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 threads 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

Hi John
Sounds like you have a great plan. Don't see how to improve on it.....but I
am no cab mount expert.....
Good luck
Pete

"John2005" <johnjmechanical@yahoo.com> wrote in message
news:be8a3587-f36f-4335-ac0f-35c155492023@i76g2000hsf.googlegroups.com...
> 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 threads 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

"Pete" <kc5gxc@isp.com> wrote in message
news:1e331$48d6dacc$45126b0d$22068@DIALUPUSA.NET.. .
> Hi John
> Sounds like you have a great plan. Don't see how to improve on it.....but
> I am no cab mount expert.....
> Good luck
> Pete
>
> "John2005" <johnjmechanical@yahoo.com> wrote in message
> news:be8a3587-f36f-4335-ac0f-35c155492023@i76g2000hsf.googlegroups.com...
>> 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 threads 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
>
>

John, If the cab mounts (what you are calling brackets) have rusted
enough for the Bushings (what you are calling cab mounts) to fall
through, then I would be afraid to use backing plates to repair them.
Personally if it were my truck I would torch off the old cab mounts
and weld in new ones.
Regards,
JR

Hi JR,

Thanks for your message.

Two different people on two separate occasions at the body-shop of a
GM dealer said that even if I were to leave the cab mounts as they
are, there is no real safety issue & I could continue to drive the
truck as is .

I'm not a welder myself but I do know enough to know that anyone who
welds to a frame needs to really know what they are doing or else you
wind up with a cracked frame. The dealer wanted about $2,000 to fix it
which is more than I want to spend on the truck at present. The dealer
said they would probably need to take the cab and perhaps the bed off
to make welding of the brackets easier (I guess to weld and then grind
the weld down if needed).

The 1/4" thick steel plate that will be going under the existing stock
steel cab mount brackets is "L" shaped, and I will be bolting the
steel plate to the frame through existing stock holes in the frame
that were present from the factory. I will use three 7/16" OD grade 8
bolts to hold the brackets onto the frame. I have two center holes in
the 1/4" thick steel brackets that I had made, that will receive the
rubber cab mount bushings.

The 1/4" thick steel plate that will be going on top of the existing
stock steel cab mount brackets is angled so that it matches an angeled
portion of the existing stock bracket (it lays on top of the existing
steel bracket like a matching liner). I will bolt the new steel plate
to the angled portion of the existing steel bracket (which still seems
sound after beating it with a hammer) with two 3/8" OD grade 8 bolts.
I am also applying a good rust inhibitor to the areas of the existing
stock steel brackets that rusted through.

I agree that best way to fix it would be to cut the old brackets off
and get some steel brackets from and identical model truck at the junk
yard and then weld them on, but that's not something I would like to
get into right now.

The way I see it, the repair I have in mind would certainly be better
than doing nothing and should actually be pretty strong.

Heck, even if the steel brackets were welded to the frame, the only
thing holding the cab itself and the rubber cab mount bushings onto
the steel frame-brackets is a 12 mm OD 10.9 class bolt and this is
going through a weld nut in the cab floor, and the cab floor is
basically sheet metal. I think this would probably be the weakest
point in the system and not my repair. Luckily, the cab floor is still
in perfect condition, if the cab mounts had rusted through there, I
would really have a problem. Someday, maybe I will have the dealer fix
it just like it was from the factory but right now I think the repair
I have in mind will be good enough.

Please let me know what you think.

