Why are these pistons different ? (Pic)

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NorthernSX

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Both 700 redheads, far motor has Wisecos, near motor say "ART" on the inside of the piston.
Near motor is ported, far is not.
Why is the bottom cut out of the skirt on the far one ?
Does that not affect timing ??
 

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NorthernSX said:
Both 700 redheads, far motor has Wisecos, near motor say "ART" on the inside of the piston.
Near motor is ported, far is not.
Why is the bottom cut out of the skirt on the far one ?
Does that not affect timing ??
Click to expand...

I believe thats to allow for air and fuel to get into the case quicker at the transfer port is slightly blocked.
 
Ok, well, am I OK using the notched wisecos in the ported motor ?
Do the wisecos come like that ? Or has someone notched them?
 
I know my 600 twin comes with notches stock.
Mine is ported and those notches get bigger with porting.
But, thats not a case reed engine. Not sure if that makes a difference or not.
 
NorthernSX said:
Ok, well, am I OK using the notched wisecos in the ported motor ?
Do the wisecos come like that ? Or has someone notched them?
Click to expand...

someone made the lower skirt notchs to advance rear boost port timing, it wont hurt to use those in another engine. If it were me I would add a "boost hole" to the stock Yamaha "art" pistons as they are the oem Yamaha pistons and do not have the warm up 4 corner heat expansion problems the wisecos due. Wisecos are ok as long as you have the right cylinder to skirt clearance...and let the engine warm up well before riding it everytime. they run a lot more skirt clearance to make up for the expansion rate, usually you need .0035-.0038 with them.
 
mrviper700 said:
someone made the lower skirt notchs to advance rear boost port timing, it wont hurt to use those in another engine. If it were me I would add a "boost hole" to the stock Yamaha "art" pistons as they are the oem Yamaha pistons and do not have the warm up 4 corner heat expansion problems the wisecos due. Wisecos are ok as long as you have the right cylinder to skirt clearance...and let the engine warm up well before riding it everytime. they run a lot more skirt clearance to make up for the expansion rate, usually you need .0035-.0038 with them.
Click to expand...

I am not familiar with the boost hole either. I take it the hole increase the intake charge? How large of a hole is this? How far up from the bottom of skirt should it be placed? Would you happen to have a picture? Thank you.
 
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