How to Install the Underbed Rail and Installation Kit for Demco Hijacker 2005 GMC Sierra 3500  

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Expert Reply:

I can walk you through the installation of the Underbed Rail and Installation Kit for Demco Hijacker UMS 5th Wheel and Gooseneck Trailer Hitches part # DM8551004 but we have not had the chance to install this on a 2005 GMC Sierra 3500 yet. A standard hand drill can be used to drill the holes through the frame rail.

On some models there will be a heat shield that is attached to the bottom of the truck box that will have to have a section of it removed to make clearance for the base rail(s).

1. If the shield is below the level of the top of the frame, the base rail will have room to span across from frame rail to frame rail.

2. If the heat shield is above the level of the top of the frame, the shield must be cut and removed before the base rails can be installed.

3. Cut the heat shield just rear of the box support cross member that is at the front of the wheel well of the truck. Remove the screws that hold the rear section of the heat shield in place. A #25 torx head driver or and/or a 10mm wrench or socket will be required to remove these screws.

4. Remove the heat shield.

Use one of the following two methods to install the base rails.

1.

A) Remove the mounting bolts that attach the box to the truck frame on one side only. Raise and block the box high enough so the base rails can be inserted (with locking pin holes up) and span across and rest on top of each side of the frame. Lower and bolt the box back to the frame. Position base rails under holes drilled in bed.

OR

B) Locate a spot on both sides of the box support cross members on either side of the truck and cut and bend up a 3 1/2” wide section of the sheet metal lip where the bed and the fender well of the box join. Insert base rails (with locking pin holes up) and span across and rest on top of each side of the frame. Bend the sheet metal back down and position rails under holes drilled in bed.

2. With base rails in position below the bed and the base rail holes centered under the holes that were drilled in the bed of the truck, position one of the 1/8” thick shims under each end of the base rails between them and the top of the frame. Drop a 5/8” x 1 1/2” bolt through the top of the end of the base rail and shim. Re-install and lock the hitch assembly into the base rails from the topside of the box.

3. Measure and center the entire hitch assembly side to side and front to back so that it is as close to center of the holes in the bed.

4. Attach the frame brackets to the base rails with the portion of the frame bracket that is notched
out towards the cab of the truck. Fasten with the 5/8” bolts that were inserted in the mounting holes and secure with a 5/8” flat washer and 5/8” lock nut.

5. Tighten the bolts to the base rails ensuring that the frame bracket is up against the truck frame.

6. Using the holes in the frame bracket plates as a guide, drill the frame in the four mounting holes on each frame bracket with a 1/2” or 33/64” drill bit. You may want to drill small pilot holes to make drilling easier. It is best to install a 1/2” x 1 1/2” bolt (#04051), washer and lock nut after each hole is drilled to ensure there is no shifting of the hitch while drilling the next hole(s)

7. Tighten all bolts.

8. Re-install the rear wheels (if removed), double check all fasteners for tightness, and lower truck (if raised).

9. Unlock and lift the hitch assembly out of the box to make sure it can be removed and inserted with relative ease. If there was any shifting during installation that caused any binding, simply loosen the 4 bolts that attach the base rail to the frame brackets and realign the base rail and retighten.

10.Ensure the side rail locking pins are turned and secured in the locked position with the safety pins. Install the hitch head to the saddle bracket.

expert reply by: Conner L

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