The .015 is about right. Less gets cavitation, more loses volume. My openings in the impeller are larger than the factory or other aftermarket vanes to increase the volume.

Frank/Rebop

Bristol, In ( by Elkhart) 

I havent noticed the difference in blade heights, but havent looked either.  My understanding is that the closer the tolerance between the end of the pump vane, and the cover casting immediately past the volute, the more water will shear into the block.  My Rebop pump and aluminum cover has only .015 or so clearance.  Measured it once but forgot the actual numbers.

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

I have have noticed a difference in height of the impeller blades on various y-block water pumps.  The difference is as much as 1/8 inch.  Could that make a difference in the performance of the water pump and impact the cooling ability of the water pump??  Has anyone experience in this or an opinion?

Thanks,  

Gary