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KX 125 92 Overbore Kit (139-144cc)

Zash

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Jun 6, 2007
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#1
Hi everyone

I'm looking for an overbore kit for my KX 125 92.
I could found some at Eric Gorr's shop but there are only for KX from 94 to 07.
Has anyone a big bore kit on his 92 ?

If I'm right Eric always use the same piston for his mod.
Does anyone know the difference beetween the 92 and the 94 cylinder ?
Is it possible to instal Eric Gorr's piston on a 92 ?

Thanks !
 
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#2
Zachs:

I do not remember from the top of my head, all KX 125 1992 and newer have a 54 bore x 54.5 stroke. 1991 and few years back 56 mm bore.
I have used 1991 pistons from KX125 to make a 140 cc from a KX 125 2003. I am almost sure that 91 was the last year to have a 56 mm piston. 56.5 will give you 140 cc. Wiseco sells the 56 .020 oversize. Certain minor modifications to the piston are required.
In general terms, all you do is find the right diameter piston with certain right dimensions such as the distance from the crankpin to the crown. The rest could be machined to mach the engine features.
It is very difficult to make good pistons, but not impossible and someguys may have their own foundry and cast tools, do not know any of them, I just buy the nearest piston to my application. You can see that in the piston application catalog at wiseco.com.
 

Zash

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#3
Thanks for the answer

The 92 was the first one with a 54.5 mm stroke.

It seems that the cylinder of the 94 for exemple has the same base gasket, so I think it's possible to change the cylinder for a 94. Now the question is: will the piston fit ?
It's possible that the connecting rod is different.
 
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#4
Zach:

Some older bikes pistons didn't have the intake boost hole carved on the piston. I know as a fact that the 91 KX125 didn't have that hole. You have to make it or find someone who can. The top bearing and wrispin is the same, the height from wristpin to crown is the same. It has a flass head vs an small convexity found on later models. This helps to keep the compression ratio low.
When you study the important parts on the piston geometry, besides the right bore, you'll find that the skirt lenght is important to close the exhaust port when the piston is at TDC, the window to open the rear booster port geometry, the side cutouts not falling into the exhaust window.
As I said, look over the WISECO spec page on piston kits and see your options. You must find a 56.5 mm dia with the stroke higher than 53. When you check the numbers for the top bearing, make sure are the same for your bike year model. Also the exhaust valves have to be grounded down to maintain 1 mm offset with the bore.
 

Zash

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#5
Eric Gorr sell a 94 cylinder (allready overbore )
I think it could be a good solution, because the base gasket is the same and the valve system seems to be the same, I would try to install the 94 cylinder on my 92 .
What do you think ? Could it work ?