Choose piston size x30 or generally any engine!

Hi there fellow engine nerds! I feel kinda dumb asking this question. How do i use a dail bore guage to decide what piston to choose for desired gap between piston and cylinder? So i just got my engine home from cylinder honing and piston change. I want to measure the gap… do i measure the piston at its largest point or what? What do i set the guage on? Lets say i have a used engine that i want to rebuild… piston is worn down and i dont have any pistons. How do i measure to know what piston to order to get a clearance of about 0.13mm? Thanks. Maby @Lborka

Pistons are measured at a specific point. For x30 it is 17.5mm from the bottom of the skirt, perpendicular to the gudgeon pin.

  • Meausre bore with dial guage and micrometer. (Lots of how-to videoes on youtube)
  • Decide on your preferred clearence.
  • Order pistion to suit, taking into account IAME’s sizing being based on tighter clearance. (Fox + Son website has a good explanation on this)
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So i want to know what a engines clearance it… i just got it from my builder. New piston and fresh hone.
And i would like to build engines with that clearance coming time… i dont get the how to videos on yt my small kid brain cant handle the dead guys woffling for 20 min. I want a point list woth what to do… Guess its my ADHD

Assuming you have a dial indicator bore guage and not just a telescopic T guagues.

  • Mic piston.
  • Zero bore guage to mic.
  • Measure bore and see direct reading of clearence.

Hold on a minute… i have a dail bore guage… i dont have a mikrometer and how do i mic something?

You’re going to need a micrometer, you cant get by with just the dial bore gage. There are ways to zero the bore gage, but seeing as you are wanting to compare it to the piston, you would measure the piston with the micrometer, and use that to zero the bore gage. Lots of youtube videos of how to do that.

If you don’t have a micrometer, a set of calipers is fine for this purpose, since you dont need an absolute measurement, just a way of transferring the piston size to the bore guage.

A dial guage is only a relative measurement device for comparing, not an absolute measure that will tell you the size in mm.

If you’re going to be rebuilding stuff and charging for it you’ll want a bore gauge, micrometer, a dial gauge and either a rotary encoder or degree wheel at the very least.

Yup. Im just trying to make a list on all special tools i need. I was previusly sending cylindr for hone and choosing size but i want to do it myself what are all tools and measuring things i need?

But if i dont have a piston and im rebuilding? What do i zero the guage to then? I have a used pison and a cylinder. And i want to get a new piston in with 0.12mm clearance…

I read a bit. So if i want to know clerance on new piston and bore gap i measure the piston zero guage and meacusre cylinder. If i want new piston i measure old piston and zero guage and see if play is more then 0.14mm if so i choose right pison to have 0.12 clearanxe?


Yes, to determine piston wear/clearance.
If you’re going to stick a new piston in, then you want to wait until the cylinder is honed first before measuring for a new one.

This video is a good example of measuring the bore. Skip straight to 9:10 if you want to get the actual measurement.

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and i have always just shaken the piston and ring back in. Is this good method or do i need some tool to make it easier.

Can i put my cylinder in the ultrasonic with water and dawn/ some dishwashing thing

Dawn works pretty well for alot of things, but not sure it would be my go to in an ultrasonic cleaner. I would recomend a good aluminum safe cleaner like Simple Green Aircraft. Make sure you use distilled water too. Most tap water has impurities and minerals that like to stick to things…

Also, if you are shopping for sonic cleaners, make sure it heats too. The heat is what makes the magic happen; without it you really just have a buzzing bowl.

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I have a cleaner… swedish water is clean af… but if you think about it dawn is baisicly degreaser. But i have been using dawn on carbs and worked well

I have no doubt your water is (generally) cleaner than ours and I bet its tasty too. But it’s tasty due to all the minerals, that you don’t want in or on your parts. Do enough loads with tap and you’ll see scale start to build up in your machine… No different than your coffee maker or tea kettle. If its in your machine, it’s on your parts.

I’m partial to our water here in the states though, I find the lead content really helps it slide down on a hot day and the PFAS chemicals taste just can be beaten.

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and what can the minerals do to the cylinder?

I bet minerals accumulate on surfaces over time. I’m no chemist but I imagine there’s some minerals that seek to bond with others somehow.

Best water I’ve ever had was South Lake Tahoe at Kirkwood ski resort. The water was from well I think, out in the middle of the mountains at 10k feet. Not like my slimy NJ tap water.

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