By
PWL216
I was the winning bidder on a pump powered by a Kohler K91R engine. Based on its specs it’s a 1961. I don’t really have a need for it at the moment, but couldn’t pass it up. Here’s what I found about the K91…
Model: Kohler K91
Spec Number: 31504A
Serial Number: 3019680
Horsepower: ~4 HP
Shaft Orientation: Horizontal (most likely, based on spec)
Year of Manufacture: 1961
About Spec Number 31504A
Kohler’s spec number identifies the exact configuration of the engine — including shaft size, PTO options, mounting patterns, and whether it’s for a tractor, generator, or industrial use. While full public spec decoding guides are rare, the “A” suffix usually designates a factory revision or minor configuration changes.
Spec 31504A is known to have been used on early Wheel Horse tractors, most likely models like:
RJ-58
RJ-59
Suburban 400 or 550