Shown below is a genuine Ford Model A water pump repair kit. Before I found this kit, I went through an expensive process of buying and trying reproduction water pump shafts only to find that they were basically junk for a variety of reasons.
I have bought shafts from many of the major reproduction parts suppliers and typically found that the outside diameters and hub tapers were the wrong size and shape. I have documented some of those reproduction parts problems on another web page.
This page details the original specifications and requirements for the water pump shaft.
The original Ford detail drawing below clearly shows the diameter, taper, and hardness requirements for the water pump shaft.
I wonder why it is so hard for suppliers to make such a simple part to print?
The important point to note is that the outer diameter was specified to be finish ground to a final dimension of 0.624 to 0.625 inches.
In order to achieve this proper finish dimension, a larger 0.630 to 0.634 inch diameter rod stock was used as the starting point to reach the correct final size after grinding.
Unfortunately, all the reproduction parts I've checked were just produced from nominal 5/8 inch rod stock. This nominal stock diameter is actually already less than the required 'finished' size, and typically measures only 0.621 to 0.623 inches in diameter.
This method of reproduction shaft manufacture using undersize available stock materials may be expedient and lower cost, however it is dimensionally wrong and produces junk parts.
The excessive clearance allows the undersize shaft to vibrate at a lower frequency in the front roller bearing and allows more clearance in the rear bushing. This clearance also increases the shaft and bushing eccentricity and increases the difficulty in sealing the rotating shaft, especially with modern lip seals.
Another point to note is the shaft hardness requirement of Rockwell C 45-55 on the steel shaft. In actual fact, most reproduction shafts today use 300 series stainless steel which is much softer than the hardened carbon steel.
Some reproduction shafts are made from precipitation hardened 17-4 austenitic stainless alloy. That is a much better choice than the typical 300 series since it can withstand the galling and brinelling effects from the roller bearing much better, and is a closer match to the original Ford specification for strength and hardness.