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50cc to 90cc conversion.

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As title states , I have a 90 cc parts motor and a good 50 cc Bottom end. I know I need to use the crank from the bigger motor. Will this be a direct swap? Will I have to "clearance " the original 50cc case? I need helpful hints in advance. Thank you

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Chances are, the 90cc cylinder may be 69mm tall, 6mm taller than the 50 one. If that is the case, then it's a good possibility that the crank from that engine has a 49.5mm stroke, which means you'll probably have to line bore the 50 cases for the crank to clear. You'd have to swap the engine studs due to the taller cylinder. You'd have to use or get a longer cam chain, also due to the taller cylinder. Also keep in mind that the 90 may have a different engine stud bolt pattern, and that the cylinder and head may not slide all the way down onto the 50 lower end because of that

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If you try sliding the cylinder down onto the 50 lower end first, that'll determine whether or not you need to go any further. Just make sure you have the two centering dowels (part #5 in the link below) in place on the lower end to ensure that the cylinder will line up and sit flush on the cases.

centering dowels

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How's the conversion process coming along?

If the wrist pin is 14mm, there are pistons still available, but I think the bore size is very limited.

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I have been busy with work and I think I will now wait until the riding season is over to complete the swap. However the bigger cylinder slides right down over the stock studs. I have to remove the flywheel on the 50 motor to see if it will fit the big bore. If it does I will do it shortly

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You should remember to use the 50 stator plate also, you can't mix-n-match flywheels and stator plates between Honda and Chinese models.

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