Identify A New Piston Size

I feel like an idiot for having to ask, but at a rummage sale I ran across an old NOS Honda piston in most of it's original box. Wouldn't you know, the only part of the box missing was the top flap:doh:, where they used to put the doggone part number. For 25 cents I couldn't leave it there.

Now, I have this piston and no idea what it fits. It measures about 50 mm across at the skirt, and 50mm tall. It has "N1" and ".5" stamped on the crown, so all I know about it is it is a honda four stroke small bore 5 thousands over. Has to be from the 60's or at latest 70's judging by the box. Any ideas? A 90cc? 100cc?


I'd say your in the ballpark. 50mm x 50mm x 1 cyl = 98cc

Yeah, that piston could be for allot of things though. .5 I believe is a half mm.

Dog walker's right. .5 is .020" roughly.

Are there any numbers anywhere? What does the wrist pin and wrist bearing bore measure? I've got some Wiseco bearing numbers around somewhere, if it matches any of those you can track it down easier.

Edit: that doesn't make any sense, but measure the wrist pin OD and the piston hole diameter and width and post 'em. If I can help you, I will.

0.5" would be 500 thousandths, a huge piston size increase which most cylinders couldn't support.

Edited by Smacaroni

On the outside of the piston below and to the left of the wrist pin hole on one side it has "ART". Cast inside above the skirt, running vertically by the seam are the numbers "158" - gotta look real hard to see them. Also on the inside on the opposite wall above the skirt by the wrist pin boss is stamped "4" with a "^" over it.

I noticed there were numbers written on the OEM plastic bag the piston was in, but they are very worn and all I can make out are "21 ? 3 or 5 ? / ? ^ or x 5 0" (red numbers are ones I could not make out, "?" means there was something I could not even guess at). The string makes no sense as a piston part number.......I'll try and measure the other parts you suggest, but i don't have a good metric ruler here.


Woah, you found a NOS piston for $.50?! Awesome.

My $.02 is it's a CT90 or S90 piston. Also fits a ATC90.

dratv_2152_104016115 Does it look like this?

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