Maintenance & Technical > KX500 Original

Stock vs. aftermarket piston and engine balancing.

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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.

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