All:
Here are some pictures I took last night of the oil pumps. Got a used one from my landlord at the shop. His area is behind mine and we both are total Spridget addicts and racers.
This picture shows oil pumps in row. Starting at the left; New "concentric", then old/used Hoburn Eaton, then new "Powertune", and last one is a three bolt fixing 948/1098/Cooper 'S' 1275 type. Note the different type of bolting to the block. 4-bolt and 3-bolt fixings usually use metal lock tabs. 2-bolt is usually a lock washer.
This picture is of the old used HE pump showing the worn out rotor. See the scores from debris caught between the rotor (cam driven part) and the scroll. Also look at the flat worn on the rotor face. The trailing face as the same wear. This pump goes in the recycle barrels. Aluminum in one and steel in the other. Beer money when full.
Here is the money shot. The one showing where I measure for scroll to housing clearance. 0.006" in my upper limit.
This is the last one showing the Powertune oil pump with measurement. This oil pump appears to be the County brand offering and looks to originate in the UK at a oil pump manufacturer who makes pumps for a number of applications and suppliers. I've got the same pump in a different box without the "Powertune" in the body.
Hope this benefits all viewers. Oil pumps are the heart of the engine. Finding a good used pump in a 36+ year old engine happens, but not often. Usually, debris and use has taken its toll. A new one is cheap insurance for an engine and the typical enthusiast rebuilder who isn't into engines on a weekly basis. I usually have 3 or more engines going at a time.
Take care and enjoy,
Mike Miller