JonG, you are absolutly correct. The neoprene/Viton seal was introduced for the 56 model year and its introduction is announced in the 56 pass car shop manual. So you would see that seal in 235s with casting dates consistant with the 56 production run. I have a september 55 motor that came with the neoprene seal and the 848 head with makes sense as the assembly plants were shut down September 56 for retooling and the casting dates on motors were generally a week or so ahead of an assembly date. That being said, I am still not 100 percent sure whether the truck line, or some of it, got the seal for 56. The problem being is that I have never had an unmolested never taken apart truck 235 or 261 in my hands. Being the work horses that they were and are, those motors often had an in chassis ring job, valve job, rear main seal replacement. The trucks needed to work so no time to waste.
The 55 motors on the other hand were still using shims, the rope seal and if stick shift, most still used the milk bottle lifters and the shorter push rods. The issue with the 55 rear main cap is that it was never designed for, and thus, never machined for, the hard seal. It was instead machined deeper which is why installing the neoprene seal in a 55 and earlier motor generally does not work and/or requires shimming, and even then, it still may not work.
Last edited by Dragsix; 10/28/2021 1:33 PM.
