Piston Vs Cylinder Clearance

[ailll1]

A friend of mine is in the process of rebuild his ES and has some questions that I can't answer either, so I thought I'd ask experts.
Let's say he buy a 74.5mm piston, what size does he has to bore the cylinder? Does the piston measurement showed is in fact the measure that the bore has to be to fit the piston? I understand that the cylinder has to be a bit bigger than the piston but by how much?

Never done a top end rebuild on anything but I am rebuilding my old yz125 so this info will be really helpful for me too.

Thanks guys!

[ironchop]

First, download a free manual from this site for your friend for the correct answer....don`t always trust what you hear on the net and that goes for this forum as well...be warned

Second, he needs to measure the bore with a PRECISION measuring instrument....like a dial bore guage....then he has to check for taper and oval bore (measure it on all sides AND top to bottom. When he figures out what his current MAXIMUM bore diameter is, ONLY THEN should he buy the piston the next bore size bigger....best bet is take it to a Pro to be measured if you don`t have the right tools...this step is CRITICAL to keep you from buying a piston you can`t use.....He may just need a hone if the cylinder bore is still good (not scratched, gouged, worn, oval,or tapered) and a fresh set of rings.

Third, NEVER.....and I mean NEVER buy a piston AFTER the bore/hone job....a true professional machinist needs the new piston to bore TO THE PISTON`s ACTUAL SIZE and get the CORRECT piston-to-cylinder clearance.....then when the bore is complete, he needs to set the ring end gap which varies from piston manufacturer to manufacturer (for example Wiseco gives you a small pamphlet with a new piston with a ring gap table so you get the correct gap for the application (example: race, trail, etc.)

These are my opinions and it`s the ONLY way I do a topend and I`ve never had a problem.

[RubberSalt]

What ironchop just said, measure, buy piston, get bored - you need the piston before the bore. I just got done measureing my ported cylinder, It's ready for the 1st oversize.

[atc500x]

If I'm correct the clearace must be set to 0.002

[ailll1]

Alright, I'll get measurements with instruments we got at school and I'll post back.

But, Maybe I wasnt clear enough(im not native english): If he buys a 74.5mm piston, will the piston be 74.5mm? Or thats gonna be the size of the cylinder for the piston to fit. I dont want to chop to much meat just to have the piston fit, I'd rather buy the next size first time.
Sorry guys if I'm having difficulties understanding but I got some great info until now.

[Dirtcrasher]

Should say in the piston sheet but I believe it's more like .003-.004"s which I'm fine with on an air cooled thumper.

IMHO, the old air cooled 250R's need more like .006 thousandths........

But, IDK what you have.

[pablo]

ailll1,
You are correct. The size on the box refers to the bore size. The piston will be smaller.

Example: I bought a Wiseco 87-89 TRX250R piston size 67.50 mm. The piston actually measures 67.430 mm.

[ridered86]

@pablo
Wow, glad I got to see that post! I was worried about all the measurements, etc. If I may add a question within a question, how high can the dome on the top of the piston be? i.e., how much clearance should I have? (It's a 200, by the way)

[atc500x]

Honda service manual give 0.0006 to 0.0018 clearance.Service limit to 0.004

0.0006 is way to tight for me.I would go to 0.002 on a new bore
Ring gap is 0.012

[atc500x]

IMHO, the old air cooled 250R's need more like .006 thousandths........

But, IDK what you have.

According to the Honda service manual I have,the air cooled 250r clearance is 0.002 to 0.004,service limit to 0.006

[ailll1]

Thanks pablo! That's what I needed to know mostly.

Looks like my buddy is not ready for a top end rebuild on his ES.
Anyways, I took measurements on my yz125 and the biggest bore I could get was 56.601mm. Pistons go by .5mm increement. So if I'm right, I need a listed 57mm piston?
I found a guy who owns a machine shop and he told me the same thing you guys told me: he needs the piston to bore/hone my cylinder.
Thanks again for all the help!

[Dirtcrasher]

According to the Honda service manual I have,the air cooled 250r clearance is 0.002 to 0.004,service limit to 0.006

Either no one drills the exhaust bridge or has no clue how to jet. We have thread after thread of air cooled motors seizing - after break in which is why I suggest .006, maybe .005 would be fine but .002/.003 would have allot of seizures around here, too tight IMHO.

It also depend on whether the piston is cast or forged which is why WISECO seems to want a bit more room to grow........

[pablo]

I don't want any misunderstandings with the different numbers shown above between inches and millimeters.

In my post #7, .07 mm is just under .003 inches.

[Dirtcrasher]

Yep, I was typing in inches.

Again though, a forged and cast piston expand at different rates as well as air cooled versus liquid cooled cylinders; I always check with the manufacturer and tell the machinist what I want.

I've seen tons of new builds on here blow up even with proper jetting and it's usually a cold seizure or too tight clearances.