I live down the bottom end of NZ where it gets cold enough for me (we are on our 4th day of 6deg frost right now).
Micheal; I feel that the main reason apart from trash like circlips/plastic off timing gears/ stem seals etc of
oil pump drive failure, is people that jazz the throttle on start up & or HV type
oil pumps that have been fitted for the wrong reason. many owners/builders fit an HV pump to a motor that has seen better days to prop up sagging
oil pressure, some time later the motor gets rebuilt but the pump looking brand new is refitted , to a motor which doesnt really need it. Chances are the promotion hungry salesman also told the guy that he needed a HD drive to go with the big pump. This is about where I enter the picture after the owner has visited every wannabe mechanic to cure any of the following ill's that he has created.
1. The oil filter blows out the ring when they rev it on start up.
2. It keeps shearing the roll pin in the dist gear.
3. Its on it's 2nd dist gear since the rebuild.
4. His mate has told him the drive gear on the cam is badly worn.
5. The lifters pump up real easy on cold mornings & and the motor dont rev like it used to.
I have many HV pumps that have been removed from 'new' motors for the above reasons.
The secret if you can call it that is to build the motor with the right clearances & on the FE enlarge the oil galleries so that the oil pressure you read on the gauge is actually being supplied to the bearings etc rather than just being 'created' by the restrictions in the gallery. Then use a pump [and cooler if necessary] to ensure that the oil pressure can be maintained at all times. Done in this manner you will usually find that you will have a reliable combo without having to resort to HV pumps etc.Defintely use a full screen pickup, that bypass hole on the original was only for old hi mileage purposes on stock engines where people used to change the car rather than the oil!!
Scufty, I think the circlip probably jumps out or fails after a long period between starts & the lifter has collapsed allowing the pushrod to knock it out or during an overrev with weak valve springs.
Hope I have not got to far off topic.
Cheers Jac Mac