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rusky

wiseco racers choice piston alters squishband?

11 posts in this topic

I had a prolite and racers choice single ring pistons sitting on my desk so I decided to take some rough measurements. Here's what I noticed.

Pertaining to the single ring piston:

Its longer when measuring from bottom of the skirt to the dome.

Ring groove is slightly closer to piston crown and the ring is a little bit thinner (less drag).

Piston has a larger diameter (tighter clearance).

While the length from wristpin to dome is higher, wristpin to skirt distance is shorter. Wouldn't this suggest that the piston is allowed to travel further therefore reducing the squishband? Or at the least, bump up the compression a bit?

Currently on my phone, will post numbers and pictures when I get to a computer tonight.

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Could it change much without causing problems? Not any mention of it in the instructions. Unless every Yamaha yz250 was the same, I dont see them changing it enough to matter.

It is a hell of a piston, though.

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Whoa good catch rusky. I just measured the old wossner piston for my 144 against the wiseco racers choice. Rusky is right, the whole piston is moved up a skosh. The racers choice is over .5 mm taller from the wrist pin. I had squish where i wanted it with the wossner. I have a wiseco prolite in it now but did not measure it. I gotta start measuring stuff better!

Cant really tell if the crown shape is differnt to help compensate for the height difference between the wossner and racers choice, but i doubt it can help much with over .5mm differnce?

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Whoa good catch rusky. I just measured the old wossner piston for my 144 against the wiseco racers choice. Rusky is right, the whole piston is moved up a skosh. The racers choice is over .5 mm taller from the wrist pin. I had squish where i wanted it with the wossner. I have a wiseco prolite in it now but did not measure it. I gotta start measuring stuff better!

Cant really tell if the crown shape is differnt to help compensate for the height difference between the wossner and racers choice, but i doubt it can help much with over .5mm differnce?

From what I could see when comparing the ProLite and Racer's pistons is that the Racer's piston was both moved up a little and had a more convex dome. This leads me to believe that back when the YZ250 was raced, and the single ring piston was used, instead of milling the head to adjust squish, they just made a piston to do all of the above. Just a guess but seems quite probable to me.

The higher compression would definitely explain the feeling of having more bottom end.

Going into the garage to take pictures, will be back.

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I have had seizures with the racer's choice when installed in new or freshly plated cylinders that are on the "tight" end of spec...

Which led me down the same rabit hole - they are larger in dia than a pro lite...

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I decided to do away with pictures and just post my numbers, so here we go.

Racer's Choice:

TiN ring free gap - 5.0mm

ring groove to crown - 3.55mm

diameter - 66.08mm

length - 79.45mm

wristpin to skirt - 5.90mm

wirstpin to dome - 32.13mm

ProLite:

chrome ring free gap - 6.9mm

ring groove to crown - 3.07mm

diameter - 66.0mm

length - 78.19

wristpin to skirt - 5.45mm

wristpin to dome - 31.49mm

As a note, these are rough numbers. To get precise measurements I would need much better equipment than a simple digital caliper. Yet you can still see some noticeable differences.

Edited by rusky

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Great info!

Especially noting the difference in "pin-height" (pin to dome dimension) of each piston.

Interesting thing in comparing the Racers Choice vs performing head modification is that the Racers Choice piston will also retard the port timing events on ALL ports and powervalve by the same stroke length, but NOT the same number of crank degrees.

For sure a guy with head mods needs to be very careful with measurements if he wants to switch to the Racers Choice.

A solution to that may be to run a 0.5mm thicker base gasket, or a double base gasket. That will correct the squish clearance if your head is set up for the OEM or Pro-Lite pistons "pin-height" and it will correct the same offset in port timing.

Rusky, have you noted where the ring end-gap is aligned on the piston?

Is it still on the back between the ports like the OEM and Pro-Lites?

I have not tried the Racers Choice... After seeing your dims, I'm thinking maybe getting one for some experimentation in the future.

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Rusky, have you noted where the ring end-gap is aligned on the piston?

Is it still on the back between the ports like the OEM and Pro-Lites?

Yup, same place.

I urge you to give this piston a shot if you haven't yet, the difference will surprise you.

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I had rb do my head and measured with the racers choice in place. My god, what an engine.

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