Firstly, as has been seen already, the piston has a chunk out of the skirt which means its not perfect. The bore isn’t too bad, and it’s not been rebored. The head does need cleaning and the barrel need honing, and have already acquired a new piston and ring [[From North West Vintage Cycle Parts, for $99.00]].
Unlike the MX250, the SC500 has a larger external rotor (and uses a standard puller). I’d had this apart already on the hunt for a spark and it was clear then that the seals were knackered and this is what prompted a complete engine strip and the need to break the cases.
The clutch doesn’t too bad and should be okay for future use. The SC500 has a balancer shaft and its removal is straightforward, though installation needs to be done carefully in order to align the balancer correctly. The 1968-77 250-500cc Clymer manual is pretty good and gives you information on the different variants and options .
The kickstart and gear selector mechanisms are the same as the MX250 and fairly straightforward to strip down (and fairly easy to put back together). You can’t detach the kickstart spring and remove the whole assembly as one, and you’ll need to remove two circlips from the shaft, and then using the kickstart lever move the shaft round anti-clockwise until it disengages from the spring.
I’ve a template made up for the crankcase bolts and this is useful if even to ensure that you’ve all the bolts removed before you try and split them. Again, split the crankcases is the same as the 250, using the two threads for the crankshaft seal retainers. Very easy split.
What was obvious straight away was that the main bearings and the primary drive bearing had a lot of corrosion, so now only were they difficult to remove from the cases (lots of heat, cold day), but will need replacing. The primary drive bearing is retained in position by a large circlip and I have a feeling finding a replacement bearing won’t be straightforward.
The seals are also slightly different from the 250; I think the rotor side is maybe the same, but not clutch side.
Big end seems okay, so no need to split and rebuild, I hope. More to come, but at now at the stage of knowing what needs to be done. Some engineering decisions to be made, whether to balance the crank (and leave the balancer out), nicosil line the barrel with the new piston etc and also what to do about the electronic ignition.