need some ? answered on piston size and clearance

[feartheride]

I have a kawi 750 small pin motor which we just tore down due to a bad crank.
cylinder bore is within .0005" taper and out of round,bore size is 81.5 m.m. (3.1695")

Usually with a wiesco piston the bore is a standard size and pistons are .004 approx. under the standard bore size
In this case the used piston measures 3.165" which is .008" clearance ,to much

so with the bore being right on 81.5 ,after a quick hone it will likely be about .001" over,if I order new pistons 81.5 will the clearance be right. I know with how wiescos are it will leave me .001" more then what would be ideal for a stock motor. But I want to use pro-x.

no sence in reboring if it is not needed,when I can just give it a hone..

 

[SurferSmurf]

i spent some time on the phone with a tech from wiseco a while back cuz i had a situation like this. he recomended a clean up hone and said the extra clearance would be ok and recomended due to the greater heat being generated from the higher compresion since forged pistons grow more. i also know from experiance that a motor will continue to run strong being much farther out of tolerance (not necessarily safe). my first ski was a 650sx and if it would not have gotten a clog which over heated the motor and warped the head i never would have known that the cylinders were .017 to .019 oversize due to wear. it was still running great.

 

[feartheride]

I spoke to wiesco tech couple years ago and I thought he told me that there pistons are made to fit nominal bore sizes ( so if the piston is designed for a 81.5 mm bore then the piston will be 81.4,/.004" smaller then the nominal bore size ) .but from what i have just measured with these pistons ,that doesnt seem to be the case.My ? is more ,if that is the case will I be able to buy 81.5 mm pro x pistons for this 81.5 mm bore.After a small hone the bore may be .001" over 81.5

What size are 81.5 mm pistons ?

 

[feartheride]

bump

 

[feartheride]

just looked up the wiesco ring # for an 81.5 mm bore, part# is 3169 =3.169" or 81.5mm...

so it looks the piston has shrunk about .0035-.004"
whats up with that

Does any one have an 81.5 new pro x piston they can measure

 

[sjetrider]

These do vary in size even when new bro. If you are going to run .010 clearence I highly recomend buying the weisco's and letting it warm up before every ride. Weisco's expand much more than Pro-X pistons. I see where your going but if someone measures a new 81.5mm ProX u could end up with one a little smaller. I have seen varience in them. Dont know why but they do vary. .006 is about the largest tol. I would run with a pro-X and .011 with a weisco. I have pulled a good running motor apart and found it running with weisco's at .012.

 

[feartheride]

Just spoke with wiesco and as I thought,they tell me that it is normal for a new piston to collappse..The spec size on a new 81.5 mm piston is 3.1648" my used ones are 3.1615,so they have collapsed .0033"

So what I'm saying is even if I bought new pistons at 3.1648 " and put them in a fresh bored cyl. 3.169 " (81.5 mm) ..After running it the piston will collapse say .0033 (like mine did).. you will end up with a clearance of .0075 "

 

[sjetrider]

should be NO POBLEM.

 

[feartheride]

wiseco said they did testing with all pistons cast and forged and found after a short amount of time they all collapse rather quickly,forged ones a little more then the cast ones

 

[feartheride]

should be NO POBLEM.

Hey what are you running in your fleet for motors these days

 

[sjetrider]

Last 2 weeks been rough. New 904 ADA billet , 5 mil with 951 pistons motor hung a ring (not enough chanfering GRRRRRRRRRRRRRRRRRRRRRRRRR!!!!!! Broke a sleeve too over holiday weekend. Then just melted an 8 year old crank in a ported 760 I been running for a LONG time (just put a new top end in it after daytona just to freshen her up and guess it was too much for the crank LOL) Also cracked a sleeve in that mess. Rough season LOL. But in a few weeks I will have TWO new motors moooooooooHAHAHAHAHAHAHA!!!!!! And a lot lighter wallet to lug around.

Hopefully WDK will get my new carbon prepreg hull done soon to go with the big motor.

God he is frustrating to deal with sometimes, better be worth it in the end, Usually is.

 

[augustaf]

wiseco said they did testing with all pistons cast and forged and found after a short amount of time they all collapse rather quickly,forged ones a little more then the cast ones

I did my own testing and found that those forged pistons are not dimensionally stable, depending how much heat they saw. First they grew in size necessitating about .002" greater bore clearance then shrunk after awhile. I have never had that happen with the customary cast pistons.

 

[sjetrider]

I did my own testing and found that those forged pistons are not dimensionally stable, depending how much heat they saw. First they grew in size necessitating about .002" greater bore clearance then shrunk after awhile. I have never had that happen with the customary cast pistons.

this is why they recomend near double clearence. price you pay for light weight pistons. taht and you have to warm em up pretty good.

 

[feartheride]

Last 2 weeks been rough. New 904 ADA billet , 5 mil with 951 pistons motor hung a ring (not enough chanfering GRRRRRRRRRRRRRRRRRRRRRRRRR!!!!!! Broke a sleeve too over holiday weekend. Then just melted an 8 year old crank in a ported 760 I been running for a LONG time (just put a new top end in it after daytona just to freshen her up and guess it was too much for the crank LOL) Also cracked a sleeve in that mess. Rough season LOL. But in a few weeks I will have TWO new motors moooooooooHAHAHAHAHAHAHA!!!!!! And a lot lighter wallet to lug around.

Hopefully WDK will get my new carbon prepreg hull done soon to go with the big motor.

God he is frustrating to deal with sometimes, better be worth it in the end, Usually is.

Wow that does suck,hope you get some trouble free years with the new motors ...

Good luck with the hull ..... hope he comes through in time to get some time on it this summer

So what are the new motors ????