PRO TIPS Q&A: UNIBILT CONVERSION

Peterbilt Unibilt cab supsnesion a popular upgrade for older customized Petes.

I would like to convert a 1990 Peterbilt 379 into a Unibilt. I see 359′s at shows that have been done. Do I have to reinforce the cab and sleeper or just bolt the brackets in.—Ryan Judd

I have a ’93 379 Pete and was wondering if there is a kit to make it Unibilt. I know anything can be done just wondering what all modifications it would take. — Justin Herrington via Internet

A common question among our Custom Rigs’ readers. So we went to a couple of the experts for their advice. Here’s what they ahd to say:

“The simplest way to get the Unibilt effect is to get the vin number from a Unibilt then go to a local Pete dealer and use the vin number to order a cab and sleeper “walk-thru” panel and boot to fit that particular truck. You will also need hucks to install the new panels.

Then get a cab mate for your truck cab and sleeper. With parts in-hand, install the rear cab panel and boot along with the cab mate’s air-ride system. This will give you the Unibilt feel but with a much smoother ride. If want Unibilt all the way, then you’ll need to get a rear cross member off a unibilt—or fab something up for the additional rear sleeper and cab support.

In addition, Peterbilt (www.peterbilt.com/uploaded/resources/Peterbilt_Essentials-Module3-Sleepers.pdf ) used heavier frame braces going from the frame up the  cowl / “A” pillars to the roof and added heavier stringers in both cab top and sleeper roof to help distribute the load of the Unibilt design more evenly across the frame.” 
– Adam Martin, Owner Colorado Custom Chrome; (970)284-6552

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“You will have to do much more than ‘just bolt the brackets in’, to make it a unitized cab/sleeper assy-like the ones you see on the 1994 and newer Peterbilts.

The Unibilt cabs have additional structure and reinforcements in the cab & sleeper corners, there is a steel bar that runs completely across the top of the sleeper opening to provide extra rigidity and strength and Unibilt cabs have a steel windshield mask on the cab, rather than the aluminum mask like they used 1988-1993.

All of this is part of the structural integrity package that allows the cab & bunk to be bolted together with no ‘under cab/under bunk’ support, other than the air bags and suspension under the rear of the sleeper.

I suggest 2 options: Just install the large unibilt opening between the cab & sleeper and leave the cab/bunk mounted solid to the frame. This is fairly quick, easy and not too costly. The ability to slide the driver’s seat back and gaining the larger opening really ‘changes’ the cab interior. It will seem like an entirely different truck to you.

The other option is to install the large unibilt opening, install the unibilt bracketry with some extra interior structural support in the corners of the cab & sleeper, and lastly run a ‘junior I beam’ (approx 3 ½” tall, can be alum or steel) from the front cab mounts, completely under the cab & bunk to the rear sleeper air suspension which you will have to somewhat modify.

We have used the sleeper suspension out of an International or KW when doing jobs like this. It works better for this application than the ‘outboard’ Peterbilt type, and it is all hidden under the bunk! This is a costly, time-consuming upgrade that requires a good amount of fabrication. But, the truck will ‘ride like a dream when you’re  done!” – Bryan “BOSSMAN” Martin, Chrome Shop Mafia