RE: Converting RC engines to Control Line
from what I was told, 52 was non sandcast, 6 bolts, 3 backplate, no reinforcements no crank webbing. 53 had webbings 55-56 had webbings, re-inforcements and somewhere next they added the bar in the exhaust. And that's about all I know other than all mine snarl better than yours on 10% less nitro.