21A rods set up on a 4" Merc crank

[Ronnieroadster]

Hank from what I have been told by the fellows who once ran the 21A rods in the old days for their 4 1/8 stroked cranks the rods worked perfectly. My personal experience using 21A rods was also with the Merc crank stroked to 4-1/8 I was burning alcohol in my restored altered. The rods worked fine my sets were also NOS.
Bored and Stroked {Dale} has lots of experience in what I call the modern era using the 21A rods in a Hot Rod flathead Ford engine application. As he wrote above that's the way to go.
Ronnieroadster