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MG MGA - Bearing clearances/Plastigage

I don't have access to a workshop manual at the moment, but was hoping I could get the bearing clearances for all the rod and main bearings on the 1622 motor. Specifically, I'm trying to see what size of Plastigage I need to buy to verify my clearances.

J DeRienzo

David DuBois

Awesome. Thanks, David!
J DeRienzo

.001-.0027 is the spec I have listed. I try to keep it as close to .0015 as I can.

R J Brown

Isn't the crank to mains clearance .0005..?
G Goeppner

Generally, no. "Official" specifications were:
1500 main bearings .0005-.0020
1600 main bearings .0005-.0020
1622 main bearings .0010-.0027
1800 main bearings .0010-.0027
1500 rod bearings .0001-.0016
1600 rod bearings .0010-.0025
1622 rod bearings .0010-.0025
1800 rod bearings ????

You can see with sequential development of the engine that diametrical bearing clearances were increased. The changes can be viewed as the later spec's superseding the earlier specs, as these are essentially all the same engine type evolving (improving) over time. So when rebuilding an engine today, any of these models, one should favor the later specifications.

If the machine shop can control the dimensions to tighter tolerances, then shoot for the lower end of the range, .0010-.0020 diametrical clearance. These days it is easy to be spot on within about .0002 range for crankshaft regrinding. Commercially produced bearing liners are not quite that accurate, and they are affected by accuracy of the con-rod big end machining. To hold tighter tolerance on the big ends than the factory spec's requires re-machining of the con-rods (shave the cap a bit and re-bore the big end).

Small end of the range at .0010 is the required minimum to allow sufficient oil flow for cooling the bearing. Large end of the range is (was) to allow for mass production manufacturing tolerances. All of these numbers were originally derived during the era of standard use of straight-weight 30-weight (non-detergent) motor oil.

In the modern age of semi-synthetic "multi-weight" oil, it is common to run thinner oil (like 5W-30) with smaller bearing clearances, in the range of .0005-.0015 (in modern cars, not vintage engines). This requires higher precision machining, now possible in mass production. The primary impetus for using lower viscosity oil is slightly reduced internal friction resulting in slightly lower fuel consumption.

Difference in fuel consumption between old spec and new spec is quite small, so hardly worth going to the effort for a vintage engine. This is only significant if you drive the car a lot to use lots of fuel. Most of us like to use 20W50 oil, particularly to protect the "flat tappet" camshaft, so you should stay with the old spec for larger crankshaft bearing clearance. One thing you should never do is to run 5W30 oil in an engine with the vintage (larger) bearing clearances.
Barney Gaylord

Thanks for all the comments. The clearances were spot on and I am hopefull in that even if the motor doesn't work or has problems, we've given it our best shot this time and I've personally verified the clearances and assembly procedures.

I also appreciate the additional information on the oil weights. I can't remember what we were using for the oil, but it'll be nice to have an idea of what to use when we get this fired up.
J DeRienzo

This thread was discussed between 10/11/2011 and 15/11/2011

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