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2 ring piston vs. 3 ring

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What is the difference between the 2 ring listed race only piston, and the 3 ring? And why is the 2 ring listed as race only?

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2 ring pistons have a shorter service life. They need to be replaced at a much shorter interval than 3 ring pistons.

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The 2 ring piston has one compression ring (instead of two) and one oil ring.

2 ring pistons (four stroke) have less drag and can help performace, but, as stated above, must be replaced more frequently. The do cause less wear on the cylinder though.

It's not a good idea to switch from a three ring to a two ring piston without replacing the cylinder. The 3 ring piston will have different wear patterns near the top of the cylinder and can cause problems with the oil ring not sealing @TDC.

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it may have to do with less reciprocating mass also. Of course, as mentioned, the cost of that reduction would be more frequent need for replacement.

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it may have to do with less reciprocating mass also. Of course, as mentioned, the cost of that reduction would be more frequent need for replacement.

There can be other benefits to switching to a two ring piston but in order to take full advantage there needs significant design changes.

Switching to a two ring piston will allow the wrist pin to be located higher on the piston and allow for a longer connecting rod to be used. Not something that can be done without a custom piston and connecting rod.

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There can be other benefits to switching to a two ring piston but in order to take full advantage there needs significant design changes.

Switching to a two ring piston will allow the wrist pin to be located higher on the piston and allow for a longer connecting rod to be used. Not something that can be done without a custom piston and connecting rod.

There may be a simple answer that makes me feel stupid for asking, but...

If you move the pin up, and get a longer rod, you haven't really changed the stroke or compression... because the stroke is determined by the crank, and the compression is determined by the stroke and combustion chamber size....neither of which change.

What is it I'm missing? Strength?

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There may be a simple answer that makes me feel stupid for asking, but...

If you move the pin up, and get a longer rod, you haven't really changed the stroke or compression... because the stroke is determined by the crank, and the compression is determined by the stroke and combustion chamber size....neither of which change.

What is it I'm missing? Strength?

By changing the rod ratio (center to center length of conn rod divided by stroke length), you can improve the performance without changing engine displacement.

This happens for a few reasons;

A longer rod increases dwell time at TDC and improves efficiency.

Provides a more favorable mechanical advantage with the crankshaft by reducing the angle of the rod in relation to the crank.

Moves the point of peak piston velocity closer to 90 degrees ATDC/BTDC.

Increasing the length of the connection rod will improve power output throughout the RPM range without making any other changes.

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