A couple of things that you might not have completely right.

I don't remember Doc's C Service. But his Racing C, was the same as my dad's Evinrude. It was a 6042 Model, Hex Head motor. We called it a "Six Stud".

In those days Racing motors like the 6042 Evinrude and the PR 65 Johnson were built for alcohol burning race motors. They both had detachable heads, and roller and I think needle bears on the crankshaft.

Service C's were either Johnson P50's with detachable heads, or the EVINRUDE SPEEDITWIN cast in heads. My dad had a P50 that was fast but when he blew it up he quit racing C Service. C service motors had brass bearing on the crank.

C Racing and C Service were both 30 cubic inches.

Nitro was legal in either class but we never ran nitro as it was corrosive as hell and my dad had seen the result ruining carbs brass jets and the like.

I won the 1967 John Ward Trophy Race (500 CC World Championships) driving Harry Bartolomei Konig, with Nitro added. Only time I ever ran nitro.

The Evinrude 6042 cylinders had a large content nickel mixed with cast iron and when Doc "Blew Over" the cylinders were much stronger than the aluminum crankcase.

My brother spilled his DeSilva Runabout in C Racing Runabout, and blew 8 ears off Hubbell cylinders.

Racing and Service were two different classes but fuel isn't what made them different.

Thanks for posting, I feel close to Doc when you do....Doc started Evinrude/Johnson's racing Division with Charlie Strang.

Add: WE USED A ACID FILLED BATTERY, "HOT SHOTS" WERE TOO EXPENSIVE.