GMT400 upgrades. It’s a popular subject for almost anyone that owns a GMT 400 like me. If you’ve ever been over on the GMT400 forum then you already know that’s true. The forum posts confirm that people are asking about GMT400 upgrades. From suspension and engine to interior and accessories, there are a lot of aftermarket parts available for these trucks. In this buyer’s guide, I’ll try and cover the most common and popular GMT400 upgrades.
What is a GMT400?
I think before we talk about GMT 400 upgrades, we need to talk about what a GMT400 is. A GMT 400 is the chassis for the Chevrolet C/K and GMC Sierra full-size pickup trucks that were produced from 1988 to 1998 by General Motors. The C (RWD) and K (4WD/AWD) full-size trucks had this classification for the first time.
Engineering, Build, and Design
Pioneer Engineering on 9 mile Road in Warren, Michigan, was the primary location where the GMT400’s design team at General Motors Truck & Bus worked on the vehicle. A.O. Smith Automotive Products, Dana Holding Corporation, and Tower Automotive constructed the GMT 400 frames. GM produced GMT400 models in Fort Wayne, Indiana, Pontiac, Michigan, Flint, Michigan, Oshawa, Ontario Canada, and Janesville, Wisconsin.
- Gasoline 4.3L-V6, 5.0L-V8, 5.7L-V8, 7.4L-V8
- Diesel 6.2L V8 and 6.5L V8 engines
- TBI fuel injection came on gas engines from 1988 to 1995
- CPI (central point injection) came on the 4.3L-V6, 5.0L-V8, and 5.7L-V8
- SFI came on the 7.4L-V8 from 1996 to 2000 (sequential multiport fuel injection)
GMT400 Upgrades and Performance Parts Guide
If you’ve been on the GMT400 forum, then you’ve inevitably seen a post similar to this example before:
“Hey everyone, I recently purchased a 1994 Chevrolet C1500 4×4 with the 5.7 TBI. What upgrades can I make to the truck to improve its horsepower and performance? I’m considering installing an exhaust system and I’d like to do a lift kit. Does anyone have any suggestions for what I should do?”
If you can relate to the above example in any way, then this GMT400 upgrades and performance parts guide is for you. Below, you will find all the common and popular GMT400 upgrades and performance parts. If I’ve missed something on this list, send me an email and let me know. I want this to be a very complete list and resource of GMT400 owners.
This list is in no particular order.
GMT400 Suspension Upgrade
When it comes to GMT400 suspension upgrades, you’re either going to lift the truck, which is the route I chose, or you’re going to lower the truck. While I do think some people are only interested in repairing and rebuilding the suspension, most seem to be looking to lift or lower these trucks. Lowered trucks are cool, but here in Oregon, lifted trucks are king.
Below, I’ve listed the most common GMT400 suspension upgrades for lifting and lowering your GMT400 series truck.
GMT400 Lift Kits
- Rough Country has the most options for the GMT400, especially if you have a 2WD truck.
- BDS sells a 6″ lift kit for the GMT400 but only the 4WD models.
- Tuff Country has some lift kit options, but only for 4WD models.
- Skyjacker sells a 6″ suspension lift for 4WD trucks.
GMT400 Lowering Kits
- Belltech offers complete lowering kits mostly for the GMT400 2WD
- McGaughy’s has a wide variety of lowering kits for 2WD and 4WD models.
- Maxtrac also has GMT 400 lowering kits, but for 2WD only.
- Ground Force has 2WD lowering kits but their site is hard to navigate.
- RideTech offers three types of suspension upgrades for the 88-98 C1500.
GMT400 Coilover Suspension
- QA1 has coilover suspension kits for 2WD trucks.
- Elevated Concepts has coilovers and suspension parts.
It’s certainly clear from what the above companies offer that most lower 2WD trucks and raise the 4×4’s. I guess I really went against the grain by lifting my C1500. I think only Rough Country has lift kits for the 2WD GMT400.
GMT400 Big 3 Upgrade
Not only will the GMT400 Big 3 Upgrade boost the performance of the audio system in your truck, but it will also improve the performance of the complete electrical system (which can lead to improved fuel economy or additional horsepower). To put it another way, updating the “Big 3” is usually always a wise decision. This is something I definitely intend to do.
What is the Big 3 upgrade?
