You can play the eBay game but who wants a used piston in their new build?
All is not lost.
You can use a 89-04 Piston in your 83-88 KX500 engine.
There are some differences to be aware of.
83-87 are a dome top single ring design.
88 is a domed top with two rings.
89-04 are a flat top with a two ring design.
Bores and strokes are the same.
But here is what you need to be aware of.
First, take a look at the 89-04 KX500 piston in the 85 cylinder.
As you can see, with the piston at TDC there is about .015" clearance to the top of the deck.
Now look at the 83-88 piston:
This combo only nets about .004" of clearance...
So, the 83-88 dome piston will net you more compression...
Depending on how your jetted and the quality of the fuel you can get, this may not be a bad thing to have lower compression.
But if your like me, you'll use better fuel if necessary to get the performance you desire.
If this is the case, you will need to deck the cylinder about .010" and have the squish changed on the head.
But before you do anything, take careful measurements as mine are for reference only.
Remember, Motorrad says that each K5 engine is like a snow flake.
They are all a little different.
And in my experience he is right
Besides, if I was going to deck this cylinder I would probably do it so that my squish is about .045-.050".
You better be taking good care of your motor to get your squish in this range