Rear main seal orientation for 331 engine

[Caddy Wizard]

Bob,

I can't understand the difficulty with your 55 FW rear main seal.  I just have never had a problem with the rubber seals (but have had trouble with the rope seals).  I have installed the rubber rear main seal in over a dozen Cadillacs between 1949 and 1956 and none of them leaked even a little.  I just did the job again on my 1949 Fleetwood over the past two weekends (it had a rope rear main seal from about 30 years ago). So far, the Best brand rubber rear main seal seems to be completely drip-free.

[Caddy Wizard]

Bob,

I have done the job about twelve times now.  My most recent experience with this (last month) on a 49 Fleetwood resulted in the same leaking new seal as you have experienced.  The new (brown) seal is leaking.  I'll have to redo the job (which irritates me to no end).

[Jay Friedman]

As Art Gardner wrote, until his most recent experience with his '49 he never experienced a leak with a rubber seal.  My experience has been the same on my own '49 and other cars. This makes me wonder if there has been some sort of manufacturing flaw in the recently installed neoprene rubber seals as recounted by various posters to this thread. The beauty of the rubber seals has been that, unlike the rope seals, no preparation of any is needed; you just stick them in. So something isn't right. 

[Caddy Wizard]

As Art Gardner wrote, until his most recent experience with his '49 he never experienced a leak with a rubber seal.  My experience has been the same on my own '49 and other cars. This makes me wonder if there has been some sort of manufacturing flaw in the recently installed neoprene rubber seals as recounted by various posters to this thread. The beauty of the rubber seals has been that, unlike the rope seals, no preparation of any is needed; you just stick them in. So something isn't right. 

The guy at Terrell Machine Co. says that they buy their rubber seals from Best Gaskets and I have always used Best Gaskets seals -- I have never had one leak until now.  He also says that the groove for the seal in the block and main cap probably wasn't milled with great precision and repeatability back in the day, since they were going to fill the groove with a rope seal and trim it to length (with a little protruding for "crush") after installation.  As originally manufactured, the groove was roughly made and the seal was installed in a manner to fill and fit the groove.  We are now installing new rubber seals made to a specific dimension, which may or may not perfectly match the outside diameter of the groove. So the rubber seal can be a hit or miss proposition in the not-so-uniformly-machined groove of the engine. So we need to watch for this in installing the seal and perhaps adjust its fit with a shim.

I never thought of the above before, but it sounds right and makes 100% sense to me. I will have to check the rubber seals from now on to ensure that they are protruding a little proud of the flat surfaces.  If I happen upon an engine where that is not so (which I am guessing is the case with my 49 FW), I will use the thin cardboard shim that sometimes is supplied with the rubber seal to make it fit better in the groove (tighter). You slip the cardboard into the groove and then the seal goes between the cardboard and the crank.  In short, you are reducing the outside diameter of the groove slightly.

Interesting...

[Roger Zimmermann]

I used a shim on my '56 de Ville because I had the impression the seal is not near enough from the crankshaft. I don't remember the thickness; it was maybe 0.2mm (0.008"). I would not use a too thick shim and absolutely no cardboard because oil will come at it; it may create a mess.

[Caddy Wizard]

I used a shim on my '56 de Ville because I had the impression the seal is not near enough from the crankshaft. I don't remember the thickness; it was maybe 0.2mm (0.008"). I would not use a too thick shim and absolutely no cardboard because oil will come at it; it may create a mess.

The seal sometimes comes with a shim made from gasket material (a type of cardboard).  In any event, this is all news to me -- I have never had a problem with the rubber seals until now.  Apparently, most of the time, the rubber seal will seal fine, but apparently once in a while the groove is too big and you will need a very thin shim.