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MG MGB Technical - Engine mounts packing plates

My question applies to the V8 engine but can also apply to the RB Bs. What is the purpose for the packing plate(s) when fitted behind the engine mounts ? I'm guessing it's to do with steering column clearance as they always seem to be fitted on the steering side of the car. Or are they used to level the engine laterally ? Any info is very welcome as I'm about to fit another engine in the next or so. Rgds, Barrie E
B Egerton

It's the get the engine at the correct height in the engine compartment. Whether that is to with the steering column or some other alignment I'm not certain, it is usual to have to pack the right side bit more over time as the rubber compresses. It could be more likely to do with the very tight clearance at the top of the engine to the bonnet than the steering column (for that the higher the engine the better). The radiator filler plug and carb damper caps have so little clearance to the bonnet that the latter at least have cut-outs in the sound proofing pad stuck to the bonnet, but mine still shows rub-marks.

There are two parts to it - shims as required between the rubber mounts and the 'top' of the chassis brackets, and the location plates underneath the chassis brackets. There should be enough shims to prevent the mount studs resting on the bottom of the brackets slots, but not so many that you can't get the location plate installed the correct way round - this is with the hole in the *lower* of the two possible positions. If the studs bottom then the rubbers will be in shear and tension instead in compression. As the engine is lowered in - actually twisting a bit to get first one stud in, then twisting the other way to get the other stud in as you can't lower both directly in, then centralising - you may have to tap the bottom plate of the rubber mounts down a bit with a drift to get the studs low enough to get the location plates on.
PaulH Solihull

It's for downward gravitational pull that might overstress the rubber in the mounts. There is no rigid connection of the engine to the chassis and without the mounts, the whole car would vibrate uncomfortably. The engine rests in a "V" with the mounting points more or less 90 degrees opposed to one another. The gravitational force of the engine pulled downward should be as close to perpendicular in relation to the center of the mounts as possible. Otherwise, the rubber in the mounts is constantly being stressed laterally moreso than perpendiculary and the rubber eventually separates from the stress. Hope that makes some sense. I'm far from a theoretical physicist. Obviously, a few fractions of an inch or millimeters of motor mount thickness can dramatically change the direction of stress. The factory tolerances between these points differed as well as affected by age and mileage on the car. I haven't worked with the V8 shims, but the standard RB shims are actually just the base portion of the motor mounts with the rubber removed. Thus, if you're replacing mounts, you can make up your own shims as needed from the old mounts.
Rick Penland

This thread was discussed on 27/03/2011

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