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View Full Version : 300EX piston in a 250X



dieselpower1973
01-01-2007, 01:37 PM
Just curious if anyone else has ever put a 300ex piston in a 250x. The piston size is the same 74mm, the skirts are identical, and the location of the piston pin relative to the piston crown is identical. The only differences that I see are the 250X piston having a convex crown versus the 300EX piston having a concave crown and the 250X piston also has a few extra holes for oil flow. Has anyone run this setup?

Mxjunkie
01-01-2007, 01:39 PM
Just wondering why would you want to run the 300ex piston when the 250x motor runs better then the 300? We had a 87 250x but we ran a wiesco piston.

dieselpower1973
01-01-2007, 02:07 PM
I have a brand-new 300EX piston in my toolbox and recently picked up a '92 TRX250X that needs a top-end. I compared the pistons side by side and don't see any reason why I couldn't run the 300EX piston. The 300EX piston looks like it might have just a hair lower compression due to the shape of the dome. Just looking for input.

zack221
01-01-2007, 02:13 PM
i thought the topend on those two where the same but the bottom end was diff.

burkebrothers
01-01-2007, 02:37 PM
the two are interchangeable, the 300 piston is lower compression thats why everyone running 300's use the 250 piston

dieselpower1973
01-01-2007, 02:59 PM
Kinda what I figured...

I'm keeping the quad, so I'm sure it won't be the last top-end I put in it. I just thought that since I have the 300EX piston this time around, I might as well put it to good use.

dieselpower1973
01-01-2007, 03:31 PM
I've looked at several threads about this, but they contain so much incorrect information about these quads that I couldn't trust anything they said.

The most valuable piece of information for me was to slip the old 250X piston onto one end of a wristpin and the new 300EX piston onto the other end of the wristpin and do a side by side comparison. After comparing the two, I don't see why it wouldn't work.

Sjorge450R
01-01-2007, 04:59 PM
well you have a 300ex piston just sitting there. No one knows wether or not if it will work or not. Why dont you just try it and find out?? LOL. I myself still have no idea about why they wouldnt work.

dieselpower1973
01-01-2007, 06:05 PM
I'm 99% sure that the piston will fit. I guess the real question is, how will the concave piston crown and the resulting lower compression affect the 250X? Not that it's a huge difference, but it is a slightly lower compression piston.

dieselpower1973
01-01-2007, 06:23 PM
These are both OEM Honda. The 250X piston that I took out and the 300EX piston that I'm putting in.

williams_179
01-01-2007, 07:34 PM
We used to race 300ex for several years and we ran the 250x 12:1 pistons to bump up the compression on the 300s. They are all the same just different compression. Hope this helps.

dork
01-01-2007, 07:37 PM
you finally answered the question to this...that was the best thing to do, stick the wrist pin in each and measure the compression heights. yea, it will work if thats the same. the 300ex had a lower compression to offset the extra heat created by the extra cc's, hence the dish in the piston. 8.9 vs 9.4:1. and the height difference was taken up by the rod, 123 vs 127mm.

dieselpower1973
01-01-2007, 08:31 PM
So we've come to two logical conclusions:

1) A 300EX piston will fit the bore/stroke of a 250X

2) A 300EX piston has lower compression than a 250X piston (due to the differences in dome shape)

So what can I expect, performance wise, from the lower compression ratio I'll be running with the 300EX piston? Do you think the difference will be enough to notice? Will it be hard to start?

If it seems like I'm running this topic into the ground, I apologize. I have nothing better to do while I wait for an affordable cylinder head to pop up on eBay.

williams_179
01-01-2007, 09:23 PM
you probably won't be able to notice any difference and it will not be harder to start. I may have a head if you need one but it will need valves. if you are interested let me know.

dieselpower1973
01-03-2007, 09:44 PM
How much for the cylinder head?

250X_project
01-04-2007, 08:42 AM
Dork has is 100% right. The Honda and Wiseco pistons have different part numbers because of the compression. Also the 250X cam is actually better performing than the 300(interchangeable yet diff. specs), so you will be fine power wise.

Plus now if anyone ever asks again you can put this hole topic to rest for once! ;)

2muchquad
01-04-2007, 07:08 PM
I guess a lot of readers...er posters dont know how to use the "search" option:)