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Terry Burtz, Calif · 12-18-2018, 02:23 PM

The Lucky Star,

A discussion about rear main seals is similar to discussions on this forum about what oil to use.

I suggest that you educate yourself by talking with people who run at Bonneville and the machine shops that build high performance engines. Don't be afraid to ask lots of questions.

Welding on a crankshaft is not a "do it yourself" modification.

Crankshaft welding needs to be done by a crankshaft rebuilder that has a crankshaft welding machine that utilizes a submerged arc. After welding, the crankshaft needs to be straightened to eliminate axial and radial runout of the flywheel mounting flange. Next the crankshaft needs to have the welded areas machined to within .020 in. of final dimensions, and re-straightened as stated above. Once straight, the crankshaft is now ready for final machining.

As originally designed, the dowel pins locate the flywheel and are loaded in shear. The 4 bolts provide the clamping force and are in tension. In my opinion, this is the best design.

Be wary of a machine shop that says they can machine the face and diameter of the flywheel mounting flange to make things run true. Machining the outer diameter to run true only opens an interface that was previously a tight fit. Machining the face of the flange to run true introduces bending forces along with tension in the 4 attachment bolts. Changing the dowel pins to bolts puts all 6 bolts in tension, shear, and bending.

Since I manufacture seals along with many other parts for our beloved Model A's, I am hesitant to comment because I don't want my response sound like an advertisement. I am a manufacturer and do not sell retail.

On a stock engine (gravity feed to mains and dipper to the rods), the rear main slinger system is adequate unless there is excessive blow-by, the crankcase is overfilled, or the rear main clearance is too large. In the Model A era, small amounts of leakage were acceptable since most driveways and garages had porous surfaces.

When an engine is modified to supply pressurized oil to the rear main, the volume of oil supplied will overwhelm the stock slinger design and a positive seal will be required to prevent leakage.

Positive seals rub on mating diameters to prevent leakage unlike slingers that throw excessive oil into a collection groove that collects and returns the excessive oil to the oil pan.

Surface finishes on the slinger areas of Model A, Model B, and aftermarket crankshafts are similar to the surface finish on 150 grit sandpaper.

This rough surface finish will quickly wear any rubbing seal that attempts to seal against it.

For any seal to work, it is necessary to provide a smooth rubbing surface that is concentric with the rear main journal.

One type of seal that has been used is a packing (asbestos rope, Teflon rope, rubber
impregnated cork, wool, and many other materials. The stock front seal for a Model A engine is a packing, and the first engines designed with a rear main seal used packing type seals.

Packing type seals work until they wear and the preload against the smooth rubbing surface is lost. If the seal rubbing surface is not concentric with the bearing, seal wear will be much faster.

Modern engines use a radial lip seal. This seal design utilizes a flexible lip that is preloaded against the smooth rubbing surface. Radial lip seals are able to flex and accommodate small amounts of runout. The ability to accommodate runout depends on several factors including resiliency of lip material, length of lip, initial lip deflection, RPM, and temperature.

Radial lip seals can be either 2 piece or 1 piece. 2 piece seals have one piece fits in the rear main cap, and the other piece fits in the cylinder block. This type seal needs to have a little crush where the seal halves meet to provide an oil tight interface. 1 piece seals stretch over the flywheel mounting flange and have no mating surfaces.

Radial seals can accommodate much larger runout than packing type seals, but there is a limit.

Regarding runout, one interesting failure was a pressurized engine that didn't leak when driven around town, however had a rear main leak when driven on the freeway. During a forensic analysis to determine the cause of the leak, the engine was removed from the car, placed on an engine stand and pan removed. Pressurizing the rear main cap drain tube to 60 psi did not result in a leak. Disassembly of the engine and checking for runout of seal rubbing surface showed .010 in. TIR, and the rear main journal also had runout. At slow speeds around town, the seal was able to flex quick enough to prevent a leak, but at high speed, the seal lips were whipped out causing a leak.

The engine above with the leak had a Burlington crankshaft that was machined to provide a sealing surface, and then drilled to deliver oil pressure to the connecting rods. Drilling the crankshaft relieved stresses and the crankshaft was distorted unbeknownst when the engine was assembled. Subsequently, the crankshaft was straightened and lasted a few years with no leaks before it broke.

I am confused regarding the comment by BRENT in 10-uh-C as to how thrust (fore and aft movement of crankshaft?) has any effect on a radial lip seal, would like to thank Dave in MN for the endorsement, but would like to know why dead-on concentric is needed. The seal mentioned above with .010 in. TIR that leaked at high speed was in San Jose, CA. MN is a much colder climate and temperature will have an effect on resiliency.

I'm available through PM on this forum, or ask your parts retailer for my contact information.

