Rear Main Seals, Theory and Reality One of the most troublesome leaks on XK and V-12 engines built prior to ’89 is a leaking rear main seal. Some of you probably read Dick Russ’ article in the Jaguar Journal of March/April ’95, or its reprint in British Car of Dec ’95, on sealing up the rear main of a V-12 E-Type. His procedure will also work on the XK. How many of you know that Jaguar officially took exception to Russ’ solution (actually the idea of Phil Long of Terry’s Jaguars)? They said that if the rear seal actually touches the crank, as it did on American engines that used a rope seal, the warranty would be voided because the seal would cause heating that would wipe out the rear main bearing.

Jaguar really didn’t like rear main seals in that era, they didn’t even have one until 1962. They used a slinger ring built into the crank and a spiral groove that supposedly directed oil back to the slinger and into the pan. Bill Price tells me that the leaking was so bad on some Mark II’s that Jaguar apparently did a dealer retrofit where a small catch basin was brazed to the back of the sheet metal pan and copper return tubes installed to drain the excess oil back into the pan. He’s seen this on at least 3 Mark II’s that he’s restored, the latest model being a ’64. Starting in ’62 a rear seal was included. The spiral was made much shallower and an asbestos rope, much like the hemp rope seal on American cars, was installed. You can tell it was a retrofit by the way the top part of its retainer bolts to the XK block.

The approved factory procedure for installation of the Jaguar seal is much different than for American cars, where the seal is essentially packed against the crank to prevent any oil that made it past the slinger ring going out the back. American cranks have a smooth surface for the dog turd (that’s what old time mechanics affectionately called it) to rub against and, properly installed, they didn’t leak for 100,000 miles and they didn’t cause the rear main to overheat.

The Jaguar crank has a shallow spiral where the seal resides and I agree that if you pack the factory seal, or one from a 460 Ford, tightly against this spiral you will get heating and probably turning and, fairly quickly, destruction of the seal. Jaguar says that you must size their seal with tool JD-17 (the V-12 also requires adapter JD-17B-1). The result is 1 to 2 thousandths clearance between the seal and the crank.

So much for the theory. Every Jaguar engine that doesn’t leak (and a few that leaked a little) that I have disassembled had a seal that has been touching the crank. Some had run over 100K. A few did have a scorched rear main. Some had a seal that has rotated 90° . Obviously Jaguar did not glue the seal in place as Russ suggested you do with his Ford replacement. It's also obvious that the factory was fudging on the specs so that the engine didn’t leak too much. Many rear main bearings were replaced under warranty in the early 80’s. Was there a connection? The Jaguar slinger ring wasn’t really tall enough and the seal wasn’t far enough away from the rear main on the V-12. Poor fundamental design.

So what is the correct procedure? Well, you could be on the safe side, size the seal so it doesn’t touch the crank and put up with a little to a lot of leakage, depending on how hard you run your car and how much you park on steep uphills. Or you could take your chances with the Ford seal, which is much softer that the Jag asbestos and impregnated with graphite to boot. If you are running increased oil flow and pressure along with a full pass oil cooler, I’ll bet you won’t burn the rear main. But that means you’ve extensively modified the engine so why don’t you at least have the crank grinder take the spiral off the crank or, preferably, modify the engine so it will take a modern neoprene seal.

In late ’87 Jaguar retired the XK engine. The new AJ-6 has a thoroughly modern seal and doesn’t leak. Starting with engine #8S66784 in the XJS and #7P02073 in the XJ12 Jaguar fitted a neoprene seal. This took place during the ’89 model year. These new V-12 seals don’t leak when properly installed, but sometimes blow out the back of the block, causing a catastrophic flow of oil.

Which brings me to one of the major causes of a rear main leaking, especially on the early V-12’s. A pressurized crankcase will cause an engine to leak, even if the seals are all in good shape. Normally a crankcase should be under a slight vacuum, whether it has the old draft tube or the modern PCV system. If the rings are badly worn or broken, allowing a large amount of blowby, the crankcase will be pressurized. This doesn’t mean it can only happen on a high mileage engine. Running an engine really hot can cause the rings to lose their tension and blowby will be the result. The only solution is a ring job. If something is wrong with the PCV system (stuck valve, loose or cracked hoses, hoses incorrectly hooked up) the crankcase can become pressurized. It’s just possible that the PCV system has been modified or even eliminated by some previous owner in the mistaken belief that it was taking horsepower away from the engine. Again, the V-12 suffers the most, because its system is marginal, even when everything is new and functioning correctly. Towards the end of it’s life (‘94-’96) Jaguar modified the V-12 ventilation system with a proper valley breather in an attempt to meet smog regulations and still keep the crankcase ventilated. When these engines get old and tired they will have problems because that valley breather will be overwhelmed and impossible to service without a major teardown of the top of the engine. Since most of the last V-12’s seem to be burning up from malfunctioning Marelli ignitions this may not be a problem after all.

Errata Recently I wrote that a possible problem with a poor running V-12 could be a hole in the vacuum advance module. Some V-12’s of the late ‘80’s had a tiny hole in the module on purpose so that the engine idle speed could be stablized. If you apply a vacuum to these modules they will not hold it, but they will move if the vacuum is sufficient. If the module will not move under vacuum and you hear a hissing, replace the module. Also, please note> The timing on a V-12 is to be set at 3000 rpm, not idle. A scary propostion as the timing mark is under the engine at the damper pulley. If you have an early V-12 do not assume that the timing mark is in the correct position. Note that it has adjusting screws! Setting this mark by putting the piston A1 at top dead center is not very accurate because a piston hardly moves for 10° of crank rotation when it’s at TDC. A much better way is to use a dial indicator on a piston that is midway on it’s stroke where you get maximum motion for minimum movement of the crank. Think about it.

Reminiscing How many of you read Road & Track? In the Aug ’00 issue under 30 Years Ago they quote Cyril Posthumus on the newly introduced XJ6, ". . . uncannily silent, gloriously swift and safe as a house . . . A REMARKABLE BARGAIN- about one-third that of contemporary accommodation- and performance-comparable machines." I sure hope they can say the same thing when the current XJ6 replacement comes out next year.