Order of tightening front/rear motor mounts?

[XPAGnut]

I am new to this site and the yahoo mail exchange. Question about my TC.

For various reasons I have removed the engine/gearbox, and am now preparing to get things back in place. My question is on the motor mounts front and rear. The rear mounts of course bolt onto the back part of the gearbox at the rear cover plate. I have not found on my car the typical cracking or repairs to the rear plate. I plan on tightening up the rear mount's first, and then the fronts. Does this sound like the correct order?
Tony-
UT

[Steve Simmons]

Hi Tony, welcome! I don't think it really matters which order you tighten them in. I like to get all the bolts in place and finger tight, then snug them all up. Usually gearbox first but only because I like to get the hard part out of the way first.

[XPAGnut]

Thanks Steve. Actually what I was talking about is the adjustability potential built into the front mounts. Never seen it talked about. More involved than tightening up bolts, in this case.

The rear (gearbox) mounts allow for no adjustment, unless you are real sure of yourself and get to bending them. The problem of "gearbox rear plate breakage" is apparently rampant in the TC's judging by the number of "kits" available to strengthen the ears where the rear mounts attach to it. I wonder if the cause of that breakage is not simply due to people not knowing about the adjustability built into the front mounts, and therefore not taking advantage of it, rather than any inherent weakness of the rear plate casting?

The "finger tight, then snugging up" method works for a lot of things to get them aligned before final tightening, I agree. But due to the weight involved with the engine/gearbox, if advantage of the engine hoist and sling are not utilized, to adjust alignment within the front mounts, after the rear mounts have come together snugly, one could well end up with with the rear mounts twerked to the limits of their rubbery capacity in one direction, ready to snap the rear plate ears with just a little engine torque encouragement, in that same direction. I think if the MG manual had mentioned the need for alignment, and showed where in the front mounts that alignment was to take place, rear plate ear breakage would be practically non-existent in TC's. With no attention to this "detail," the odds of breaking the rear plate ears is a crap shoot. Or so it seems to me.

[Steve Simmons]

I've never suffered a broken plate myself. The only adjustability I see would be by shimming the front or rear. This shouldn't really be needed assuming the front plate on the engine is straight and undamaged, as are the gearbox mounts on the frame. You would have to shim one end or the other quite a bit to introduce (or relieve) any strain caused by misalignment. The two mounts are so far apart that a shim in the front would equate to a very small degree of upward rotation at the rear. The rubber can easily flex to make up for any slight misalignments.

My method of installing is to drop everything in place and let it sit under its own weight before tightening anything.

[XPAGnut]

The rubber cannot easily flex if it is out of adjustment and already at one extreme of its flexibility. That is what I am shooting for is all the critical rubber of the front and rear mounts in a neutral state of flex when things are tightened up. Stresses beyond the flexibility of the rubber, due to misalignment, is what breaks the gearbox rear plate.

I would strongly advise against the use of any shims at the front mounts. Shims were not the adjustability "built in" at the front mounts I am talking about. I too was talking about "dropping" the engine in place and letting it sit under its own weight before tightening the front mounts. First you do tighten the rear mounts, and raise the front just off the mounts and let engine/gearbox settle; to find where it wants to sit right in the chassis engine mount hole. It is right and left maladjustment at the back mounts that will break the rear plate. Also some fore and aft movement, but that will "adjust" itself into place all by itself. The small amount of misalignment in front translates to a much larger amount of misalignment at back. Or, misalignment right and left at the rear mount can be corrected by adjustment of engine/gearbox location at front mounts.

Begging the question is why did MG design the front mounts with this adjustability potential built in, if it is unimportant?

[Steve Simmons]

I'm not suggesting to shim it. If shimming is necessary, something is wrong. No adjustability is built in. Everything should line up just by setting things in place, as the mounts were welded in place using a jig so that it is all precisely in line with the engine and gearbox mounting points. So long as all hardware is in place before tightening anything, all will be well. Any slight amount of slop at the mounting holes will not affect anything. At the front, the long bolt along with the indexed bottom piece place everything correctly and hold it there. The only movement will be vertical (down) as the rubber mounts get old.

The rear plates tend to crack because they are a cheap, weak alloy. I've seen newly manufactured rear plates that were much stronger. People also crack them by removing the engine alone, without supporting the gearbox. All the weight is then placed on the rear mounts and the plate wasn't designed for that.

[Duncan M]

The original motor mounts rubber bits were made up quite differently from the currently available ones. That collat standing up against rubber in pic below is made from steel and is original

[Steve Simmons]

That original part is much nicer than the current reproductions. Thanks for posting. I would guess that the slightly smaller rubber is to allow for expansion when you squish it down by tightening the long bolts. Especially if the rubber was softer.

[Duncan M]

The part of the rebound rubber that fits up through the hole in chassis mount has the steel shoulder bushing on it, so it never expands to fill the hole. The bottom part does expand when the nut gets tightened and compresses it some. I think a technical drawing found on the net (not sure where it came from originally) allows us to imagine how engine torque movements could damage the rebound rubber commonly sold today, without the steel shoulder bush. They usually have just a flat (horizontal in drawing) washer, or nothing. This drawing does show the shoulder bush (look close) but it does not seem to indicate the steel bush fit into the "hole" is a loose fit. btw, The shoulder bush's I rescued from the original's were re-used on the rebound rubber's I made. The second pic down from top is deceptive to size, but the top part of rebound rubber fits snugly through the center of the steel shoulder bushing. Final 3rd pic, above, shows arrow pointing to where shuloder bush fits up through the hole in chassis front mount.

[XPAGnut]

Well thanks guys, are the two holes for the top mount, where the two little bolts go, as shown in the pictures from Duncan looking about right? That is the adjustment I was thinking, but those holes on my car look much larger. And compare that to the rear mounts where both mating mounts bolt holes are just big enough for the bolt to pass through. Could the rear mounts originally being so easy to bend, that Steve touches on be so that they can be adjusted so as not to put stress on the rear plate? Something to think on I suppose.