12-18-2018, 02:23 PM	#11
Terry Burtz, Calif Senior Member Join Date: May 2010 Location: Campbell,CA, USA Posts: 319	Re: Best Rear Main Seal The Lucky Star, A discussion about rear main seals is similar to discussions on this forum about what oil to use. I suggest that you educate yourself by talking with people who run at Bonneville and the machine shops that build high performance engines. Don't be afraid to ask lots of questions. Welding on a crankshaft is not a "do it yourself" modification. Crankshaft welding needs to be done by a crankshaft rebuilder that has a crankshaft welding machine that utilizes a submerged arc. After welding, the crankshaft needs to be straightened to eliminate axial and radial runout of the flywheel mounting flange. Next the crankshaft needs to have the welded areas machined to within .020 in. of final dimensions, and re-straightened as stated above. Once straight, the crankshaft is now ready for final machining. As originally designed, the dowel pins locate the flywheel and are loaded in shear. The 4 bolts provide the clamping force and are in tension. In my opinion, this is the best design. Be wary of a machine shop that says they can machine the face and diameter of the flywheel mounting flange to make things run true. Machining the outer diameter to run true only opens an interface that was previously a tight fit. Machining the face of the flange to run true introduces bending forces along with tension in the 4 attachment bolts. Changing the dowel pins to bolts puts all 6 bolts in tension, shear, and bending. Since I manufacture seals along with many other parts for our beloved Model A's, I am hesitant to comment because I don't want my response sound like an advertisement. I am a manufacturer and do not sell retail. On a stock engine (gravity feed to mains and dipper to the rods), the rear main slinger system is adequate unless there is excessive blow-by, the crankcase is overfilled, or the rear main clearance is too large. In the Model A era, small amounts of leakage were acceptable since most driveways and garages had porous surfaces. When an engine is modified to supply pressurized oil to the rear main, the volume of oil supplied will overwhelm the stock slinger design and a positive seal will be required to prevent leakage. Positive seals rub on mating diameters to prevent leakage unlike slingers that throw excessive oil into a collection groove that collects and returns the excessive oil to the oil pan. Surface finishes on the slinger areas of Model A, Model B, and aftermarket crankshafts are similar to the surface finish on 150 grit sandpaper. This rough surface finish will quickly wear any rubbing seal that attempts to seal against it. For any seal to work, it is necessary to provide a smooth rubbing surface that is concentric with the rear main journal. One type of seal that has been used is a packing (asbestos rope, Teflon rope, rubber impregnated cork, wool, and many other materials. The stock front seal for a Model A engine is a packing, and the first engines designed with a rear main seal used packing type seals. Packing type seals work until they wear and the preload against the smooth rubbing surface is lost. If the seal rubbing surface is not concentric with the bearing, seal wear will be much faster. Modern engines use a radial lip seal. This seal design utilizes a flexible lip that is preloaded against the smooth rubbing surface. Radial lip seals are able to flex and accommodate small amounts of runout. The ability to accommodate runout depends on several factors including resiliency of lip material, length of lip, initial lip deflection, RPM, and temperature. Radial lip seals can be either 2 piece or 1 piece. 2 piece seals have one piece fits in the rear main cap, and the other piece fits in the cylinder block. This type seal needs to have a little crush where the seal halves meet to provide an oil tight interface. 1 piece seals stretch over the flywheel mounting flange and have no mating surfaces. Radial seals can accommodate much larger runout than packing type seals, but there is a limit. Regarding runout, one interesting failure was a pressurized engine that didn't leak when driven around town, however had a rear main leak when driven on the freeway. During a forensic analysis to determine the cause of the leak, the engine was removed from the car, placed on an engine stand and pan removed. Pressurizing the rear main cap drain tube to 60 psi did not result in a leak. Disassembly of the engine and checking for runout of seal rubbing surface showed .010 in. TIR, and the rear main journal also had runout. At slow speeds around town, the seal was able to flex quick enough to prevent a leak, but at high speed, the seal lips were whipped out causing a leak. The engine above with the leak had a Burlington crankshaft that was machined to provide a sealing surface, and then drilled to deliver oil pressure to the connecting rods. Drilling the crankshaft relieved stresses and the crankshaft was distorted unbeknownst when the engine was assembled. Subsequently, the crankshaft was straightened and lasted a few years with no leaks before it broke. I am confused regarding the comment by BRENT in 10-uh-C as to how thrust (fore and aft movement of crankshaft?) has any effect on a radial lip seal, would like to thank Dave in MN for the endorsement, but would like to know why dead-on concentric is needed. The seal mentioned above with .010 in. TIR that leaked at high speed was in San Jose, CA. MN is a much colder climate and temperature will have an effect on resiliency. I'm available through PM on this forum, or ask your parts retailer for my contact information.