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Installing GMT900 cab mounts in GMT400s and GMT800s

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by: Aloicious

Oct. 13, 2011

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Do you want to improve the ride quality in your GMT400 or GMT800 truck? Here we’ll be examining how to install newer GMT900 rear cab mounts onto a 1996 GMT400 extended cab truck. However this should work for most all GMT400’s and GMT800’s that have 6 total cab mounts. (which would be extended cabs and crew cabs) As of right now I do not believe this will work on regular cab trucks which only have 4 total cab mounts. However if someone with a regular cab truck would like to try this, please let me know how it goes.

First off, let me say that I performed this install with my 1996 extended cab C1500, and although I believe it will work with most other GMT400’s and GMT800’s, I have not tried it on anything else, so Please let us know if you attempt this with your truck and how it goes so we can keep this article as updated as possible.

Well, let’s look at what you need to get. This swap involves the rear-most 2 cab mounts (one on each side of the truck). We’ll need a total of 2 of the actual cab mounts from a GMT900 truck (one on each side), we’ll also need 2 lower mount cushions (again one for each side), as well as 2 cushion washers (one per side)…we’ll re-use the stock GMT400 mount bolts, and we’ll also keep the front cab mounts as is.

Here are the GM part numbers as well as the total quantity you’ll need to get again:
Upper Mount 25791031 - Qty 2
Lower Mount cushion 25791032 - Qty 2
Washer 15854745 - Qty 2

The upper mount however has been superseded by a different part number of 25994881…below is a picture of both part numbers, the mount on the left is the original P/N 25791031, the mount on the right is the newer P/N 25994881. You can see where the red arrows are pointing which shows the difference between the two. In this swap I used P/N 25994881 (the one on the right in the image) for both sides. I received 1 of the #25791031 mounts and found it difficult to find another one. Either part number should work though.

GMT900 body mount differences

As you can see they’re slightly different, I feel that the newer part # 25994881 cradles the mounting bolt a little better, and helps align it somewhat, but in reality either one should work fine.

Here is what the lower mount cushion, and washer you’ll also need look like:
GMT900 body mount cusion and washer

The total cost I paid for all the parts needed was $120 shipped to my door, it may be slightly more or less depending on how much you shop around.

So now let’s look at the stock mounts. Here are the front 2 mounts on the driver’s side (the passenger side will be identical except it won’t have the parking brake cable running below it.)

Front mount:
GMT400 front Cab Mount

Middle mount:
GMT400 Cab Middle Mount And here’s the stock rear mount, this is the one we’ll be replacing:

GMT400 rear cab mount

Before we start removing stuff let’s get the tools we’ll need, you’ll need a 15mm socket, possibly a pneumatic impact wrench, possibly a small grinding tool, possibly a hammer, torque wrench, floor jack, block of wood or something to extend the jack’s range, wheel chocks. We’ll talk about all the steps to replace one side, you’ll just simply repeat everything for the other side of the truck.

So let’s chock the wheels before we start, also make sure your parking brake is fully applied (safety first). Now go around and loosen all 6 of the cab mounting bolts with the 15mm socket (you may need a breaker bar, or impact wrench to get them off, mine were on there pretty good), don’t remove any of the front 4 bolts, just loosen them about 1” with the bolt still attached. You can remove the rear 2 mounting bolts, (it may be difficult to remove the driver’s side bolt/washer with the E-brake cable in the way, but you should be able to shimmy it out of there).

Now get your jack in place and ready to lift the rear portion of the cab off the frame. You will likely need a good block of wood or something to extend the height of the jack, my jack did not have enough range to lift the cab up enough, but the size of the block, and if it is needed will depend on your jack. You’ll want to find a safe place to have the jack so it won’t damage the cab floor. Once you have a good sturdy and safe jack placement, start raising up the cab (not the frame, only the cab), it will lift the cab off the mounts, you don’t need to go up too far, but enough to get the rear upper mounts out, that is why we loosened the front 4 mounts about 1” to allow this movement, but we kept the bolts in place to help keep everything aligned too.

Once you get the upper cab mounts out you may have a small metal ring in the hole of the frame. It is part of the mount and needs to be removed. In this image you can see this ring in place with the mount itself removed:

GMT400 rear cab mount removed

I had to tap it out with a hammer. Once you get that ring removed you’ll have a hole with 2 flat edges in the frame, the flat edges of the hole are to keep the mount solidly in place. You can see the ring removed here:

GMT400 rear cab mount removed also

I had some rust that was preventing the new mount from sliding in very well, and the paint on the mount itself was also pretty thick and preventing it from seating, with a small grinding tool, like a dremel, I cleaned the hole in the frame, and removed some of the paint from the mount itself, it was still not going in well, so I clearanced the hole just a tiny bit, it’s likely you will need to do the same, just be aware not to take too much material off, you want the mount to fit snugly in the hole. Here you can see the upper mount fitting through the frame hole correctly:

GMT900 mount on GMT400 frame

After you’ve got both upper mounts in place you can lower the cab back down and make sure it is sitting on all the mounts correctly. Now you take the lower mount cushion and washer with the original mount bolts and install them. The lower cushion will have a flat side and a rounded side, the flat side will sit against the frame, and the rounded side will get cradled in the washer which has a crevice for it. You may need to push a bit to get the bolt to start threading, but once it’s in it’ll compress the lower mount cushion a good amount.

Snug up all 6 mounting bolts lightly, then get out the torque wrench. The specs for GMT400 mounting bolts are roughly 55-60ft lbs, where GMT900 mounting bolts are 44ftlbs plus 170 degrees. They both should be all in all pretty similar, I torqued mine to 60ftlbs in 3 passes going 25ftlbs, 50ftlbs, and finally 60ftlbs for all 6 mounts. I will go re-check them at 60ftlbs after a week or so of driving to ensure they are staying.

Here is the fully installed driver’s side GMT900 mount:

GMT900 mount installed on GMT400

GMT900 mount installed on GMT400 also

Here is the passenger side, that white paper is just part of the factory sticker with the part # on it that I couldn’t get all the way off, I didn’t worry too much about it being on there:

GMT900 mount installed on GMT400 again

Here are the parts of the original mount that were removed:

Parts of old GMt400 cab mount that was removed

The original mounts were fairly rusty, but the rubber was in good shape, so the improvements from this swap are more than just from replacing old mounts.

These GMT900 mounts help a with ride quality and are an improvement over the stock GMT400 (or 800) mounts. Will it make your truck drive like a Bentley? No, but for their relatively low cost, they do improve the ride a worthwhile amount, the most benefit you’ll feel most in daily driving will be over things like the expansion joints in the highway pavement, etc. it is a good upgrade, I wouldn’t hesitate to do it again.

Here is a HUGE thread about people who have installed these same mounts, mostly on GMT800 diesel trucks. you can read their take on it as well.
http://www.dieselplace.com/forum/showthread.php?t=272124

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