MG T-ABC

Recalling those words of wisdom, I took a hard look at both the Moss Oil Seal, and the GM rear oil seal. I looked at the MG engine block, and wondered, "Why would MG (Morris) put a seal in the back of the engine that didn't work?". Then it came to me. Morris put a rear oil seal (Thrower Plate) in the back of the engine. This design dates back to the mid-thirties, but probably was part of the Continental engine design, when the engine was initially designed, in the late tens. It worked well at that time (the tens), it just wasn't state of the art in the post-war years.

What they incorporated in their design, was an Archimedes screw to return the oil back into the block, that had worked it's way to the rear-most portion of the crankshaft. The Archimedes screw is at the back of the crankshaft, then an oil barrier (plate), then the large flange for the flywheel mounting. In the engine block, at the back of the rear main bearing cap, was a flange lip that would surround one half of the Archimedes screw. The thrower plate mounts directly to the rear of the block. It supplies the other half to surround the Archimedes screw. The thrower plate butts up to the bottom of the rear main bearing cap, and is secured by two pins and by three 6mm x 1.0 bolts. The design worked reasonably well, as long as the clearance between the Archimedes screw and the rear main bearing cap/ thrower plate was kept to a minimum.

What happens is that the crankshaft flexes. Remember, these weren't balanced crankshafts, or flywheels, or other moving parts. A small amount of unbalanced weight, over the years, moves the part in an elliptic, rather than a circular motion. Add the weight of the flywheel attached to the rear of the crankshaft, and you can either increase the unbalanced weight ( if both unbalanced weights are placed on the same side) or move the imbalance by installing the flywheel having a greater imbalance that that existing in the crankshaft, to the opposite side of the crankshaft's imbalance. In any event, the wear at the rear main bearing is greater than either the centre or front main bearings. This wear allows for the Archimedes screw to move outward into the circular surfaces of the rear main bearing cap/thrower plate, enlarging the opening. With a larger opening (clearance), the Archimedes screw becomes inefficient. No where in the workshop manuals does it mention to check clearance at this point. Jim Bigler, Jerry Felper, and others knew that the thrower plate had to be butted against the rear main bearing cap. In fact, they would advocate removing the two pins, and tap the thrower plate up against the rear main bearing cap, with the cap torqued to spec to the engine block. Bud Quist took this one step farther. So, what's the solution?

1. Have all parts of the engine balanced. Then, mount the rear main bearing cap to the block, torqueing the nuts to spec. Install the thrower plate, using the three bolts and the two pins (I would remove and discard the pins. D. DuBois), plus the gasket, and gasket cement. Do not tighten snug the bolts. Tap the thrower plate at the left and right pin corners, to butt the plate to the rear main cap. This also bends the pins, helping to keep the plate against the rear main cap. With this completed, now tighten the three bolts snug.
2. Smear a thin coat of J-B weld on both of the semi-circular surfaces.
3. Install new main bearings into the engine and main bearing caps.
4. Using Teflon tape, wrap two turns around the Archimedes screw portion of the crankshaft. Press the tape into the screw threads. The Teflon tape acts as a release agent, and helps to remove the J-B weld from the screw threads.
5. Lay the crankshaft in the block.
6. Install and tighten to spec the centre and rear main bearing cap.
7. Rotate the crankshaft in the direction that it spins, two or three complete revolutions.
8. Remove the centre and rear main bearing cap.
9. Remove the crankshaft. Pull the Teflon tape out of the threads. Use a dental pick, or other tools, and remove any remaining J-B weld residue.
10. Remove any excess J-B weld in the rear main bearing cap, oil thrower, or engine, not on the circular surfaces.
11. Let J-B weld sit for 24 hours.
12. Repeat steps 4-10.

This completes the repair of the circular surfaces. The Archimedes screw now has a minimum clearance to work with. When assembling the engine, place some gasket cement on the flat surface of the oil thrower. This seals the circular area. Almost all oil will now be returned to the engine. You will have a small leak, better than when the engine was new. The cost of this repair is about $5.00.