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Discussion Starter · #1 ·
Hi all,

I am restoring an 85 TRX 250 Fourtrax for my friends son. As I have been stripping it down for a complete top end overhaul and cam chain replacement I noticed when I pulled the jug it Mic's out at 74mm. Now as I'm shopping pistons I see 66mm was the standard bore size of an 85 year model. After checking both engine and frame serial #'s I do indeed have an early run 85 model.

My question...was there a big-bore kit available for these back in the day?
Or, did another Honda model have a larger bore that was a direct fit?

Basically, I need to figure out what piston to buy that would take me .025 over so I can have the jug bored.

Any help is appreciated!

Super Moderator
4,647 Posts
I don't know where you got your info, but the OEM Honda service manual (part number 61HA802 for 85-87 TRX250 Fourtrax) specifies the stock bore and stroke as 74 x 57.3 for a displacement of 246438.152cc

Yes, there were lost of companies making Big Bore Kits, back-in-the-day (Powroll, Cycle Pro, Shindy and others). If you want a big bore kit, there's a 2mm over Shindy kit on Ebay 85 87 Honda ATC250SX ATC250ES TRX250 Piston Shindy Big Bore Kit 260cc MX Ahrma | eBay, but, if you go big bore, you have to have a special head gasket that may not be available. I would advise just boring to .5mm (.020in) which will make it a 249779.648cc - 2mm over will make it a 259939.146cc

Honda did not have a bigger bore that was a direct fit.

Also, most cylinders of that era with a stock bore are worn to the point that a .25mm (.010in) oversize bore is not sufficient to clean the bore accurately. Consider getting a stock .50 (or larger) piston - look for piston part numbers 13103-HA0-305, 13104-HA0-305 or 13105-HA0-305 and the rings to match (you can find them on Ebay)

Have a good day
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