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Demoman1
12-24-2019, 12:22 PM
I'm getting a cylinder head with a 68mm bore what size piston should i use for it?

ps2fixer
12-25-2019, 01:13 PM
FYI, heads don't get bored unless you're boring out the spark plug hole or something. The cylinder where the piston goes is normally what's refereed to as the engine's bore size which I'm assuming you're talking about. The stock bore of the 85 ATC250R is 66mm so it's bored about 2mm over stock. Generally speaking you buy a piston, then have the cylinder bored to be within spec. Per the service manual, the piston to cylinder clearance is 0.060-0.095mm so if you buy a 68mm piston and the piston vs cylinder size isn't within that range, it will need to be bored again or another piston sourced. Normally oversized pistons are just a tiny bit bigger than stock bore, like thousands of an inch bigger, seems like someone was trying to increase the engine size a little by going quite a lot larger. Not sure what the limits are for the stock cylinder sleeve.

Short answer, you put in a 68mm piston in a 68mm bore, anything else is out of the tolerance range.

Here's an example of "normal" options, clearly your engine isn't exactly stock anymore. Oversized pistons range from 0.25 to 0.5 over sized which I assume is in mm.

https://www.partzilla.com/catalog/honda/atv/1985/atc250r-a/crankshaft

I'm not exactly an expert on boring cylinders, so maybe someone can chime in and give good advice where to source a piston from or what to check to see if the engine will hold together in it's current state etc. If you want to go back to stock, you could always buy another cylinder that's the standard size bore. Most likely you'd have to go used and fair chances you'll still have to do cylinder work and piston to cylinder matching.

Good luck with the build.

350for350
12-25-2019, 09:24 PM
I've seen Wiseco pistons listed at 69mm. That makes more sense than just two oversizes. Especially on something like a 250R. Of course Partzilla just lists Honda parts.

deathman53
01-02-2020, 09:07 PM
you can go up to 69.5mm, but the sleeve is really thin. I believe 69mm is 120 over, 68mm is 80 over. I would spec the the cylinder and get the next size piston for it, don't trust that the person says its 68mm(80 over), it well could be, but its worn out.