Replacement of rear cab mounts on a 1991 Chevy K1500 short bed pickup

Hi everyone,
I have a 1991 Chevy K1500 short bed pickup. I'm replacing the rear cab mounts (rubber bushings). I have a Chilton and it looks like a fairly
straight forward job, but I have never replaced cab mounts before.
Does anyone have any tips or suggestions before I do the job that might make it go smoother ?
I purchased the new cab mounts from the dealer, along with two new cab mount bolts. The steel brackets that the rear cab mounts are bolted to have rusted through, (rubber cab mount bushings are falling through the steel brackets) and I will be putting a 1/4" inch thick steel plate on the top and bottom of the existing steel brackets.
I purchased two 10.9 class 12 mm bolts that are 1/2 inch longer than the stock cab mount bolts, in case the stock cab mount bolts are no longer long enough, since I added 1/2" inch to the stack-up with the two 1/4" thick plates.
One thing I noticed about the stock cab mount bolts (GM part # 15704266) is that they have what looks like a yellow nylon patch on about a 1 inch long portion of the threads about 1/2" from the end of the bolt, which I assume is to prevent back-out of the bolts once their torqued down.
The very end of the stock cab bolts also have some small grooves running axially through the first three threads. There are 4 or 5 grooves evenly spaced radially around the bolt. The very end of the bolt almost looks like it's supposed to clean the threads of the weld nut on the cab floor, almost like a thread chaser.
If I have to use the longer bolts that I purchased, can I just put some blue removable loc-tite on the end of the bolt to prevent back- out ? I also have two 10.9 class split lock washers, along with some regular round washers, I could use under the heads of the bolts as well.
The main thing that worries me is that when I go to take the existing cab mount bolts out, they may be rusted to the weld nut on the cab floor and either strip or break off the weld nut.
When I go to put the new cab bolts in, I had considered putting some grease on them to keep the thereads from rusting to the weld nut on the cab floor, however, that would seem to defeat the purpose of the nylon patch on the stock bolts, and if I use the new longer bolts that I purchased, I could not use loc-tite on the threads if I use grease, although the loc-tite itself could possibly prevent rust.
Depending on which bolts I use, do I have any options for preventing rust where the bolts thread into the weld nuts on the cab floor ? Do I need to worry about it ?
The rest of the truck is in great shape, and the funny thing is, the front cab mounts are almost in perfect condition. I'm not sure why the rear cab mount brackets rusted through, especially since they are located further from the road (less salt) than the front cab mounts.
Why do these cab mounts rust through, the metal is fairly thick just like the frame ? Do the rubber mounts hold in moisture and/or salt ? I have always tired to keep the salt hosed off the undercarriage in the winter. There were times I drove through some deep water, but if that was the problem it did not effect the front cab mounts.
In addition to any feedback you may have on my questions, I would appreciate any general advice that may make the job go easier and/or come out better.
Thanks John
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I got the rear cab mounts replaced along with the steel brackets I made and everything went fine for the most part. The cab mount bolts did not have a bit of rust on them where they entered the weld nuts (I was very surprised).
After replacing the upper and lower rubber bushings, I centered the round steel retainer under the lower rubber bushing as best as possible, then I torqued the bolt down to 55 foot pounds just like the GM service manual said, however, it seemed to smash the lower bushing much thinner than the original bushing. Other than the fact that it seemed at least half as thin as the original bushing, everything looked fine, the right side looked perfect and the left side lower bushing had a 10 or 15 degree portion of the rubber that seemed to slip or walk out from under the retainer, but nothing serious. The new upper mounts seemed perfect, cab at correct height, etc..
After driving, there was no change to the left side, but the right side of the lower bushing had a fair portion that seemed to slip or walk out from under the steel retainer. As long as it does not get any worse any time soon, I think it will be OK, it did not slip all the way out, I can't see inside of the bushing or anything, but it does have a portion (probably at least 45 degrees) where the retainer either cut into the bushing or that part of the bushing just walked out or "bulged out" from under the retainer. Probably more likely that the retainer cut into the bushing since I can't see inside of the bushing, if a portion of the bushing walked out, I could probably see inside of the bushing. Perhaps a little cutting of the retainer and a little walking of the bushing occurred.
I wonder if I either was sold the wrong GM lower bushing by the dealer, or if the GM part changed a little over time and is no longer an exact replacement (I had that happen with the connectors of a coolant temperature sensor in the past), a NAPA aftermarket was a better fit and better choice.
I doubt GM makes a heavy duty bushing. If it gets worse I will have to check into an aftermarket replacement or perhaps a Urethane aftermarket bushing since those are tougher than rubber.
After everything was torqued down, I did spray some valugard rust inhibitor on the parts, the stuff does penetrate, but I can't believe that it could get between the steel retainer and the lower bushing (after it had been torqued to 55 foot pounds) and then cause a portion of the bushing to slip out. A 12 mm OD cab bolt torqued to 55 foot pounds is exerting a fair amount of force, as evidenced by how much it smashed the rubber bushing.
Any ideas why this may have happened ? If it becomes necessary to replace, any suggestions for a quality aftermarket replacement for the lower bushing ?
Thanks John
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John2005 wrote:

Didn't you add some steel to the mounts? That would be the reason for the extra deformation of the bushings. They are designed to only crush down on the thickness of the factory body mount. Since you can remove the new parts how about pulling the plates and just welding one to the remains of the factory mounts.
--
Steve W.
Near Cooperstown, New York
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Hi Steve,
Are you saying that the steel piece which is integral with the upper cab mount rubber bushing, and which extends down from the underside of the upper bushing, is supposed to "bottom out" on the round steel retainer that is under the lower rubber cab mount bushing, when the cab mount bolt is torqued down ?
If that is the case, I did not know that the retainer under the lower rubber bushing was supposed to make contact with the lower steel portion of the upper bushing, otherwise, I would have added a 1/2" inch thick spacer in-between the lower bushing retainer and the lower steel portion of the upper bushing, to account for the thickness of the two 1/4" thick plates and/or brackets that I added.
I'm not going to do any welding, but if the lower bushings deteriorate, I may add a spacer or sleeve inside the lower bushing, and/or go with a polyurethane lower bushing which should be stronger.
When I torqued the bolt down, I put some blue loc-tite ( # 2240) on the threads, and also used a grade 8 split lock washer under the bolt head along with a regular round grade 8 washer. As long as the bolt's don't loosen, and the lower bushings don't deteriorate too bad, I'm just going to leave it like it is, I'm tired of crawling on the ground and fooling with it. I need a lift.
Thanks John
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