Forged Rear Main Question

[RonLey]

All,

This is a curiosity question concerning the use of the forged rear main. I have a approximate August 28 Roadster. When was the forged rear main phased out and I would guess the cast rear bearing was to save cost and weight.

Is there any advantage to the forged rear main?

Thanks
Ron

[Tom Wesenberg]

I think the cast was started during mid 1929, but would have to look in the SERVICE BULLETINS to be sure of the date. The bolt length needs to match the cast or forged cap.

Forged is stronger, so the cast was made thicker to compensate, so I don't know if one has an advantage.

[Fullraceflathead]

Theoretically the Forging should be stronger but considering the Horsepower it see's I cant see how you could ever stress it to it's breaking point short of an actual Hydraulicing of the cylinders.

[d.j. moordigian]

Just off hand,...I think the pipe was changed at the same time as the cap change..
But I can't swear to it...

[MikeK]

If you do Babbitt a steel cap can be tinned prior to pouring. The bonded Babbitt will not crack off in chunks, a big plus on the thrust faces. Of course the Babbitt person would have to know this and be instructed to carefully tin all steel cap surfaces first.

Cast iron caps cannot be tinned and must rely on mechanical lock. When the Babbitt starts to develop grain boundary fractures it goes fast and falls from the thrusts in chunks.

If you are going to line-bore for insert bearing shells the cast iron caps will cut the same as the block and not induce the tool deflection you experience boring a hole that is half cast, half forged.

[jetmek]

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forged caps will deform at the bolt ears and not sit flat without resurfacing.. the earliest ones didn't have a web between the bolt ears and rear slinger area. cast makes a more ridged cap.