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MG MGB Technical - SPACER?

The abandonrd project thar won't go away...........OK boys and girls, here we go. Victoria British page 100, Illustration number 4, part number 5-5001. Why do I need it? Same as Moss page 6, illustration number 91a, no part number given. Do I need to re-mount the engine? What the heck is it for? Thanks, Tom TC 2850
Thomas McNamara

Tom,

I'm told that yes you do need it. But I thought one was needed on the left side only. Something to do with the rotational forces of the engine - perhaps somebody else will remember more than I do. On my car (that uses the earlier style engine mounts) I found myself lacking the spacer on the day I was replacing the engine, so I shaved the rubber off an old engine mount and used that.

You don't need to pull the engine back out. Just put a sturdy jack and plank under the sump, put a little lifting pressure on the engine, remove the four bolts attaching the engine mount to the pad on the frame-iron and raise the engine just enough to slide the spacer in between. A couple of undersized (1/4") elongated bolts through the holes will help keep it all in alignment as you ease the jack down.

FWIW,
Allen
Allen Bachelder

OK, so I need the darn "spacer thing.Is it made of steel or rubber? thanks, Tom, TC 2850
Thomas McNamara

Tom,

The spacer is a steel piece - identical to the metal base of an engine mount. That's how I made one by shaving the rubber part off an old engine mount. ' Think I "shaved it by cutting the rubber off with a hacksaw - cutting as close as I could to the metal base. Then I cleaned the rubber residue off the base with a wire wheel. The metal base then becomes the spacer.

' Hope this helps.

Allen
Allen Bachelder

Thanks Allen, I'll get one on order. Tom
Tom

I think it is down to the design of the square engine mount system. Because it is a system of round studs going through round holes with the faces angled into a 'V', and the tolerances which were normal in the distance between the chassis rails and hence the mounts, there was always a chance that the studs would hit the sides of the holes before the faces of the rubber mounts touched the faces of the chassis rail mounts. Spacers tokk up this variation on an 'as required' basis. With the later round rubber mounts their studs fitted into slots in the chassis mounts and so could encounter a wide range of tolerance on chassis mount spacing. Nevertheless, spacers are used on the drivers side of those as well to clear the rack shaft, but at least the two rubber mounts are the same size. On the RV8 the passenger side mount is significantly thinner to get adequate clearance between the top of the engine and the bonnet. The full-size is still needed on the drivers side to clear the rack shaft as before.
Paul Hunt 2

One million times - the spacer plates are to compensate for variations in tolerance of the welded on mount on the frame. True for both rectangular and round mounts. Not all cars had them, and they are much more common on round mount cars. They appear on one or both sides, and not necessarily the same number per side, though the number is usually only one different from side to side, with the greater number on the LH side. I've seen up to three on round mounts (from the factory); none to one is more common on rectangular cars.

If your engine clears everything without them, forget it. The only decent clue is that the mounts should appear to be "square" as installed with the engine weight on them - the rubber sides (all 4 or the full circumference) should be perpendicular to the metal plates. If it really needs spacers badly, the mounts will be in too much tension/shear, and not enough compression, and will fail prematurely.

FRM
FR Millmore

Tom-
Just to elaborate on what Fletcher has already pointed out, when new front motor mounts and their brackets are installed, inspection usually reveals that the assembly is already bending toward the block. That means it is prestressed in compression, and as the engine rocks, the stress cycles from compression to tension and back again, ultimately leading to fatigue failure. This condition is at its most severe on the Left Hand bracket, since torque effect causes that side of the engine to lift under acceleration, whereas the Right Hand bracket tends to remain in compression, except during hard engine braking. If you fit a spacer of approximately 1/8” (.125”) thickness between the bracket and the block at the large bolt, you will prestress the bracket in such a way as to prevent the cycling through zero, which reduces or eliminates fatigue failure. This compressive preload also keeps the rubber mount plates parallel, greatly increasing the life of the mount itself. If the mounts are correctly shimmed, then the force on the rubber mounts will be at right angles and they should not sag, even over a long period of time. The need for these shims is determined by the dimension across the mountings in the chassis, which varies due to build tolerances, just as Fletcher said. You can determine if they are needed by examining the mounting rubbers - the sides should be at 90º to the ends when under the weight of the engine. If they slope towards the engine at the top, then you need to add shims. If they slope away from the engine, then you need to remove shims.
Steve S.

Tom,

You've just heard from three of the best heads in the hobby. Follow their advice and you can't go wrong.

