Here is the factual history: older engines were designed with wide-machined tolerance dimensions, some were a little closer, some wider. THAT is why rope seals were used by the OE designs.. to accommodate the wide machined tolerances of the seal grooves. As tolerances would be tightened for production, costs would go up.... OEs were always, then looking to keep costs down. The early rope seals were made of soft-conforming asbestos (rope) with impregnated graphite. Also, there were multiple cross sectional sizes of the rope as used by the OEs, that would be pre-determined what was best for the wide tolerances of the specific engine's seal grooves.
In the mid-late 80s, asbestos was materials were replaced by non-asbestos materials... and many of those included Kevlar raw material (with impregnated graphite)... BUT... the non-asbestos seals were NOT as conformable as the original asbestos... many different seal materials were evaluated, the best choices prevailed with the major gasket companies within their offered gasket and seal kits. However, the new non-asbestos rope seals were difficult to be cut flush at the parting halves of the block/cap. In fact, certain serated cutting tools were found to be best, NOT the old traditional pocket knife... remember, now the materials had Kevlar within them -- tougher than steel!
Also, the packing of the rope would benefit by using the factory recommended radiused tool... difficult to find in the aftermarket... as well as OE tools.
The BB side seals also had the issue with variable machined dimensions. A pre-sized rubber seal (regardless of what the material is), will not always fit properly within those side seal channels. The BEST choice, as per OE design, is the use a soft swelling fiber strip (as per OE) that is to be soaked in mineral spirits ... so to allow it to swell upon installing into the side channels. The better gasket companies will include those fiber seals with instructions to soak in mineral spirits. IF you attempt to use RTV injected into the side channels, you must also use a thin water-soaked insert that allows the RTV to cure with the water of the insert. IF you only insert RTV, then it will likely not fill all the cavity due to air purging against the injected RTV, and it will not cure.... it needs air (at least via water) to cure.
Molded rubber crank seals for the BB will work most of the time, but it may not IF the groove is factory machined beyond a reasonable dimensional tolerance. Also, be certain to install any rubber shaft seal with the helix grooves facing away from the engine.. so that it allows the helix grooves to pump oil beyond the lip back INTO the engine... that's how its designed.
I'm a former Fel-Pro gasket engineer and I know the history of these older engines, and wrote the instructions within the gasket sets.
