JC
The light weight frames were from the first Coopers, the 4 speed Enduro model, they never came here, my MX frames are basically the same as the later MX frame, the only difference between the frames are that the early frames had bolt on foot peg mounts and solid rear motor mounts, later model had welded foot peg mounts and floating rear motor mount that used the swing arm bolt and modified swing arm, later Enduro model used the same basic frame, motors were the same except for closer 5th gear on the MX and the MX barrel had different porting specs to the Enduro model, my MX bikes came from the factory fitted with the Yamaha GYT DT1 single ring piston, later bikes were fitted with the MOTO ISLO copy pistons of the DT1 but used Yamaha rings and gudgeon pins, I use Yamaha DT1 2 ring pistons in my MX and Enduro bikes but you have to take 4mm off the skirt both sides of the piston and the change the postion of the pins in the ring lands area so the ring gaps miss the ports, all Coopers are hand ported and radical in the port timing, DT1 head gaskets are used but base gaskets from the DT1 don't fit, must use MOTO ISLO base gaskets, early bike had cush drive rear wheel hubs later built model had solid rear wheel hubs (stronger).