The Big 3 is named after the three primary power lines that link the alternator, engine block, and chassis to the battery. Most charging systems should be upgraded to reduce voltage dips by using 0 gauge 100 percent oxygen-free copper power wire, which has a larger conductive surface area and can carry more electrons with less resistance. Adding the Big 3 Upgrade will often eliminate light dimming, voltage dips, and assist raise the charging system to or near 14.4 volts.
What are the main benefits?
The Big 3 is a really good addition to the GMT400’s electrical system, and a definite must-have for anyone running high-draw electrical accessories such as amplifiers, winches, aux lights, etc.
Big 3 Upgrade Kits
While I have listed some places that make Big 3 Upgrade Kits below, you could buy and cut your own wire if you’re so inclined.
- GP Car Audio has a Big 3 Kit a nice kit with custom options.
- CE Auto Electric Supply has Big 3 Upgrade Kits specifically for side post batteries.
- Sonic Electronix offers two different kits.
- Sky High Car Audio also has a few different Big 3 Kits.
GMT400 Brake Upgrade
The most popular GMT400 brake upgrades include:
- GMT400 rear disc brake conversion
- GMT800 brakes on GMT400
- GMT 400 brake master cylinder upgrade
- GMT 400 big brake kit
- GMT400 front brake upgrade
- GMT 400 brake booster upgrade
GMT400 rear disc brake conversion
There are many ways you can go when you do a GMT400 rear disc brake conversion. I think a kit is the way to go. Below you will find GMT 400 rear disc brake conversion kits for 5-lug and 6-lug trucks. There are many more kits than what I listed.
- Little Shop MFG has a kit for 5-lug trucks at a reasonable price.
- Little Shop MFG also supplies a kit for the 6-lug GMT400.
- Willwood offers disc brake kits but they aren’t specific on the site.
- Cunningham Machine makes billet aluminum Single Caliper Disc Brake Adapters.
You can find more GMT400 rear disc brake conversion kits on the Summit Racing and Jegs websites. Do a search for “rear disc brake conversion kits” and then put in the year, make, and model of your truck. Seems like you can find more kits on these sites, than on individual websites. You can also do a Google search. Here are the results from a Google search which contains ever more kits and information.
GMT800 brakes on GMT400
Fitting NBS GMT800 brakes to an OBS GMT400 seems like a good idea until you start reading enough forum posts about issues that can come up. I think when and if you decide to upgrade the brakes on your truck, stick to kits made by a reputable company.
GMT400 brake master cylinder upgrade
While this seems to be a popular upgrade, any information you find will be found in online forums.
- Little Shop MFG has a brake master cylinder upgrade. They seem to have brake parts for these trucks.
- Pro Performance has several upgraded master cylinders to choose from.
GMT400 big brake kit
Here are a couple of places offering GMT400 big brake kits.
- Pro Performance offers high-quality kits at an affordable price.
- Little Shop MFG has Willwood big brake kits for 2WD.
- Little Shop MFG of course supplies the 4WD big brake kit as well.
- StopTech offers big brake kits and race kits.
Like the rear disc brake conversion section, there are more kits available on websites like Summit Racing and Jegs versus simply doing a google search on the subject. It seems like Little Shop MFG has been around a long time and has a lot of parts for the GMT400 when it comes to brakes.
GMT400 front brake upgrade
A front brake upgrade and a big brake kit are not the same. A front brake upgrade consists of better brake rotors and pads. A big brake kit increased the diameter of the rotors and require new calipers. Below I’ve listed some helpful resources.
- Power Stop has a Z36 brake upgrade kit.
- EBC has a large selection of front brake upgrade kits.
- Hawk Performance has Talon rotors and performance brake pads.
- SummitRacing.com and Jegs.com websites have the largest one-stop selections.
GMT400 brake booster upgrade
Not a lot of information out there on this at first glance. You have to do some digging within the forums. Seems like if anyone is doing a GMT400 brake booster upgrade they are using NBS parts.
GMT400 NBS Master Cylinder Upgrade
A great low-cost upgrade to the GMT400’s soft spongy brakes. By upgrading to a NBS master cylinder, you may replace the squishy brake pedal with a firm brake pedal. The OBS trucks are notorious for having a mushy brake pedal. There’s lots of information and videos on the subject online.
