within .0005 in. all the way down and will cut to within .0002 with the setting fixture to bore size.







Another issue besides noisy topend and high risk of cast piston breakage the jetting and performance was horrible when the ring gaps are huge
and causing blowby into the crankcase!!



4 Pokes don't get hurt as much with blowby as it just normally vents to atmosphere, with a 2 stroke the charge in the crankcase is the next power cycle and you don't want spent exhaust blowby there as it's kills power very bad and will be hard to set the carb jetting correctly.
I always say...how hard and far can you run with a plastic bag on your head?
So many RD's that has tuning/jetting issues in the past were non existence after a proper bore job and ring seal with less rocking and tighter ring gaps.


My pricing is not as cheap as most shops... but due the the time I spend, my equipment and desires to make all my bore jobs be bored and finished honed to only the next size up to save your cylinder bores life as you don't want to ever resleeve RD cast-in cylinders.

Here's Dave's finished piston clearance @ .0019



Final finished cylinders...




Chuck

[Czakky]

Most interesting tip yet

[350GUY]

This is valuable info. Thank you Chuck for sharing, very kind of you.

[85RZwade]

  yes, thank you Chuck for sharing your hard-won knowledge. Invaluable! 

[m in sc]