What year did Ford start using the low head and how hard are they to come by.
Michael, the low head was used from the beginning of T production to the later part of 1917. Compression on the high head was slightly lower and the head had a little larger water capacity.
They are somewhat rare, but I see some at the Chickasha Ok swap meet most years.
About the first three million Model T's came with a low head. My 1917 torpedo (block casting date towards the end of June 1917) was one of the last cars made in the 1917 model year. It came with a low head originally.
Low heads come up occasionally on Ebay.