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XR75 - How big can I go?

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My kid is done with his '78 XR75 so naturally I'm taking it from him. 😢

He used it as a playbike, spare MXer in case his KX65 died, and sometimes mini-harescrambler (he liked it better in the woods than his little KX). A year ago I did a fork swap from an MR175 and added a set of 15" NOS S&W gas shocks from one of my evo Bultacos. It's now at just under 9 inches of travel on both ends - plenty for Dad to have some fun on.

Anyway, now that it's Dad's again - I'd like to add some HP. Will an older XL/XR100 top-end bolt on? If not, how limited am I with a new bore on the stock cylinder?

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An XR100 motor will bolt right in and you can go upto 150cc with it

Check out Powroll's website they make parts for the XR75.

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Thanks. I'm aware of going the Powroll route but have access to several 100 motors that are just sitting there...

Looking to get by cheap on this one. 😢

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Yeah, if you have access to xr100 motors then use them. They are easier to make faster. But, if you want to make your 75 faster, it's pretty much the same process. You can make it a 121cc with a bore and stroke. If someone would ever try a klx110 big bore pistion, then you can make it a 140cc. But mainly, cams, carbs, and high compression pistons. There are plenty of companies making the stuff. If you're really want it to move, then stuff a XR200 in there. It can be done fairly easily. My friend has one and it's scarry fast.

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