300 to a 200

[Zman]

I do understand that the head protrudes into the cylinder. In order to get a reasonable squish I would have had to cut the head mounting surface or cylinder base about 1.5 to 2mm. I did not look at port timing when I did it, but I can say there were no or interference or clearance issues with the assembly. I did this over last winter while we were rebuilding a 200 engine and unfortunately I lost my notes on the exact measurement.

[Jakobi]

Cheers Zman. Going with the 250 jug on a 200 bottom end 'over square', as you said you'd need to machine from the cylinder. It would be important where you take it from in terms of port timing though, and also I'm not sure but I think you'd likely be up for the price of a replate on top after doing such work.

Going the other way is probably easier as in you can have a spacer machined up to lift the cylinder, as long as the ports are where they need to be.

[Zman]

Hello Jakobi, I suppose the head could be machined on the mounting surface like you would to fix the compression and squish.

[pscook]

I would guess the exhaust port would be well below the top of the piston at BDC, so you would want to raise the roof of the exhaust port a bit. Unless you want killer grunt, but I don't know if that can be achieved with such an over-square engine. Maybe the exhaust valve could be timed to help with top end, I don't know. I'm not smart enough for that math.

I would be looking at the smallest piston possible on the longest stroke for a myriad of reasons, mostly for grunt. But you would have pretty much no top-end, I would think.

[Pinittowinit]

Ok the 300 bore and stroke is 72mmx72mm
The 200 is 62.5mmx65

If you raised the cylinder 7mm that should make the stroke the same. So 62.5x72. Then of course is the exhaust ports. But as it sits like that it should have loads of bottom to mid and fall in its face on the top correct?

[Elvis74]

To switch from a 300 to a 200 you need:

Crank shaft
Rod
Cylinder head
Piston
Cylinder

[Pinittowinit]

I'll be the test dummy if anyone has a 200 cylinder and piston.

[Pinittowinit]

Originally Posted by Elvis74

To switch from a 300 to a 200 you need:

Crank shaft
Rod
Cylinder head
Piston
Cylinder

So the 200 crank will drop in a 250-300 bottom end?

[Elvis74]

The basic engine is the same from 200 to 300 cc.
The 125cc is totally different.

[rosco]

You would only have to raise your cylinder 3.5 mm. I would guess that port timing would be your biggest issue.