It depends on the size of the crankshaft pulley. Normally, the low mounted water pump engines have the big crank pulley and the 7" water pump pulley that bolts onto the pump. Early 235's ands 216's run the smaller crank pulley and the high mount water pump with the small diameter pulley pressed onto the pump shaft. High mount pumps can have either a 3/8" or 5/8" belt, depending on the year of manufacture. Jerry
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OK. The engine is ‘55 block which has a low mounted water pump. Therefore, I should measure the damper/crank pulley to see if it’s 6 5/8” and if so, go with a 7” water pump pulley. And also measure the width to figure out if the damper pulley (or engine) is from a car vs. truck so that I get a water pump pulley of the correct width.
The late pump's capacity (gallons per minutes) is 72% higher than the earlier pump (at the same engine speed), so it's efficient even when the pulley sizes turn it more slowly. Using the exact sizes isn't needed to insure adequate water circulation (close is close enough), but it lets you use original belt size.
The water pump manufacturer should be able to tell you the recommended RPM and you can do the math from there. If you spin it too fast it will cavitate. High flow water pumps generally want to spin slower. On my 292 I used a FlowKooler high flow water pump that can spin 5000 RPM without cavitating. I altered it for the A/N fittings and smoothed the casting. I have a custom made damper with an 8 rib pulley on it. My water pump pulley started it's life as a reverse rotation Ford pulley and I cut the grooves in it and redeemed it. I bought a spacer and cut it down as required. I countersunk the bolts because it's close to the blower belt. For some reason I felt the need to fill up all the holes. Then off to powder coating ($20). Keep in mind to allow .004" to .005" for the powder coating. I realize most people aren't as crazy as me but the principles are the same. Thank you for your time.
As I just said, the late (larger) pump turns 95% of engine speed with the original pulleys. The early (smaller) pump turns 140% of engine speed with the original pulleys. This information is all in the factory service literature.
