Maintenance & Technical > KX500 Original

Piston/rings 1985 kx500

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Zack:
I suspect one could use the ‘83-‘84 piston in the ‘85.

For my ‘85 KX500 I did decide to swap to the ‘88-‘04 piston. My cylinder is bored to .5mm oversize already so my hope of installing a standard size cast piston is a no-go. My cylinder is being cleaned and plated for a .5mm oversize wiseco piston and I am expecting things to work out just fine.

I did contact Tom Morgan about the suggested modifications to the engine when swapping to the late KX500 piston and he priced the work at $129. Tom told me he needed only the cylinder head to do the work. I have heard of guys doing this piston change without doing the head modification and never missing the potentially lost power. Since it is only the cylinder head to remove, I believe I’ll try it as-is and make a judgement after I ride it- if I need more power I intend to send the cylinder head to Tom.

 Anyway, the cylinder and piston should arrive in a couple weeks, I expect to ride the big kx again shortly thereafter.
I will keep you guys in the loop.

sandblaster:
I think that the guys who said it did not affect the power had worn out engines.
I've seen that statement more than a few times.
With a fresh bore, piston, and rings I'm sure it felt just fine.
Let's look at Approx piston dome cc's.
Wiseco 3.25cc
Wossner 3.9cc
Stock 86 double ring 18.5cc
Stock 85 single ring 14.5cc

You can swap any of them around with the correct head and or cylinder cut.
But to think you can drop in a 3.25cc piston dome in a bike that was made for a 18.5cc piston dome and not loose any power...
Well... I could find some bridges to sell you  :-D

Zack:
Thanks for the piston dome cc!

Well lost power to be sure.
I wonder if a recreational rider would ‘miss’ said power though.

If the cylinder head mod is only to mill down the head to restore lost compression (~11.25cc worth) then I could likely perform that job myself.
I thought the job might Include setting a squish band angle on the head as well as fix lost compression.

Does anyone know what the cylinder head mod actually consists of?

sandblaster:
Yes, the squish and dome has to be completely re-done.
Every .010" of height you remove from a 86mm bore is approx 1.5cc

Zack:
Update

I got my engine back together. It is currently assembled with the 86.5mm wiseco piston.
I opted against the cylinder head modification for a few reasons, the main reason being impatience.
The engine runs great! Engine compression is on the low side of useable- 130 psi. But certainly much better than the 100psi it had before the rebuild
I honestly can’t complain about the engine at all. Starts pretty easily, idles nicely, revs out nicely, plenty of power.
I still have the cylinder head mod in-mind for later, maybe late this year. For now, I intend to enjoy my kx500.

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