Allen
Allen Bachelder

I know in previous threads that it was stated that the shims for rubber bumper cars were for clearance of the steering shaft on certain cars when built in the factory.
James

The abandoned project spacer task continues....
OK folks, I got the spacer, jacked up the engine, put the spacer in and now the engine sits too high for the bolt holes to match. Should I try somehow to compress the rubber mounts? Seems like a diffacult job.

Before starting I did look at the rubber being "square" as mentioned and saw that the rubber "leaned" forward. I think this equates to the drive train being out of true. I can square it up by holding the tranny in place while tightening.

At this point I think I'll go spacer-less.

Tom, TC 2850
Tom

FWIW, I don't have spacers in my 74. I didn't have any front or backward alignment problems.

Clifton
Clifton Gordon

Tom-
Might help to know if it is a rectangular or round mount car, though the principles are the same.
The spacer(s) only alter the up-and-down, in the plane of the mount, shape of the rubber.
It is common to have to tap the lower mount plate down a bit to get the bolts in when things are correctly spaced. Do this with a small punch and the weight of the engine on the mount.

Fore and aft changes are due to:
Bad gearbox mounts, or
same incorrectly assembled on the crossmember, (the crossmember brackets can be installed backwards, and things fit badly!) or
Incorrectly adjusted or assembled restraint rod, or accident damage - bent frame brackets.
On round mount cars, bent or cracked engine mount (engine side) brackets (VERY common) will also throw the rubber out-of-square - the engine bracket plate MUST be parallel with the frame bracket plate when loaded.

FWIW, the earliest B manual does not show spacer plates.
The later (Bentley) one shows one plate, fitted to LH side. That's for rectangular cars, primarily 68-74.

The round mount book does not mention the plates at all, though most of these cars had one to five spacers.

As I was doing dozens of these monthly as far back as 1965, on first time clutch jobs, I can assure you that I remember the first time I saw a spacer (ie, there were none on the earliest cars), and that they became more common and more numerous as time went on. I attribute that to wear on tooling and/or sloppier work as time progressed.

FRM
FR Millmore

Thanks to all for your collective wisdom. I bolted the engine in last night without the spacer, now that I have one. :( The car is a '73 roadster that I purchased unassembled. It has rectangular motor mounts.

I have restored a TC and a TD but this is by far a more diffacult undertaking, because I did not disassemble the car. This board is great! It has helped me a lot on all three projects.

Regards, Tom
Tom in cloudy upstate NY

As has been said the spacers are to align the studs with the holes to cope with variations in the chassis rail spacing. If the studs sit on the edge of the holes and there is still clearance in the mount then you need a spacer. If you can't get the studs in the holes and the mounts are up tight, and you have a spacer in place, then it is too thick. However I still maintain that the round mounts with the slots in the chassis rail mounts cater for chassis rail spacing on their own, spacers then are only required to clear the rack shaft.
Paul Hunt 2

Paul-
Not true. The round mounts have slotted stud holes for ease-of-assembly purposes only. When correctly assembled, the stud will be all the way at the bottom of the slot, and there are offset square washers fitted on the bottom side which ensure that the studs are so positioned, and which will not fit in place if the studs are not bottomed. If the square washers are missing, the studs still go to the bottom of the slots. Failure to position them this way will result in failure of the rubber bond in the mount, as it will be in a tensile shear state, rather than the compressive shear designed for. If the mounts are not bottomed in the slots, the stress on the already weak engine side brackets is increased, making them even more likely to break - and they sure don't need any help in that! Exactly the same combination of stresses occurs if the mounts need spacer plates and they are not fitted.

FRM
FR Millmore

Maybe the 4-cylinder is different. I've changed mounts a couple of times in my V8. I had no problems getting the washers on, and the studs were not bottomed even when there was a thinner spacer than currently on the drivers side. In fact I wasn't able to lower the engine straight down, but had to twist it to one side to get one stud in its slot first, then twist it the other way to get the 2nd stud in. The mounts I took out showed shear stresses, and when I put the new ones in I could see the same stresses developing, as they stopped sliding down the angled faces of the chassis rail mounts due to friction before the engine was fully supported. I put a drift on the top edge of each mount and tapped them down further until they were 'square'. The workshop manual does mention the square washers (locating plates) and their handing. But if it is the studs that are taking the load, be it the bottom of the slot or the hole in the locating plate, the mounts won't be fully seated and surely it is then that the rubber will be under shear stress and not compression. And in fact I can't see anything that will stop the square plates sliding down the inside of the mount as far as the studs will take them, so the studs will never rest on them, only the bottom of the slots, and only then if the rubber becomes compressed enough to allow them to get that far.
Paul Hunt 2

This thread was discussed between 05/04/2008 and 12/04/2008

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