Yamaha/Wiseco Piston Help?

[GIJoeBob]

I have a 1980 Yamaha IT125 and I am putting in a new piston, rings and having the jug kicked up a size.

The piston that is in the bike now is a Wiseco 236P6, which is as far as I can tell a 3 over piston. I ordered a Yamaha 4 over piston (part number 3R9-11638-00) and it is actually smaller than the 3 over Wiseco piston.

The Yamaha piston is NOS and I suppose that the wrong piston could be in the box. Is there a way I could verify that the Yamaha piston I received is the correct one apart from the number on the box?

Any help is appreciated.

Thanks.

[cr480r]

measure your parts next time.. a Wiseco P6 for any machine is .060" over or 1.5mm oversize...

[GIJoeBob]

measure your parts next time.. a Wiseco P6 for any machine is .060" over or 1.5mm oversize...

I knew that. My understanding is this-

1 over is .5mm larger than stock
2 over is 1mm larger than stock
3 over is 1.5mm larger than stock
4 over is 2mm larger than stock

Is this not correct?

[cr480r]

I knew that. My understanding is this-

1 over is .5mm larger than stock
2 over is 1mm larger than stock
3 over is 1.5mm larger than stock
4 over is 2mm larger than stock

Is this not correct?

Atv/motorcycle piston typically go in .25mm increments... with some sizes missing on certain applications...

wiseco example: P=std., P1=.25mm, P2=.50mm, P3=.75mm, P4=1.0mm, P6=1.5mm, P8= 2.0mm


Notice how P5(1.25mm) and P7(1.75mm) are missing? On some machines they never or used to offer them, and on more popular machines they still do.. Thats why its always important to measure your bore if you dont know what size it is before ordering parts.. Your new 4th overbore size yamaha piston is probably 1.0mm oversize and was in the correct box...

[GIJoeBob]

Atv/motorcycle piston typically go in .25mm increments... with some sizes missing on certain applications...

wiseco example: P1=.25mm, P2=.50mm, P3=.75mm, P4=1.0mm, P6=1.5mm, P8= 2.0mm


Notice how P5(1.25mm) and P7(1.75mm) are missing? On some machines they never or used to offer them, and on more popular machines they still do.. Thats why its always important to measure your bore if you dont know what size it is before ordering parts.. Your new 4th overbore size yamaha piston is probably 1.0mm oversize and was in the correct box...

So the piston I started with is a 5 over and I need a 6 over?

[cr480r]

So the piston I started with is a 5 over and I need a 6 over?

The piston you had is 1.5mm or .060" over stock size... the next larger available wiseco is 58mm thats 2.0mm over or .080".. Thats will probably be the last bore for that cylinder.. Does it need to be bored?

[GIJoeBob]

I figured out the cause of my problem.

The Yamaha pistons move up in this fashion-
1 over - .25mm
2 over - .50mm
3 over - .75mm
4 over - 1.0mm

The Wiseco pistons move up in this fashion-
1 over - .50mm
2 over - 1.0mm
3 over - 1.5mm
4 over - 2.0mm

So my 3rd oversize Wiseco piston is actually larger than the 4th oversize Yamaha piston.

[cr480r]

I figured out the cause of my problem.

The Yamaha pistons move up in this fashion-
1 over - .25mm
2 over - .50mm
3 over - .75mm
4 over - 1.0mm

The Wiseco pistons move up in this fashion-
1 over - .50mm
2 over - 1.0mm
3 over - 1.5mm
4 over - 2.0mm

So my 3rd oversize Wiseco piston is actually larger than the 4th oversize Yamaha piston.

Your problem is ordering pistons in 1st, 2nd, 3rd, 4th oversize,etc.... always order parts by actual size or you are asking for trouble... more often than not the guy behind the parts counter only knows whats available in his current catalog... It may not coincide with what was once previously available or available from a different manufacturer or supplier.. Your 1.5mm wiseco is actually currently the 5th step in oversize in the Wiseco catalog, but it may the 3rd oversize available in your catalog or someone elses catalog given to them by their parts source.. If your cylinder needs bored you will need the wiseco 236P8 (2mm) oversize which is presently the 6th or last oversize, but at one time that size may have been the 7th or 8th over depending on how many sizes they offered for your particular bike when it was newer... If you are buying a new wiseco and your jug needs bored, you will need a 58mm(+2mm) piston unless you can find a 57.75mm(+1.75mm) piston somewhere.. Telling the parts guy the actual bore size needed drastically lessens the chance of a mis-communication and getting the wrong parts. Hope this helps

[rdlsz24]

If all else fails take it to the shop that is going to do the boring and have them tell you what size piston you need to buy. Worked for me

Rob