Xr80 cylinder head question

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Xr80 cylinder head question

This is a discussion on Xr80 cylinder head question within the Vintage Honda XR75 and JDM Vertical Motors forums, part of the General Talk category; Hi, I have a xr80 that needs a new cylinder head. I have been looking online for a used one but they are much more ...

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Thread: Xr80 cylinder head question

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    Xr80 cylinder head question

    Hi, I have a xr80 that needs a new cylinder head. I have been looking online for a used one but they are much more expensive then a used xr100 head. So my question is, will a xr100 head fit and run on a fully stock xr80 motor? Thanks for reading.


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    Quote Originally Posted by JohnnieZ50R View Post
    Hi, I have a xr80 that needs a new cylinder head. I have been looking online for a used one but they are much more expensive then a used xr100 head. So my question is, will a xr100 head fit and run on a fully stock xr80 motor? Thanks for reading.


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    Head yes cylinder no. If you use the 100 head absolutely do not use the 100 cam gear cause they are different

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    Quote Originally Posted by storminnorman07 View Post
    Head yes cylinder no. If you use the 100 head absolutely do not use the 100 cam gear cause they are different
    Thanks for the reply. Yes I have a xr80 cam and cam gear. So will those interchange with the xr100 head?


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    Quote Originally Posted by JohnnieZ50R View Post
    Thanks for the reply. Yes I have a xr80 cam and cam gear. So will those interchange with the xr100 head?


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    Yes the cams interchange. It's just the gear that doesn't. The xr 80 if memory serves the cam gear is 28 teeth and crank gear 14 where as the 100 is a 30 tooth cam gear and 15 crank gear. So if you would us the 100 cam gear with the 80 crank (14 tooth gear on it) the cam would not phase correctly. So in short you can use the xr 100 cam to you just have to use your xr80 cam gear with it. The 100 cam has a little bit more aggressive profile to it so if you got a 100 cam with a 100 head I'd use the 100 cam unless you already have an aftermarket cam.

    The xr 100 and xr 80 have the same stud spacing so technically speaking the 100 cylinder fits. Unfortunately there are other things that limit it from being a plug and play setup. The 100 oil passage for he head is located further from center of bore than the 80 cylinder. That in itself would cause catastrophic failure cause the head wouldn't get oil. The next issue is the 100 cylinder is taller as well as has a larger sleeve and of memory serves case bore in 80 isn't large enough to accept the 100 cylinder granted in the hands of a crafty machinist you can force the 100 cylinder to work but when you factor all the cost in machining you way cheaper just to sleeve your stock cylinder or bore it to the next size bigger.
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    It would phase right every once and a while though! (only joking don't do it)

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