Anthony

There are a number of tools you can use. The easiest one to make, is a thick brass plate with a slot cut down the middle. A thin U shape. You slide this under the little end of the conrod (with piston removed). It needs to be long enough to bridge the gap in crankcases and sit flat on the base gasket both sides. This stops the crank from spinning when you turn the bolt on the end of the crank. This works whether you undo it or tighten. The use of brass prevents damage to conrod.

The cheap way is to get a thick sturdy piece of cloth and feed it between the two gears on the crank and back of clutch. This jams the gears and stops everting spinning and you can undo or tighten the bolt.

Cheers

Dan