Oil Pan and gaskets 1st Gen SB

[PbFut]

Sorry this got a little long.
I have had sealing issues on the rear main and pan since I bought the truck a year ago. The leak was pretty bad so right off I pulled the pan to swap the rear main seal. Believe it or not the upper section of the 2 piece seal was missing. That did not leave a feeling of confidence in my new purchase. To be on the safe side, I used a FelPro offset RMS in case the crank was scored. I had a heck of a time getting the correct oil pan gasket. The gaskets I received seemed too short in the rear arch of the pan that was on the motor where it mates to the rear cap. I found a FelPro 1885 and it fits the pan perfect so I thought all was good. The area stayed dry for about 4 months. Then I pulled the motor to do a deep clean and new suspension. I decided to do a top end refresh. After a cam challenge all turned out well. A few weeks later I start getting a drip again in the RMS area. Pulled the pan and to me the area around the main seal looked dryer than the pan area. Swapped in a new seal and then started thinking about the trouble I had with gasket last time. So I ordered a simple Dorman pan part number 264-104 off Amazon and a new 1885 gasket the other day. The 1885 gasket fits my old pan but not the new Dorman pan. It’s obvious the new pan requires one of those other gaskets that did not fit the old pan last time. Given the missing upper section of the rear main, I am thinking this old pan and 1885 gasket is designed for a different configuration. Is there a rear cap with a slightly larger radius that mates to the oil pan. I dry fit the new Dorman pan without a gasket and it seems to fit correctly with a very slight rock. Not nearly as much as the space the 1885 gasket has to the new pan. I am now thinking this recent leak is due to an incorrect pan and gasket but I cannot find anything relative to a pan having a larger radius in the rear. I looked up the 1885 part and there is no reference I found relative to the rear cap/seal area. Obviously this old pan has a larger radius as does the 1885 gasket because the gasket for sure does not fit the new Dorman pan and its clear those gaskets I had from months before would fit. But looking up a FelPro 1885 it says 1st gen 2 piece seal.
Sorry for such a long winded question post/post. I am sure this is a simple answer for someone here. Given the engine was missing the upper half of the rear main seal when I got it, I now think the pan is wrong as well and the new Dorman part correct. If I get the correct matching gasket, I think this leak journey will finally be over. But maybe for future reference to others, someone can step up and answer what this 1885 gasket and old pan I have is for.

[Richard2112]

"When it comes to pan gaskets, the two-piece small-block rear main seal engines are not all the same. Up to around 1974, Chevrolet used a thin front seal between the timing cover and the oil pan. From that date and later, Chevy added a much thicker front seal, which predicated a change to the oil pan. This causes much confusion when enthusiasts are swapping parts. Most aftermarket oil pans are designed to use the thicker front pan seal."

https://www.enginelabs.com/engine-te...evy-oil-leaks/

[PbFut]

Richard2112 I am referring to the rear arch of the pan , not the front.
More info, appears that I am just being crazy paranoid.
I went down and purchased a Felpro os34509T gasket most commonly brought back by parts search tools for a 1st gen SBC.
It is in fact the same gasket in the FelPro #1885 box. The difference is that the os35409T has a set of bolts and washers along with 4 plastic install helper studs included with the gasket. As for the fitment. My old pan is just radiused a bit different resulting in a difference to how much crush will happen when tightening down. Newer pan will crush tighter, and likely eliminate the oil leak I thought was the rear main seal. Old pan, do to age, or manufacturing tolerance makes the gasket appear to fit better but likely not as tight towards the bottom of the rear arch. New pan will definitely compress tighter in the area which happens to be the location I was leaking. I will install today and see what happens.
One last observation. Each gasket I've bought has a small amount of flashing at different spots that should be removed. One had a piece on the sealing surface that would have surely created a leak. So be sure to inspect the gasket before use.

[Lee H]

Which pan are you using, the original or the dorman?

Up until 1974 the oil pans used a gasket set with a thin front rubber gasket (rubber seal between pan and timing cover), 1975 and newer and most after market pans use the thick version. The only way to know for sure is to measure the depth by placing a straight edge across the front of the pan. 1 3/16 deep uses the 74 and back thin gasket set, 2 ¼ deep uses the 75 and forward thicker gasket set. Felpro one piece part numbers are: OS34509T for '74 & back, OS34510T for '75 through '85 and most after market pans.

[geezer#99]

Maybe you’re going about it wrong.
All the caps at the rear are the same as far as I know.
I think you should fit the gasket to the block and not the pan.
If you want to check any pan for straightness or pinch points just install it without gaskets. Put a flashlight inside the pan first. The bit of light from inside will really help for checking.

[PbFut]

I think my original post/question should have been, Is there a difference in caps. I just did not express in a succinct manner. The old pan had more room in the arch area which lessened the crush of the gasket. I believe this allowed the drips. Not the replaced RMS. The new Dorman pan installed fit well, given how the gasket sat seemed less "fitted" compared to the old pan. It was obvious compared to how the gasket sat on the two pans the Dorman pan would compress the rear arch area of the gasket much more.
Installing this afternoon, the gasket fit onto the block, rear cap and front cover like a glove. The Dorman pan mated very nicely and clearly quenched the gasket on the rear arch with a nice even fit, from what I could see past the flex plate . I did not have time to finish bolting the motor mounts back down so have not started the motor. At this point, with 5 pts of oil in, no leaks. I am now thinking that the real problem (after the RMS correction) was with the original pan that was on the motor. It just was not compressing the gasket enough on the rear main cap.
I will complete the project tomorrow and report back final conclusion after running the engine for a while. One final note, to be clear, I do use a dab of silicone RTV at the corners of the cap and front cover so it is not that issue.