Back in the day, lower units had measurements. My dad made many as did Marshall Eldridge and others. There was a "SPEC SHEET" and if your lower unit measure it was legal. My dad made many lower unit for Pep Hubbell back in the day!

Why does APBA make you buy a lower unit from a company that make about 5 a YEAR???
OPEN THE RULES UP!

Does APBA want new people to race with or new people to beat? Where is APBA's Ross Chastain?