Rear Main Seals

Jake Mayock

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Rear Main Seals

I would like to know if there is a neoprene type rear crankshaft seal for my 1952 Chevy 216 pickup.


For the purposes of this paper, “rope” and “wick” are used interchangeably, and “neoprene”, “rubber” and “elastomer” are also interchangeable.

Previous to 1940 there was only an oil slinger to keep engine oil from escaping at the back of the engine. A “rope” rear main bearing seal became standard on both cars and trucks from 1940 onward throughout the 216 era. This type seal is included in all major manufacturers’ engine overhaul gasket kits currently available. The “rope” is actually graphite impregnated asbestos, although these are now furnished “asbestos free” according to Fel Pro technical department.. The two pieces of “rope” for the two halves of the bearing are furnished extra long, and must be cut to fit the crankshaft. This basic design was shared by many families of engines besides Chevrolet, and was popular into the 1970s.

Here’s the rationale for the inability to retrofit a rubber type seal: The machining at the back of the engine is not precise enough to permit use of an elastomer type seal. Machining accuracy improved with the later 235 engine, and an optional rubber seal is available for that. Fel Pro, a manufacturer of quality gaskets and seals, lists part number BS5048 fitting 1940-53 216, 1941-49 235 (this was the heavy duty truck engine), and 1950-63 235 with rope. Part number BS13363 is for 1955-63 235 rubber. So only 1955 (second series for trucks) and later 235s will accept the rubber seal successfully.

CHEVROLET SERVICE NEWS Nov. 1939 introduced the new seal:

“The crankshaft has been redesigned at the flywheel end to make provision for the new sealing at the rear main bearing. The oil slinger has been removed. The sleeve nuts which were used for mounting the flywheel have been eliminated as the flywheel is now mounted to the crankshaft on three dowels and retained by four bolts screwed into the crankshaft flange. The crankcase and rear bearing caps are machined to receive the new wick type seal. The ball check valve has been eliminated by using the new seal.”

Adler to VCCA, 1-29-98: “The rear main bearing cap is now mounted to the cylinder block with four bolts instead of two. Two locating dowels are also provided to assure proper alignment of the cap and shims when mounting them.”

“To install a new lower half wick seal at the rear main bearing, insert the packing in the groove and then roll it firmly into the groove. Packing protruding from the groove after the rolling operation should be cleanly cut flush with the surface of the cap and cylinder block. To replace the top half wick seal, the crankshaft must be removed.”

There are some tricks for obtaining an almost drip free engine. The shop manual advises to roll the rope in from the ends to the middle of the bearing cap using a round dowel, or similar tool. I suggest you roll from the center outward while pushing inward on the free end of the seal. My reasoning is that the method described in the manual will tend to thin out the center of the seal creating potential for leaks. We want to stuff as much seal material into the groove as possible to prevent voids and leaks. Seating the ends before seating the middle will thin out the middle as it is slightly stretched. Therefore seat the center first for best results.

Rolling a seal

This photo appeared in shop manuals of the 1940s and early 50s. Note the person is rolling from the end toward the middle. The rope is woven so it can stretch longer and thinner, or contract shorter and wider. We want it to completely fill its groove. I suggest you put the cap in a padded vise, roll from the center outward, and apply compression from the end with other hand. This should more completely fill the groove with seal material, and keep that oil drop off the ground.

Leakage usually occurs at the parting line between the two halves of the seal, or around the body of the seal if it does not completely fill the channel. The shop manual recommends cutting the excess off flush with the edge of the cap (use a razor blade), but some mechanics used to cut so there is excess protruding from the cap, which forces the seal tighter into its groove as the bearing cap is tightened. This would help sealing, but there is the risk of seal material interfering with the cap fully seating if some material remains on the mating surface. This could produce a loose main bearing and short life. A better system is to cut the rope flush with the bearing edge, and add a small drop of RTV silicone to the mating edges of the rope. This will aid sealing but not interfere with the cap and bearing seating. The crankshaft should be rotated before and after installing the seal to ascertain that there is a bit more drag after the seal is in place.

According to Fel Pro, a common mistake is soaking the seal in oil before installation. Soaking tends to shred the seal, shortening its life. A new seal should be lubricated with a few drops of motor oil after it is seated but before installing the crankshaft. This avoids a “dry start” which would build up excess heat which is detrimental.

The rope system usually will allow some oil seepage. Fel Pro believes rope seals can produce a drip free system if properly installed, although they admit this is somewhat of an art, and it is easier for an amateur to install a rubber seal. If one drop of oil appears on the floor this could be considered normal. Larger puddles are abnormal. Anyone attempting to “sneak” in a new seal without dropping the crankshaft should be aware that the new seal will leak if not compressed with a wood dowel. Period flat rate books show how this shortcut is done, but this method is an extremely bogus excuse for restoration. Once the crankshaft is lowered even slightly, the front seal, if not also renewed, will start leaking. The same front seal part number has been used in both sixes and eights for over 50 years. When reinstalling the oil pan another small drop of RTV silicone can be put in the corners. Be warned, however, that excess RTV silicone will find its way to the oil pump pickup and clog the strainer.
Put your dab toward the outside of the pan, so excess will extrude outward, and can be cut off with a razor blade for cosmetic reasons.

One last point. As pollution rules are tightened, it is our job to make sure they are enacted in a rational way. Some, if not all emissions inspection stations are instructed not to inspect or automatically fail a vehicle dripping any fluid. Since California recently exempted antique vehicles from emissions inspections, it should be relatively easy for us to make sure other states do the same. If you are in an area that insists on dynamometer testing on all older vehicles (Canada comes to mind) it is still possible to have a machine shop cut a groove to accept a modern seal. This could be done at the time of a complete engine overhaul.

As technical advisor for Advance Design trucks, 1947 to 1955, I remain available to field questions regarding these hardy workers.

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  1. Fabulous write up. I’ve been researching all day and this has been the most informative read. Replacing a Babbitt 216 with a ‘53 235 in my ‘50 3100. In prepping engine for install I noticed oil on the front mounting plate and below the rear main. It has been sitting on an engine stand for moths at a slight downward angle (Towards the front). Don’t want to trash a new clutch with a leaking rear main or tear into the replacement engine to reseal.

    I did see a suggestion on a forum to drive a piece of 1/8” braising rod into the upper rear seal and replace the lower. Or maybe a Sneaky Pete might work if it continues to be a problem down the road.

  2. I just came across dozens of old engine oil seals, never opened in good condition. Is there a market for these old seals, or should I just toss them? I’m going to say by the packaging they’re from the 1940s and 50s.



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