Hi shedracer,
It is vital to use the 8mm plug when fitting an oil cooler as it closes a gallery and diverts oil into the cooler. If it is not fitted the oil will just return into the crankcase. I made mine by cutting one from the shank of an 8mm drill bit.
I was a little concerned that the oil pressure would not be sufficient to feed oil back to the camshaft with the line being piggybacked to the crankcase return. All was OK however and this should be checked by removing the camshaft cover and turning the motor over by hand. If everything is done correctly and the pump is in good condition, just slowly turning the motor should push oil through the camshaft.
