Maintenance & Technical > KX250 / KX125

kx125 `90/`91 cylinder head and piston

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ihampson:
Hi, Does anybody have any pictures of either or knows the differences between the two? I have been told that i have a `91 cylinderl on my `90 125. The shop i spoke to checked the numbers under the power valve cover to find this out. The trouble is they couldn`t check what year cylinder head i have, niether could any of the dealers i called.  

I need to know which head i have as the pistons are different between 90/91. According to one shop the 90 piston is dished but nobody is sure as they dont have one to check, it would have to be ordered (and paid for) first. The piston that came out of it is a flat top but is baddly pitted (from a shattered ring i by the looks of it) and has some detonation on it. The engine is completely stripped otherwise i would check the cylinder volume at tdc. I have got this bike as a non runner and havent opened up many kx engines to compare mine against.

The head is dished but seems quite shallow to me - 4.6mm deep and 40mm diameter with a shallow slope to the spark plug hole. The only markings on it are a symbol that looks like the letter A inside a small circle with a line through the center and over the A. This symbol is followed by -1-2

I have taken some pics but i am using a camera phone and its not that good.

Can anybody identify the head or provide any info? Any help is appreciated.

mustangfury:
I have a book that compares different year bikes to one another.  It was saying that the '90 and '91 kx125's are interchangeable in all their parts but not to mismatch the different pistons with cylinder heads.  It told how to differentiate between the two heads. The 1990 cylinder head has a concave design and the '91 head has an alignment pin on it to align the cylinder to the piston head.

From the way your pictures look to me, I would say that the shop you went to was right in saying it was a piston head off of a '90.  I didn't see any alignment pin but i couldn't see the whole thing in the pictures.  My guess would be that it is a '90 piston head just from what i could see.

Hope this helps.  Good luck.

moron:
what were the numbers behind the cover please

ihampson:
Thanks mustangfury, It is a `90 head.

Have just looked at the part diagrams on partsfish.com and i didn`t see the extra locating pin on the `91 last time i looked.

That book you mention sounds pretty usefull. Whats it called and can you still buy it? Does it say anything about a `89 kx250 cylinder on a `87 bottom end? (another one of my projects)

Does anybody know the best (cheepest!) place to get a +1mm `90 kx125 piston kit? am considering ordering from U.S. as wiseco oversize pistons are quite pricey in the shops near me.

Moron - the number behind the cover is 1645 but i dont know if all 91 barrels have the same number or different batches have different numbers.

The second picture (image22) shows the hole for the retaining pin on the `91 barrel and there is no matching lug on the `90 head. Hope the pictures show what i mean.

moron:
it seems i have the opposite of you ian, i have a 90 barrel and 91 head. i have no problems with my setup and runs ok

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