Stock vs. aftermarket piston and engine balancing.

[gil2264]

   I just joined the KX500 club about a month ago.  I had the head off a week ago, and it seems like there is a lot of slop between the piston and cylinder.  I think it will be ok for a little while, but I'm going to change out the piston this winter.  I was just wondering if the weight difference between a stock (OEM) piston, and an aftermarket, ex.(Weisco Pro-lite) makes much difference in the amount vibration from the engine.  It would seem to make sense to me that the crank and lobes would be balanced for the stock piston, but a lighter or heavier piston would throw the whole works off balance and cause increased vibration. 
   I'm also planning on having the cylinder replated at Luke?s Racing (http://www.lukesracing.com/luksr/np.html). He's only about 50 miles from me so I can go up there myself to check out the shop.  Any opinions about his work would be appreciated. 
   I have some other mods I plan on making this winter also, including: Setting up the suspension, adding a flywheel weight, lighting coil, headlight, taillight, Spark arrestor...basically I'm trying to make a large KDX out of it for high altitude mountain riding.  (8,000'-13,800') and I weigh 300 lbs.
   Any advice for building to this purpose would also be appreciated.   

Thanks
   Dan Gillman
Gil2264@earthlink.net
   

[GDubb]

I cant help you with much, but I can tell you that its pretty unanimous around here that lukes racing is the way to go.  I havent read anything but good things about them and there is quite a few first handers around here that vouch for the quality of their services and customer care.

[Polar-Bus]

   I just joined the KX500 club about a month ago.  I had the head off a week ago, and it seems like there is a lot of slop between the piston and cylinder.  I think it will be ok for a little while, but I'm going to change out the piston this winter.  I was just wondering if the weight difference between a stock (OEM) piston, and an aftermarket, ex.(Weisco Pro-lite) makes much difference in the amount vibration from the engine.  It would seem to make sense to me that the crank and lobes would be balanced for the stock piston, but a lighter or heavier piston would throw the whole works off balance and cause increased vibration. 
   I'm also planning on having the cylinder replated at Luke?s Racing (http://www.lukesracing.com/luksr/np.html). He's only about 50 miles from me so I can go up there myself to check out the shop.  Any opinions about his work would be appreciated. 
   I have some other mods I plan on making this winter also, including: Setting up the suspension, adding a flywheel weight, lighting coil, headlight, taillight, Spark arrestor...basically I'm trying to make a large KDX out of it for high altitude mountain riding.  (8,000'-13,800') and I weigh 300 lbs.
   Any advice for building to this purpose would also be appreciated.   

Thanks
   Dan Gillman
Gil2264@earthlink.net
   

Stock OE Kawasaki pistons are cast. Wiseco's are forged. Cast pistons have minimal expansion, so the tolerences are tight. Forged pistons expand much more than cast ones, so the clearences need to be set up looser. Forged pistons claim to be stronger, but forged pistons as a result of the looseer clearences, "knock" when the engine is warming up. Cast pistons are generally more reliable to run. I ran one Wiseco years ago, and siezed it up after only 3 races. Not 100% sure if it was the pistons fault (as it was a fresh re-sleeve job). I've changed about 25 top ends usingthe stock OEM Kawi pistons with only one OE piston failure.

[gil2264]

I take it, that means warm up is even more critical with a forged piston.  Any favorite warm up proceedures?  for trhe kx5 seeing as it doesn;t like to idle?

[gowen]

I have used Wiseco since I started dirtbiking as a kid. Very reliable. No balancing issues at all. The warm up is critical, but should be taken easy on ANY 2 strokes. peroid.

I used to have a 250r that I gave to my cousin. It siezed a Wiseco and thought I would try going back to stock instead of Wiseco, due to the fact that it siezed.. Instead of siezing and causing minor damage, the stock OEM piston collapsed and ruined the entire motor. I never foudn why it kept locking up.. Don't care anymore. But found that Wiseco has never collapsed ever and I've seen multiple OEM and Pro-X pistons collapse. I've rebuild 20+ two strokes in the past few years. I'm still on a 100+ hour piston in my YZ250... No issues and just inspected it not long ago (Wiseco).

Take it with a grain of salt.. It's personal preference. Just like Aluminum (crap)sprockets and reed spacers (cra...*censored* )

[don46]

I run Wiesco pistons exclusively in our race bikes, I've never had a problem, but then I replace pistons every month or 2  so I may not be a good judge. The critical issue with wiesco is the ring end gap, if it's to tight it stands a good chance of catching a port and trashing your cylinder, and yes warm up is more critical, I tend to warm the bike until the cylinder is warm to the touch and then ride moderately easy for a couple of minutes.

[FuriouSly]

Hmmm..  I am by no means a gearhead, so anything I write below can be ignored, but this post is interesting so I must comment.

Is this topic for two stroke or four stroke pistons?  Very important starting point.

Forged pistons need more expansion tolerances (up to 10X) but are stronger and more able to withstand high compression motors.

Main difference would be the cold running motors... piston ring blow-by and noisy pistons until forged pistons reach full expanded diameter.  Make sure you warm up a forged piston fully.  Mostly a four stroke issue having cold loose rings.

Another difference is a high performance or stock motor...  forged pistons are stronger and are needed for higher comp motors.  IF you are replacing a stock motor/stock piston use the stock cast piston.  The bottom end will be more balanced for the stock stuff.  IF you are putting a wiseco forged piston in then you should plan on a balance job on the bottom end since the forged piston is heavier than a cast one.  The cast will last as long if not longer in a stock motor than a forged one will in a built motor.

Lastly...  the cast piston is actually better for a rider that has a stock bike and is not really into the whole baby your bike scenario.  What I mean is, a cast piston is better fitting throughout the temperature range.  Cold start is quite and more forgiving on warm up.  The cast piston holds the temperature in the combustion chamber more consistently than forged, resulting in better/more even burning of the mix.

Sly

Oppss.. left something out.

Cast pistons are capable of being made from different materials/alloys than forged pistons.  Cast pistons can hold heat more consistently and expand less due to the compounds in their makeup.  They tend to be a bit more fragile upon their breaking/failure point.  They are generally lighter as well.

Forged pistons are made in such a way that the forging process which makes them stronger does not allow them to use the same materials that cast pistons do.  Thus the trade-off for the expansion downfalls.

[don46]

I am referring to two stroke when I'm talking about end gap. You should measure the ring end gap prior to installing, if there isn't enough the rings expand, forcing them out, when this happens they can get caught in a port causing severe damage. When installing the rings there are explicit instructions for measuring end gap. I've seen more than one cylinder destroyed because of this, typically on high performance race bikes. On most occasions I've had to file the rings to get the proper clearance.