While this appears to be a great upgrade, make sure you do your homework first. Seems like there are a lot of little nuances to consider. Do a google search on “gmt400 NBS master cylinder upgrade” and you’ll see what I mean.
GMT400 Steering Upgrade
If you do a google search for the term “gmt400 steering upgrade” you’ll see information about the GM IFS steering fix which is said to improve GM steering woes. There’s talk on the forums about upgrading the tie rod ends and YouTube videos on replacing the factory steering box. And of course, there’s the XJ steering shaft upgrade, but I went with the Borgeson 000936 Steering Shaft. Watch the install video.
The most common and popular GMT400 steering upgrades are outlined below.
GMT400 steering shaft upgrade
Seems like there are only two ways to go here with the steering shaft, which is also known as the intermediate shaft. You’re either going to get yourself a Jeep XJ steering shaft circa 1988 to 1994 or you’re going to buy an aftermarket replacement steering shaft as I did from Amazon.
- Cunningham Machine makes a nice U-joint style steering shaft/intermediate shaft.
GMT400 steering box upgrade
There are some options out there for the GMT400 when it comes to steering box upgrades. I suspect even just a new steering box on these old trucks would be an upgrade as most as surely worn out and tired.
Here are two aftermarket alternatives:
- AGR has a sweet GMT400 steering box upgrade for 88-98 OBS C1500.
- Redhead Steering Gears has steering box upgrades for the GMT400.
GMT400 rag joint upgrade
I would think that most of the slop and play in the factory system has to do with the OE rag joint which is rubber and will wear out and decrease in performance over time. The Borgeson 000936 Steering Shaft I purchased on Amazon has that original rag joint setup. The upgrade to this is a steering shaft with a universal joint instead. If I ever upgrade the steering shaft that will be the route I go. Looks like Boreson has a U-joint steering shaft as well.
GMT400 tie rod upgrade
Doing a Google search for “GMT400 tie rod upgrade” doesn’t return any promising results. If you have a K-Series GMT400 Kryptonite Products appears to have some nice-looking suspension parts including tie rod ends. For the 2WD trucks like mine, there’s the Prothane Suspension Polyurethane Suspension Bushing Kit for 88-98 C1500/2500/3500. Nice!
- Cunningham Machine makes steel tie-rod sleeves in raw or black powder coat finish.
GMT400 Lower Steering Column Bearing from Wolff Engineering
I was told that Wolff Engineering out of Texas has the best lower steering column bearing on the market for the GMT400. So I visited the website and sent an email asking a few questions. Within hours I got an email back from Kevin Wolff. Super nice guy and very helpful in telling me what parts fit the GMT 400. The parts look very high-quality!
The M109SS Lower Steering Column Cartridge Bearing covers the years from 1969 to 1995. This bearing will remove any play at the lower column.
In his email, to me, he said he was unsure of 1996, 1997, and 1998 years, but it sure looks like the GM109SS Lower Steering Column Bearing would be compatible. He went on to say that he hasn’t been able to find any solid information on the fitment for those 3 years, but it seems GM did make the column style change in 1996. He usually tells customers to refer to the OEM pictures he has included on the product page before ordering to make sure they select the right one.
GMT400 IFS Upgrades
There doesn’t appear to be much current information online about GMT400 IFS upgrades. In the forums, most of the talk is about doing a SAS (straight axle swap) or some old parts from Cognito Motorsports that don’t seem to exist anymore. At one point Cognito Motorsports sold a truck and SUV Pitman/Idler Arm Support Kit and a pitman arm and idler arm brace kit. I went over to their website, but they don’t seem to be offering any parts for the OBS GMT400.
The GMT400 has an independent front suspension (IFS) instead of the old straight axle. This design is not without its flaws. While the center link, pitman arm, and idler arm are the critical components, worn ball joints, A-arm bushings, and other front suspension components can also contribute to poor steering. Any components found to be excessively worn should be replaced.
GMT400 Headlight Upgrade
Do a Google search for “gmt400 headlight upgrade” and there is no shortage of products to be had. There are many style lights to choose from. You can go the original route as I did and purchase OE replacement lights that just freshen up your truck. Or you can add various custom styles to your GMT400, such as LED headlights and taillights.
Here are a few places to buy lights:
- LMC Truck has a great selection of lights. I got my lights here.
