I'm putting together a spare engine ready for a project bike. Sitting in my pile of spare parts I have a pair of 78 100RS pistons and a pair of UK spec 81 100RS. Both 9.5:1. I also have a spare pair of heads for each year to go with them. Apart from the 40mm exhaust ports the 78 heads are identical to the 81 heads. The UK spec 81 engines were rated at 70 BHP while the 78s were rated at 67 BHP. I've got enough engine, clutch and gearbox parts to have the engine as either a heavy or light flywheel. I'm leaning towards a heavy flywheel. The bottom end is an 81RS so I'll be keeping the bean can. I'll need to use the 38mm heads because the bikes GS based so I'll need the gs exhaust. The question is which pistons to use? Normally I'd just whack the 81 pistons in but I've seen plenty of mention of squish bands being better on the earlier pistons. What do you reckon guys. A worthwhile mod or just stick with the 81 pistons?