Have a read of this to see how to add pictures

Just to post this to be part of the record for searching help for others. I had a problem with a Clausing Colchester with the lead screw shear pin shearing while threading. Took a year of off and on again detective work to find the cause. Always threaded with the lathes motor (naturally), seemed to be irregular in happening. One time I tried to thread by pulling on the chuck around and found a tight spot. Marked the ways and the spot repeated when the lead screw got tight. Could not see under the rack so disassembly was called for. Found Someone had the apron off and had jacked it into place and mashed one rack tooth over partially. You could not feel it when turning the handwheel to advance it towards the head stock. When the lead screw was driving the apron the back lash was not be taken up and the gear would bind then shear the pin. Hard to find because I never turned it over by hand and it seemed random because of the different lengths of material threaded. Posted this in case it helps some one out.