Increasing stroke by swapping crankshaft

Is it possible to swap out a crankshaft from a xj900 to a xj650? ....I would like to keep the bore the same...just add stroke...?
Thank you in advance for any help

 

The 650 wrist pin and pistons will fit the 900 crank and connecting rods.
The cylinders will interchange as long as the 650 is YICS.

The 900 stroke is 8.1 mm longer, so you might need to fabricate a shim about 4 mm in the shape of the base gasket.
You will also need a larger combustion chamber so your total shim thickness may need to be a little thicker than 4 mm.

The motor would need to be assembled and the combustion chamber measured with fluid to know for sure the thickness of the shim required.
Buy a bunch of base gaskets and add or subtract them until you have the dimensions right, then fabricate the shim.
This is all a guess, the 900 pistons are actually shorter from wrist pin to crown and the 900 connecting rods are shorter than the 650 so the shim might not need to be so thick.
If you're lucky it might all just fit together and give you a CR around 9.2:1
An extra base gasket might be all you need.
As I said, you won't know until you assemble it and measure the chamber volume.

The cam chain and tensioner I think would accommodate the extra height

This would make a 754, probably a real torquer.
However, it would be so much easier just to find a good XJ750 motor.

Setting a different crank into your 650 cases requires the Plasti-gauge process and could require new bearing shells - not cheap.

 

Whilst it’s possible, that doesn’t mean it’s easy without some engineering knowledge and expertise (and cost).
On the cam chain thing @Simmy , are they all the same length across the range?

 

I compared the cam chains from the 650Turbo to the 900. They are the same as far as I can tell.
The cylinder block between these 2 is almost the same height, I think it was within a mm difference.
The 900 has shorter pistons and shorter connecting rods so overall dimension of the motor is pretty close to the 650.
I strongly suspect it will all bolt together, then the CR would need to be determined.

 

Does the 650 block HAVE to be yics?

 

YICS block needs a YICS head.
non-YICS block needs a non-YICS head.

 

Have you thought about using an XJ750 crank with oversized pistons? This will make your 650 an 820cc....

 

There aren’t any good oversized pistons on the market, not even a decent big-bore kit anymore. He’d have to look at pistons from other bikes/ brands.