Let me see if I can help some of you guys that have had issues with the Fel-Pro one piece pan gaskets, either part #1884R or the "stock" replacement part #OS30061-T. Both these gaskets are listed for the identical applications. BBC "1-piece" for 1966 through 1990. We've spoken to F-M a number of times and tried to point out to them a "major" inconsistency in the dimensions of the "stock" 4 piece “factory-style” pan sets and their “aftermarket” 1 piece ones.

Let me start with the dimensions for the stock gaskets, these are the "old-style" pan gaskets.
The side rails on these gaskets measure .090" thick, the rear "rubber" measures .260", and the front "rubber" (this is where most of the leaks are occurring by the way on the 1 pc. deal) measures .150" thick. (Remember this .150" number, this is the important one)

Now let me give you dimensions on the 1 piece gasket part # OS30061-T.
The side rails on this gasket measures .180" thick, the rear incorporated
"rubber" measures .370", and the front incorporated one (again this is where most of the leaks are occurring) measures .300" thick.

If you take a pad and a pencil and do some simple math, you'll find something just doesn't add up here!

The difference in the sides is ..090" exactly, the difference in the rear "rubbers" is .110" exactly (this is close to the sides as you can see), yet the difference in the front "rubbers" is .150". It appears they "dropped" the entire 1 piece gasket approximately .100" (due to the design itself) with the exception of the front area. They dropped it here .150", some .050" add'l? We tried to find out why, to no avail. That creates the issue! The overall pan depth (from different vendors) also comes into play here, a “shorter” radius will tend to make the fit tighter, a larger radius will be less prone to leak (not as much “squeeze)! And I'm only talking "thousandth's" here, not "inches"!

What this does exactly is try to "squeeze" the front out due to the additional clamping force, and the fact that oil does "penetrate" between the gasket, the pan, and the timing cover (with this type of gasket this could be considered "normal-seepage" over a period of time), it causes the front section to eventually "work" itself out, especially with some small amount of “crankcase pressure” pushing on it from the inside.

Next time you do an assembly lay the pan in place and notice how far the pan sits from the timing cover, it's quite a "spread". Almost can't start the front pan/timing cover bolts! The add'l thickness at the "rear" is less prone to leak due to the fact it sits in the main cap "rails". These rails aren't available in the timing cover area, the gasket here is merely "crushed", as it just sits flat.

We've spoken to Fel-Pro numerous times to no avail, they simply have their own preconceived notions that because their engineers designed this gasket, it MUST be correct. PLEASE DO NOT TAKE THIS AS "HAMMERING" or
"BASHING" FEL-PRO! It isn’t about this, it's merely a tip!

We have 2 units we did recently with the 1 piece gasket installed using a very some "sealer", along with the silicone gasket, on the front section, and so far it looks promising. It goes back to the use of a very fine film of "The Right Stuff" in the front area before placing the pan in position on final assembly. We feel the "adhesive" in the sealer will help "stabilize" the front section thereby keeping the oil out from between the pan and the cover.

IMPORTANT:CRANKCASE PRESSURE comes into play here slightly also, but we feel the gasket should still do it's intended job.
Thanks, Gary in N.Y.

(Add) On the aftermarket "race" BB blocks you have to be careful with respect to the length's of the 2 rear corner bolts. These are "blind" holes and the bolts MAY bottom out before the gasket is "crushed"! I saw this posted here awhile ago and figure it would be the proper time for a reminder. I had one of our own units leaking at the rear on the dyno! Try the bolts without the pan/gasket in place. They should sit on the surface flush! And be extra careful with the Bow-Ties, these 2 bolts go into "blind" holes in aluminum!

P.S. Just wanted to add here, ALL the dimensions above are in their "relaxed" state. We do realize that the "crushed" numbers will change, but we do feel they should all be relatively even going in! I'll give you one more "tip" here, if you can use the "Corteco" line of gaskets for your particular application, they are somewhat limited with coverage, by all means give them a try. You won't be disappointed here. We use the 1 piece SB pan gaskets from them most times and have never had a single issue! And they do have the BB ones on the “drawing board”!

Double P.S. This issue has been on my mind since "poster" Rowdy if I remember correctly was having this specific problem. I also believe he had talked about "crankcase-pressure" at the time. That "link" has been on my mind all this time. By using this forum as a "guage" we can "foresee" many upcoming potential problems. It's most definitely a "plus" for us building these units!

Yeah Gary, the significant difference in dimensions also prohibits cutting the damaged front section off and installing the front rubber from a conventional 4 piece gasket set. I know I tried.

Two things preceeded both of my particular failures, can't say for sure either are directly to blame, but probably contributed in a cause and affect sorta way. First, the actual material seemed to have lost it's integrity. Meaning, the damaged gasket did not have the same rigidity or texture as when new (I replaced mine with the same Fel-Pro 1 piece.....twice). I honestly question the compatibility of the material used and synthetic oils. For the record, I have my 3rd 1 piece Fel-Pro (I know....what a dumba$$), and have decided to use good old Dino Lard.

Secondly, my failures occurred shortly after retorquing my pan bolts. Not cranking 'em down, just snugging them. I have noticed that everytime I check my pan bolts, even while on the engine stand, from evening to morning, the bolts (actually nuts in my case, as I have studs) are concernably loose.

It is my belief that the comprimised "rubber" allowed the tits that locate the gasket to the cover to be easily ripped off, then the front section seperates between the ribs that provide the integrity of the front seal. In each instance, detached strips of the forward rib was visible sticking out between the pan and the timing cover, while the remaining 2 ribs, were actually squeezed to the inside of the pan, beyond the sealing surfaces of the pan and block. I attempted to reposition the front of the gasket, however, the rubber was now like a 1/2" too long and could not physically be confined to the space allowed (swollen, i.e. rubber soaked in brake fluid).

Excessive crankcase pressure has also been a problem, never more evident than when it is combined with a breach in the pan gasket. But......I'm not totally convinced that it was THEE primary cause of the gasket failure.

I installed a S-10 pan using only "Right Stuff" (no gasket) and never had any leaks or even seepage. Of course dis-assembly would be a pain. I may try it on my Big Block.