Rear Main Seal.

I am getting ready to install a rear main seal in my F-head. I have installed rope seals and neoprene rear main seals in multiple engines. My question is which seal is better for this F head the neoprene or the old rope style. And since it's a m38 A1 is there any difference in the rear main seals.

 

I went with the "Best Seal" lip type seal in my L134 on my CJ2, and it sealed it up like Fort Knox, this seal is built like a modern 2 piece rear main seal. I bought this engine as a rebuild from a buddy of mine and whoever rebuilt it years ago used one of the "improved" neoprene seals, and it started leaking almost immediately after startup and just got worse over about 1 hour of running to the point that if you idled in one spot for a few minutes you would have a pizza sized puddle on the floor.

When I removed, it was obviously mangled and had been spinning in it's groove. I tried to put a new one in that came in a seal kit I picked up when I first started tear down on the CJ2 and it was so tight I mangled it trying to spin it into place.

I bought the Best Seal rear main seal from Jeep1942.com via EBAY, and it spun right into place with very little fuss (no removing the crank), sealed everything up properly new oil pan gasket and all that and fired it up after cure time and all was well in the world. It's been about 3 hours of run time now and no leaks.

Here's the Best Seal rear main I got
https://www.ebay.com/itm/Willys-Rea...B-CJ2A-CJ3A-M38-A1-F134-or-L134-/112320472680

And here's the "improved" neoprene seal that got ate up after first start and was impossible for me to shoehorn in.
https://www.kaiserwillys.com/neoprene-rear-main-oil-seal-fits-41-71-jeep-willys-with-4-134-engine