Wanting to use smaller dia. pulley to speed up water pump on a 312. Have a 6" pulley off of a 302 sbf but it requires 3/8" belt. What have others used that will allow the 1/2" belt to be used? Thanks for any and all help.

Do not run the water pump too fast it will not give the water time to absorb the heat. How fast are you turning the engine at 60 mph. 

Tach reads aprox. 2400-2500 rpm at 65 mph. Problem is when it's warmed up and slow cruising around town, as in idling, it wants climb to 220 or so on the temp gauge. Has a six blade fan on it and may need to put a shroud on it also. Wanting to eventually put AC on it, due to the wifes health, but can't until the cooling is sorted out. 

You want to keep the pump rpm under 7000.  For the vast majority of street cars that rpm will never be achieved, even with the smallest diameter pulley that will fit without significant modifications.  In that regard, you cannot turn the pump too fast.

Re the pulley, spend a day going to some junkyards and contact the actual manufacturers, March, Billet specialties, CVF racing, etc., tell them what you need and see if they have something with a 1/2" groove that can work.  The bolt pattern and 5/8" shaft diameter is very common, so there is a good chance that something is out there.  Verify that your shaft is 5/8"

Other options are to get a 3/8" groove crank damper and generator or alternator pulley.  Another option is to have a pulley made.  Having a pulley made should be cheaper than the replacement of the crank and gen/alt pulleys.
Another thing to consider is if the damper is original, then it is time for a new or rebuilt unit and if you have to replace it anyway, go with 3/8" and get a matching gen/alt pulley.  That pretty well takes care of 2 birds with 1 stone and with the most efficient use of funds.


Lawrenceville, GA

Thanks for the suggestions........good food for thought. Time to push on!

An older Chrysler slant 6 pulley fits, I put one on my 57 Bird.