250R Piston (What is it?)

[itsallgood]

Pro- X
HM25084

Says "100" on the top.

Anyone know if this piston is anything special or just a normal rod piston, and not a long rod?

I've been out of the 250R scene for a year or so and forgot a lot about them. It's got a spacer plate on the bottom letting me know it's a stroker and/or long rod crank.

The piston measures 66.8mm

I have one cracked and need to know what to order.

[deathman53]

its probably a 86 style piston and they use the spacer plate to use a 86 piston on a 87-89 rod. you want a 60 over piston, most if not all pro-x pistons are 86 style. your piston is a 40 over piston, you want the next size, which I believe is 60 over, or 67.5mm

[itsallgood]

What does the 100 stand for on top of that piston, any idea?

I figured it was a stroker motor all along, just because the way it runs.
How do can I determine if it's a stroker crank or not without pulling the crank out?

[250R-Dee]

FYI - a longer rod does not equate to a longer stroke. The only way to change the stroke on any motor is by modifying the crank.

Post a pic of the piston in question. Maybe that will help jog some memories.

[Aceman]

My Pro-X catalog lists an

86 trx-250r piston kit as #0284 66.75 .030 overbore

^^This number sounds similar to the one you listed.

Next size up is piston kit #0286 67.00 .040 overbore

[itsallgood]

I understand you can have either, stroke or longer rod, or both although most of the time if you have a stroker longer rods come along with it. I don't have any pictures of the piston from a side view. Below are some pictures of why I'm having to replace.

Another question; how do you determine the stroke without removing the crank?

http://www.nemr.net/~bray1/images/100_0843.JPG

http://www.nemr.net/~bray1/images/100_0856.JPG

http://www.nemr.net/~bray1/images/100_0857.JPG

[novaracr70]

well if it a strocker you are going to have either a spacer that is bigger then 5mil more like 7 or you will have 2 mi machined out of head 5= long rod and 2 =for the strocker

[atvmxr]

I thought that the 100 is the um (micro meters, 10^-6 meters) increase in bore size, but that doesnt jive with above comments. 0.010in over is 25, .020 is 50, etc. so 0.040 over would be 100, but according to above comments your "100" is only 0.030 over.......