I'm helping a friend re commission his 7k miles from new RT1 360 and I'm having trouble deciphering the numbers on top of his piston. On the exhaust side next to the arrow it is stamped 75, on the intake side stamped 96. If I have to borrow a bore guage or calipers I will but curious if someone can tell me what the numbers mean. I did read here that 96 means 79.96 but what of the 75? Thanks!

 from the UK.

My initial thought is that 75, or .75, means third oversize - Yamaha oversizes go up in 0.25mm increments, but aftermarket (eg Wiseco) are different. I'm not sure that I'd expect to see a third oversize piston on such a relatively low mileage bike, though.

A good, clear photo of the markings would be great! 

Here's the pic I took today. Had to scrape the carbon off to get it, and still not very clear, but here it is. Also I read here of a 360 piston with 74 next to the arrow and 96 at the intake but no clear answer was given on that. Very mysterious.. 

96 is the standard size piston.
You're probably right in your assumption that the 96 relates to the 79.96mm size of the standard piston, it's also the last two digits of the standard part numbers on pistons.
As Alan says they come in 0.25mm increments after that.
 

This is is from my DT360 after a little clean up.  I’ve got a 72 and 96 stamped on the head.  Please advise again what the 72 and 96 indicate?

Jmckinney....  Have you checked your VIN?  Has piston been swapped? 

That doesn't look like an original 74 DT360 piston?

Maybe an expert will know.



 

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Yes . . .  from what I can see, I have no reason to doubt that's the original "standard size" DT360 piston.
If you plan on having it bored, and have trouble finding the proper oversize piston, let me know . . . I have some.
I'll be out of town until Saturday afternoon though.

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Thanks Enduronut.  On my phone I couldn't see the "step" for the dykes top ring. 

looking for help with 1973 mx 360 piston, like 81.5mm, my  single ring piston has a 1.0 on top, measures around 81mm .sounds like enduronut could help Thanks

RT1 won't have a Dykes top ring, just two thick rings, thicker than normal. Or am i am the wrong planet again. Whats the subject.