dreaded rear seal leak

[Bill Traill]

Experience and theory on the dreaded rear seal leak. In the 1/3 century I have owned TC #5221 it has covered 80,000 miles and has had one engine rebuild and two overhauls. After a rebuild in 1968 and the first overhaul in 1976, the oil leak from the rear scroll/slinger "seal" has followed the same scenario: Hardly a drop for the first 5,000 miles, a few at 10,000 miles and becoming worse from then on. At 30,000 miles it was almost worthwhile to scrape the puddle off the garage floor for re-use; the chassis was an oily dripping mess (other little drips contributed too). I assumed the main bearings and crank were wearing thus reducing the scroll clearance (on bottom side). This turned out to be false. A year after buying TC #5221 in 1967, the crank busted in the usual place necessitating a complete engine rebuild. My father and I did the assembly and followed the advice of the machinist on setting up the scroll clearance (.003"). I bought a new crankshaft and had it nitrided (yes, I read the anti-nitriding posts of a couple months ago). New bearings, pistons etc. I will add that the engine had non- standard (larger OD) cylinder liners installed in it's distant past and they needed reboring to .08". I opted for new liners. The MG liners fell right through the bores! The machine shop located "diesel engine liners" with proper ID, and the OD was machined to fit the block. At 40,000 miles (1976) the leak again had become very bad, but the old XPAG was still a willing steed. I tore the engine down to see what was going on. The crankshaft journals and bearings were hardly warn in, the scroll clearance therefore had not changed! The piston rings were goners. Cylinders had .0006" taper. I worked at an automotive machine shop at that time and the "old heads" helped me assemble the engine. No major machining, just new bearings, pistons, timing chain, cam regrind to Willy's 77 profile (nice!) etc. The shop owner told me these scroll seals work ok on Model A Fords but they don't last long on MG's for some reason. There was very little leak for about 5,000 miles (maybe a drop or two overnight). This past summer (another 40,000 miles) I decided to get serious about the leak problem. I wouldn't park the TC on anyones driveway because of the puddle it would leave....my XPAG's incontinence had become downright uncivilized!.... not to mention embarrassing. It was suggested that the cork sump/bearing cap seal had dried out and shrank, so I replaced it.....twice. The second time I used another sump. I then took the TC out for a good hot run on a twisty mountain road, coasted to a stop with the throttle set at 2500 RPM. I sat down by the roadside and watched the sump....nothing.....not a drop. I then set it to idle and went back to my dusty road side seat and within 30 seconds the drops came like ants to a picnic. I concluded that the scroll's purpose is to keep oil in the engine at idle. At higher RPM the clutch housing becomes pressurized due to the spinning clutch cover drawing air in thru the inspection cover vent and it must be this pressure keeping the oil in. I remember reading an article years ago about someone installing the inspection cover up- side-down and the air was forced out of the vent creating a vacuum that sucked oil out of the engine and transmission! You can't inspect anything by removing this cover so it's purpose may be a disguised pressurizing apparatus. I asked myself a question: What is changing over time and gradually becoming worse? It is not the scroll clearance. It has to be the piston rings wearing out and allowing more blow-by. I did not see a lot of blow-by at the tappet cover vent pipe (I had long ago opened up the cover gasket holes). But with no pressure in the clutch housing a shockwave from #3 & #4 piston must be blowing the oil out past the scroll at idle. Indeed, when I tore the engine down this past August the piston rings were shot, two broken, but the old mill had been running strong. Again, the main and rod bearings still showed hardly any wear. The crankshaft journals were maybe a couple ten-thousands of an inch smaller after polishing, passed magna-flux too. Again, no change in scroll clearance! The "diesel" cylinder liners had .0013" taper after 80,000 miles and luckly no scoring. Any thoughts on blow-by vs. clutch-housing pressure? Unless I missed something I think the model fits the circumstances. I didn't pull this all together until after re-assembly....I should have run the engine on a fixture without the clutch housing or removed the inspection cover (or sealed it) to see if the characteristics changed. Is there a way to measure this pressure if it exists? I will bet it is far less than a pound/sq-inch. (no globalist metrics here!). There it is....all the words out on the table......anagrams anyone? Bill Traill DCO: TC#5221

[Andrew Bradley]

Discounting fluid dynamics, a .003 gap around the scroll seal amounts to basically a 1/16" HOLE at the rear of the engine (.0602" diam.). So the only thing that can hope to hold the oil inside is the fluid tension and pressure differencial. Fie on that. Convert it to a lip seal and save your money for beer and twinkies and your time for the consumption thereof. Cheers..Andy