By Dean Sprague

We rebuilt my MGTD engine about 2 years ago. We became the “T” Musketeers, David Ahrendt, Corky Guenther and I. Since it was torn down I decided to update the internals to convert the XPAG engine into something more modern and reliable. Despite a few setbacks the rebuild went rather well. In fact, since the rebuild I have driven it to Maine, Louisville, Kentucky and a couple of other trips to South Carolina and Georgia and many day trips. Since then I have put over 6 thousand miles on the car and have yet to add any oil, water or hydraulic fluid between required services. This is the first classic British car I have ever owned, let alone a “T” that didn’t at least require some oil between changes. This can make you very nervous waiting for that 2^nd shoe to drop but so far it hasn’t.

We upgraded all the seals using modern materials, which included replacing the rear slinger with a rear main seal kit. We have since rebuilt two other XPAG engines, David’s 46 TC and Corky’s 49 TC using the same seals and internal upgrades. David and Corky’s engines run beautifully but both resulted in spectacular rear main seal failures. I am not sure if I was just lucky and they were unlucky but both failed. The same three “T” Musketeers or maybe Stooges (you decide) built the engines. We used the same machinist to prepare them and the same parts and materials doing everything in exactly the same way. We checked and measured everything along the way. You would think you would get the same results right?

We have since removed both engines and replaced the rear seals again with no improvement. When we consulted the vendor and read discussions on the Internet forums we found that maybe 2 in 10 rear seal conversions seem to fail. No one knows why. We could just remove the $300 seal kit (write it off) and go back to the original oil slinger/seal or we could try and figure out what is going on. In hindsight I think the smart money would have been to replace it with the original but of course, not to be defeated we opted for plan B. After extensive study, conferring with expects we determined; the seal that is used for the rear main is a carbon not a normal lip type seal. These seals are typically used on dry sump racing engines where scavenger pumps (dry sumps) are employed. These pumps tend to create some negative pressure in the crankcase so the seal doesn’t have to deal with sump pressure. When any real positive air pressure is produced in the sump caused by new unseated rings or for whatever reasons this excessive air pressure can push oil around the carbon seal and into the bell housing.

One major difference between my TD and the two TCs is that the TD has a Hi-Gear Sierra 5 speed transmission while the TC's have Datsun B210 5 speeds. The Hi-Gear bell housing incorporates a cover plate air pressure device on the clutch housing whereas the Datsun does not. The intent of this cover plate is to provide a slight positive air pressure on the flywheel side of the engine encouraging the oil to remain in the engine. On the TCs that area is at normal atmospheric pressure. How much this contributes to the problem is unknown.

Let’s begin with David’s engine. It ran very well and stayed dry for about 20 minutes then it began to drip from the bell housing creating a healthy puddle of oil both times. We tried to “push” the break-in period in hopes the rings might seal enough to resolve the problem but after about 250 miles with the bonnet off David’s car had more oil on the firewall and windscreen than a P51 Mustang. You wanted goggles just to drive it. It was time for a real solution. If you can’t stop the crankcase from pressurizing itself find a way to suck the air out. We trial tested by loosely coupling a shop vacuum around the breather pipe and running the car. It worked perfectly! The car was dry as a bone as long as the vacuum was running but not very practical without a really long extension cord or an on-board gasoline generator.

We began experimenting by creating a positive crankcase ventilation system thru the use of a PCV valve. We bought an inexpensive AutoZone one, 2 feet of copper pipe, some 11/32 vacuum hose and some fittings. We removed the breather pipe and replaced it with our contraption. We created a vacuum source by drilling and tapping in the rear core plug on the intake manifold. The leaking stopped but the RPM wandered a bit at idle and had a slight stumble just off idle. David drove it a few hundred more miles and it stayed dry! Now the trick is to lower the suction enough to stabilize the idle but still maintain enough negative crankcase pressure. David began by putting a restrictor in the vacuum line and reducing it as long as it doesn’t leak. We measured vacuum in inches of water with a Dwyer MagneHelic meter and found that we need about 5 inches of vacuum to control the oil leakage and stabilize the idle. David and Kathy went to the GOF Central in Ohio driving the TC up and back without mishap. He now has over 2500 miles on the car. The rings have seated and the engine runs wonderfully. Unfortunately it still leaks without some vacuum. So he has acquired an adjustable PCV valve from ME Wagner to provide for micro tweaking. When the PCV valve opens and closes it demands and restricts the air going into the engine thereby creating the wandering idle problem. Now we have the ability to adjust the vacuum to the minimum needed. Note for reference that approximately 13 inches of water column equals 1 inch of mercury.

Here are his results:

1. Valve cover inlet at air cleaner completely blocked with or without a restrictor plug in the vacuum line
   Autozone 1225 PCV Valve 5” H20 idle: 800 RPM
   ME Wagner adjustable PCV valve set at 5” H20 idle: 800 RPM
   Set at maximum of 15” H2O idle: 1100 RPM

2. Valve cover inlet at air cleaner completely open

Autozone 1225 PCV Valve 1.5 to 2” H2O idle: 800 RPM (rear main seal leaks)
ME Wagner valve Max @ 8” H2O idle: 1250 RPM,
Set @ 6” H2O idle: 1050 RPM
Set @ 5” H2O idle: 1000 RPM

3. Valve cover restricted with a .075 hole (using a 3D printed plug @ air cleaner)

Autozone 1225 PCV Valve 5” H2O min idle: 750 RPM
ME Wagner adjustable PCV Valve Max 18” H2O idle: 1250 RPM
Set @ 10” H2O idle: 900 RPM
Set @ 5” H2O idle: 750 RPM

The idle values shown above are the result of changes in the crankcase vacuum only. No carburetor changes were made.

He decided to set the Wagner to 10” H2O to get a little more vacuum than the Autozone 1225 valve. Now he has a minimum idle of about 950 to 1050 RPM. However the slight stumble off idle is still present. We suspect it may have something to do with the vacuum port sitting at the end of the intake manifold creating a temporary imbalance between the carbs. Dave has bought another TC intake manifold from eBay and will pull everything apart again after the driving season. The plan will be to center drill the top of the manifold and install a 90 degree hose barb to run to the Wagner PCV valve. With the new manifold installed, the plan will be to slowly reduce the vacuum until the minimum required to contain the oil is determined.

Now we are using the lessons learned on David’s engine as a base line to assist Corky. His rear main leak appears to be a little more severe than David’s. The puddle is larger and continuous. Also it seems to require more vacuum to arrest the leakage. Corky acquired another intake manifold that was already drilled so we have the vacuum coming from the center. He doesn’t seem to have an off idle stumble so I suspect that will fix David’s as well. Corky is also using the ME Wagner adjustable PCV to provide for micro adjustment of his engine. We have discovered that each “identical“ engine seems to have unique characteristics. Who knew?

Disclaimer: we are still in the experimental stages with our modifications and all conclusions should be regarded with a healthy dose of skepticism.

It all seemed so simple when we started. XPAG engines have always been eclectic and challenging and all three proved to be no exception. You may know the saying, “I never did mind the little things”. When you deal with any MG T series car be prepared for lots of little things. It’s really part of their charm.