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Notes on neoprene Rear Main Seal

Gil’s Garage

Gil Baumgartner
CTCI Authenticity Chairman

Rear Main Seal Oil leaks from the rear main seal on the Ford Y bock engine continues to be a hot subject. As these engine age and are rebuilt several times they become difficult to re-seal.

Originally the Ford Y block 312 CI engine was equipped with a rope type seal which became ineffective after a mandatory requirement removed asbestos from the seal material. In recent years a neoprene seal has been made for the 312 Y block.

. Ted Eaton has written a great article regarding installing the neoprene rear seal. (found in Gil’s Garage). His article also covers other oil leaks associated with the 292/312 Y block engine. Ted finds that as a general rule if his installation instructions are followed the early made for 312 engine neoprene seal featured in his article holds well if the crankshaft seal surface is polished. If not it will hold for a while then usually leaks before 3500 miles. It may leak sooner if the crank shaft seal surface is not perfect.

He believes as I and others believe that the early seal material may be too soft allowing the seal to wear. It has been found that a USA made modified Chrysler seal researched, developed and sold by Fordcraft, works equally well in the 312 engine and it is not as sensitive to crankshaft seal surface irregularities. I have used this seal for several years with very good results. It can be ordered from Fordcraft 972-423-4636 and many other Thunderbird parts dealers. More recently Best Gasket has introduced “GraphTite rope rear main seals, made from braided PTFE (Teflon) fibers with graphite”. The rope material is highly conformable, filling in any seal surface inconsistencies… Its naturally slick surface places less drag on the crankshaft”. I have not used the GraphTite rope seal, however successful results have been reported by others.

Installing the GraphTite seal without removing the crank shaft has the potential of stretching the top half of the seal and reducing its sealing ability. This seal best conforms to the crank shaft when the crank shaft is removed for installation.

For replacement of the rear seal with the engine in the car most of Ted’s instructions can be followed. Refer to the CTCI Restoration manual pages 352 through 356 for replacing the seal in the car without removing the crank shaft. It is very important to also review pages 398 & 399 because they contain a clarification.

Ted’s instructions also apply to the 292 engine. The 292 engine is usually easier to seal because of a USA factory made seal available from NAPA and some other major parts stores. The Neoprene seal supplied in most gasket kits is for the 292 only. The supplied rope seal is for the 312 but usually wears out quickly.


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