Kawi2strokes.com Forum

I purchased some S3 .5 over pistons that are not directionally marked. Do I just rely on the piston pin offset as a guide?

The ring pins are always on the intake side, and welcome to the site

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1975 S3 project bike
1974 H1 project bike
1972 Yamaha R5
1974 Yamaha RD350

Hello Barside, strange that they're not marked. Are they inexpensive Chinese pistons? There are some of those that have a millimeter less compression height than stock. It might be worth measuring pin to crown and comparing to stock.

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If it surges, that's normal, upshift.

Hold on just a minute. Is that some special feature with the S series???
It's definitely NOT true for a H1 and I find it hard to believe S series are any different.

Stock H1:
Top ring gap to the right of the exhaust port.
Bottom ring gap to the left of the exhaust port.

H1 Wiseco (all years):
Top ring gap centered above the intake port.
Bottom ring gap to the right of the exhaust port.

These directions are when looking in the direction of the arrow (forward).

PK

Well I am and , PK your right my S3 pistons have pins on the exhaust side totally opposite what I learned somewhere long time ago.

I beg the forgiveness of the board

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1975 S3 project bike
1974 H1 project bike
1972 Yamaha R5
1974 Yamaha RD350

The wrist pin offset forward to rear on the piston is referred to as "Sipes angle", after the engineer in the 1920's that figured out that if the pins were offset front to rear, the pistons wouldn't "slap" in their bores so bad, allowing the rings to live long and prosper, and the pistons to be 'whisper quiet' in their bores. Back then, almost all pistons were forged, and required giant piston to bore clearances, not allowing good ring sealing, from piston rock in their bores, from the excessive clearances required with the forgings.

If you measure the edge of the pin to the edge of the piston skirt, pin to rear, pin to front, should give you the Sipes angle offset, usually .50mm (.020 inch). If I remember correctly, Sipes angle offset is twords the intake side of the piston, and should be in our case, the ring pinned side, as others here have described.

Do measure the compression height of the pin in those pistons, this is the distance from the center of the wrist pin, to the edge of the piston side to dome. You don't have to get fancy, go from the top of the pin bore to the side of the top of the dome on both stocker's and these. They should be the same. Compression distance is the height of the piston dome edge to the center of the wrist pin, and determines where the dome sits in the bore at TDC and BDC, and the porting numbers in between.

Thanks for the help everyone. Here's what I found:
Stock piston ring pins are located on each side of the exhaust port.
Stock piston pin offset toward intake side. Shorter distance opposite arrow mark on piston dome.
Using this same offset pattern, the aftermarket piston ring pins are on the intake side.
Piston height is shorter by 0.5mm. Piston pin is smaller diameter with larger small end rod roller bearing. Everything fits snug on the small end of the rod with no excess tolerance.
Piston make is listed as Mitaka.

Piston height is shorter from the pin to the crown AND the pin diameter is smaller? Sounds like you should check the deck hight of the stock piston as well as the new ones to make sure you arent losing a ton of compression. Even .5mm is going to lose you some compression.