MG Car Repair: main bearings- TR6

Question
QUESTION: Howard, you have helped me in the past, thanks. I am trying to roll new  main bearings in my 1972 TR6 while the engine in in the car. I started with main bearing close to the flywheel. Not to hard to come out- was worn through to the copper. Hard to get the new one in( I do not want to pull the engine). Any tricks? Thanks as always, Gabe

ANSWER: Hi Gabriel,

I seen somewhere in American bearing replacement a plug that is installed in the oil hole in the crank and then they just rotate the crank and that pushes the bearing shell out as the crank rotates. You had to be careful to rotate the crank in the correct direction because of the tab that locks the bearing shell in place.

This system worked to replace the bearing shells without removing the crankshaft however the problem is that often when a bearing shell is worn to a point that you see brass then the crank is also worn and sometime not round so it don't take lomg to destroy the new bearings. I have built a lot of engines of all kinds Domestic and foreign. It is extreamly rare to see bearing worn to brass and not have a rough and out of round crank. Which eats new bearings very quickly.

The old saying for this is "If you don't have time to do the job correctly, when will you have time to do it over? "

It is easy to mic the rod jurnels so do or have some one mic them and you will see what I mean becasue whwn a main is worn to brass then the rods are worse.

There are no short cuts on main and rod bearings. It is not hard to pull a TR-6 engine.

Howard

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QUESTION: Of course you're right. I'll try the wrong way because it's a lot easier than pulling the engine and I'm only putting about 1500 miles on the car per year, so it should still last awhile, that is if I can roll the bearing in- not easy. Thanks again, Gabe

Answer
Hi Gabe,

If the jurnals look good and smooth and if you mic the rod jurnals, You may get away with it for some time. It all depends on the condition of the crank.

When you are finished just note the oil pressure cold and with hot oil. If the hot oil is 25 PSI to 35 PSI at idle and the cruise oil pressure is from 45 PSI on up to 60 PSI then it will probably last a good while.

The method of putting in fresh bearings that you are doing can work. You should consider using "Plasti-gauge" (avaiable at many auto parts stores) Get the "Green" not the "Blue" and with a cap (rod or main) off lay a piece of the Plasti-Gauge across the bearing shell and bolt the cap up being very careful not to move the crank even the slightest. Tighten the cap to factory torque specs and then carefully remove the cap and look at the strip of Plasti-Gauge and use the paper sheeth it came in to gauge the width that the strip was crushed to get the oil clearance. .0015" is ideal on a rod bearing. I have seen up to .003" used but oil pressure suffers. On the mains you should see from .0015" to .0025" ideally but again I have seen .0035" used in racing engines.

If you do this test and have high clearances, be sure to use heavy oil like 20Wx50 oil. plus a can of STP oil treatmemnt would help. Don't use Synthetic oil with STP. Especially don't use Syntec Castrol oil with STP as I have had a very dangerous bad experience with that oil and the Castrol company made it very clear they couldn't care less about what happened.

Howard