Thanks
John

"John2005" <johnjmechanical@yahoo.com> wrote in message
news:b1cacc4a-7e0f-4247-be6b-bed35a541f27@m3g2000hsc.googlegroups.com...
> Hi JR,
>
> Thanks for your message.
>
> Two different people on two separate occasions at the body-shop of a
> GM dealer said that even if I were to leave the cab mounts as they
> are, there is no real safety issue & I could continue to drive the
> truck as is .
>
> I'm not a welder myself but I do know enough to know that anyone who
> welds to a frame needs to really know what they are doing or else you
> wind up with a cracked frame. The dealer wanted about $2,000 to fix it
> which is more than I want to spend on the truck at present. The dealer
> said they would probably need to take the cab and perhaps the bed off
> to make welding of the brackets easier (I guess to weld and then grind
> the weld down if needed).
>
> The 1/4" thick steel plate that will be going under the existing stock
> steel cab mount brackets is "L" shaped, and I will be bolting the
> steel plate to the frame through existing stock holes in the frame
> that were present from the factory. I will use three 7/16" OD grade 8
> bolts to hold the brackets onto the frame. I have two center holes in
> the 1/4" thick steel brackets that I had made, that will receive the
> rubber cab mount bushings.
>
> The 1/4" thick steel plate that will be going on top of the existing
> stock steel cab mount brackets is angled so that it matches an angeled
> portion of the existing stock bracket (it lays on top of the existing
> steel bracket like a matching liner). I will bolt the new steel plate
> to the angled portion of the existing steel bracket (which still seems
> sound after beating it with a hammer) with two 3/8" OD grade 8 bolts.
> I am also applying a good rust inhibitor to the areas of the existing
> stock steel brackets that rusted through.
>
> I agree that best way to fix it would be to cut the old brackets off
> and get some steel brackets from and identical model truck at the junk
> yard and then weld them on, but that's not something I would like to
> get into right now.
>
> The way I see it, the repair I have in mind would certainly be better
> than doing nothing and should actually be pretty strong.
>
> Heck, even if the steel brackets were welded to the frame, the only
> thing holding the cab itself and the rubber cab mount bushings onto
> the steel frame-brackets is a 12 mm OD 10.9 class bolt and this is
> going through a weld nut in the cab floor, and the cab floor is
> basically sheet metal. I think this would probably be the weakest
> point in the system and not my repair. Luckily, the cab floor is still
> in perfect condition, if the cab mounts had rusted through there, I
> would really have a problem. Someday, maybe I will have the dealer fix
> it just like it was from the factory but right now I think the repair
> I have in mind will be good enough.
>
> Please let me know what you think.
>
> Thanks
> John


Good Morning John
If your 1/4" repair plates are bent to match the angles of the old mounts
and are drilled & bolted as you say, then I see no problem with your
repair method. I would wire brush the loose rust and paint everything up
good
with Rustbullet or some similar product before putting it together.
Your repair could last the life of the truck.
Here are some links for future reference.
http://www.rustbullet.com/ProductCart/pc/mainIndex.asp

www.lmctrucks.com

Good Luck,
JR

Hi JR,

Thanks for your additional feedback and the link to rust-bullet.

I have also heard good things about "Zero Rust" http://www.zerorust.com/.
They say zero rust is less expensive and easier to use than products
like POR15 http://www.por15.com/.

I have not used Zero Rust, POR15, or Rust-Bullet yet, but personally,
I have had very good experience with Valuguard Cavity Wax and Wax
based undercoating. This is approved by several Auto Manufacturers and
seems to work very well.

http://www.valugard.net/

At the valuguard website, they sell the cavity wax in an aerosol can
that can be used to spray inside of doors, rockers, fenders, and other
body panels, however, I have found that it also works good underneath
a vehicle body similar to an undercoating and/or underneath
Valuguard's black wax based undercoating. They also sell the wax based
black undercoating in an aerosol but you don't want to spray that
inside of body panels, only the cavity wax goes inside of the body
panels. I know of a local GM dealer that uses this stuff with high
pressure guns. Generally, I'm not crazy about most rust proofing
treatments but in my experience the valuguard products are good.

Take care,
John

I got the rear cab mounts replaced along with the steel brackets I
made and everything went fine for the most part.

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 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

> Any ideas why this may have happened ? If it becomes necessary to
> replace, any suggestions for a quality aftermarket replacement for the
> lower bushing ?
>

When I replaced the floor in my 77 shortbox, I just drilled holes in
some hockey pucks and used them instead, never had a problem.

cheers