kx125 `90/`91 cylinder head and piston

[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

[ihampson]

What piston are you using? Is it dished or flat top?
I think i need a dished piston with the `90 head. The bike had a flat top piston in before but was badly damaged through detonation and a shattered ring. I got the bike as a non runner and dont know exactly what caused this damage, would a flat top piston create higher compression and the damage to the piston?

Is the dish on your cylinder head the same as mine or deeper?

[moron]

My piston is dished. My cylinder head looks to be the same as yours apart from the extra locating pin hole.

[ihampson]

What year did you order for when you got the piston? Is it a `90?

I`m a bit confused now about the need for different pistons when the `90 + `91 heads appear the same. Could there be a difference in stroke or piston height? I remember reading about having to use two base gaskets or a thick aluminium one when using different barrels on newer kx`s.  Something to do with the port timing and squish. cant remember exactly, will try and find the article.

How dished is your piston? is your bike apart at the moment moron? (feel bad calling ya moron, esp when your helping!) or is the picture from the last rebuild?

[moron]

I got a 91 piston, and yes my bike is in bits at the moment.
conrods are the same aswell and gaskets. my piston has only a slight dish maybe 1-2 mm

[ihampson]

So do you think a flat top piston is right for my bike. The shop i spoke to could have said the 89-90 piston was flat and i got them mixed up.
I have looked on ebay but that hasnt helped. There are people selling Flat pistons for `90 + `91 , others have a flat one that only fits a `91. others are selling dished ones for 88-91.

Not sure if the pistons are designed specially to fit the years they said or if the sellers don`t know what they are talking about. I wanna be sure which piston i need incase the shop i get it from aren`t aware of the differences between the years. (whatever they may be - i cant see any)

[mustangfury]

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)

The book is called "Motocross & Off-Road Performance Handbook" and it is written by Eric Gorr.  It is still available and I got it from Barnes and Noble.  It gives you a complete overhaul on how to maintain and fix dirt bikes in general.  In the back it lists the different dirtbikes and tells a little about each, such as: weaknesses, strengths, suggested fixes, common problems, etc. It lists each bike by year and covers suzuki, kawasaki, yamaha, and honda race bikes.  It covers some trail bikes but mostly race bikes, such as the kx, kxf, rm, cr, etc.  It costs about $25 retail.  It doesn't always say if the cylinders are interchangeable or not.  It just happened to have that listed for the kx125 you were talking about.  It does not say anything about the '87 and '89 kx250 cylinders being interchangeable.  Sorry.

[moron]

ian when you bought the bike did it have the piston in the bike.

[ihampson]

The bike had been sat in a garage for ages when i bought it. The top end was stripped because it had blown.

What i dont understand is the difference between the two years and why you have to buy the same year piston as your head. The heads appear to be dished the same. The only other thing on the head that could make a difference is the depth the spark plug fits into the cylinder.

Do you think that could be the reason? If the plug comes further into one it could hit the piston  or maybe the flame around the spark would be to close to the piston crown and burn through?  I might just be clutching at straws but i dont wanna put this bike back together with the wrong piston and have it blow after a few weeks, esp as im on the last oversize and would need a sleeve if it blew up bad.  Suggestions anyone?   

[moron]

when i screw my spark plug in its flush with the inside of the head, check yours see if its the same

[kexikalle]

This is 90 which needs dished piston, right?