- Amazon is another go-to place for GMT400 light upgrades. I got my factory-style taillights here. See my video
- eBay, is pretty much the same.
- Morimoto headlights are truly high-quality.
- Trucklite and JW Speaker also make excellent 5X7 LED projectors.
- Holley sells 5×7 led that look stock, but use a Morimoto led array that comes in 2 colors.
- Headlight Revolution has an assortment of LED lights.
GMT400 Alternator Upgrade
My 1994 Chevrolet Cheyenne C1500’s stock alternator is rated at 105 Amps. To power today’s latest accessories, additional amperage is required. High amp alternators are direct replacements for the original alternator and provide the charging power required by your truck. There are many reasons for a GMT400 alternator upgrade. Here are a couple of options.
Outside the above options, you always have your local part stores like Advance Auto, Autozone, and Napa Auto Parts for OE replacement alternators. You can also go to the aforementioned Summit Racing and Jegs for more aftermarket style options.
GMT400 Interior Upgrades
The stock interior in my GMT400 certainly isn’t the worst I’ve seen, but it could use some love.
Here’s a shortlist of suppliers that offer GMT400 interior upgrades:
- The GMT400.com Forum has a great resource on OEM and custom interiors.
- LMC Truck has a great selection of reproduction dash and interior components.
- JDS Customs has some unique and useful parts for your GMT400.
- StockInteriors.com has original standard cab headliners. They also have carpet.
- Dakota Digital has some amazing custom instrument clusters.
- Classic Industries is a good resource for interior upgrades and more.
- Cunningham Machine has a billet dash overlay for NON-tach GMT400 88-91.
GMT400 Seat Upgrade
If you do a Google search for “gmt400 seat upgrade” most of the results consist of forum posts talking about seat swaps. So if you’re into the seat swap option, you’ll find plenty of information. If you’re looking for other kinds of GMT400 seat upgrades, LMC Truck is a fantastic one-stop resource. They have some beautiful. bench seats, bucket seats, center consoles, seat warmer kits, and more. There’s also a company called Atomic Performance and Fabrication that makes custom seat brackets if you want a Sparco or Corbeau-style racing seat.
GMT400 Sagging Doors
Door hinge bushings are a frequent point of failure on the GMT400. My passenger door closes properly, but I occasionally have difficulty opening it. GM hung these massive heavy doors using only a quartet of bronze bushings, which quickly wore out, causing the doors to sag and become difficult to open and close. Replacement of worn-out door bushings appears to be the solution. Often, simply replacing the upper hinge bushings will suffice. The outside door handle is a side consequence of hinge deterioration. Individuals would repeatedly yank on the handle to open the door until it snapped.
There is no shortage of GMT400 accessories or customization options. I swapped out my factory steering wheel for a Grant Classic Series Steering Wheel when I was fixing my broken turn signal cancel ring. I bought the steering wheel at Baxter Auto Parts. Here’s the install video I have on YouTube.
If you’re looking for GMT400 accessories, here are some options:
- Grant Steering Wheels has a large selection of steering wheels at good prices.
- For all other interior, cosmetic, and appearance products LMC Truck is the best source.
- Classic Industries will also offer many GMT400 accessories.
- Amazon is a growing source for GMT400 accessories. I bought my tail lights here.
- eBay may also be a good source if you’re looking for something specific.
GMT400 Performance Upgrades
Here are more GMT400 upgrades. There’s the GMT400 performance upgrades category and the sky is really the limit here. I wanted to swap out my 4.3 for a 383 TBI stroker for the longest time. I’ve decided I want to make a racy little 4.3, and I want to do the dual electric fans, underdrive pulleys, and a performance chip. For starters.
Here’s where you can find those GMT400 performance upgrades and more:
- Flex-a-Lite has dual electric fans but there are others out there. Check Summit Racing.
- Jet Performance has computer and transmission chips, pulley sets, and a throttle body spacer.
- Performance Distributor makes a 4.3 Firepower Tune-Up Kit. It’s excellent. I installed it.
- For the GMT400 with a V8 engine, you can readily find performance parts at Jegs and Summit Racing.
- The forums offer good information on engine swaps, like LS engine swaps.
- Flowmaster has performance exhaust systems. I installed a 40-series on the truck.
- If you have a V8 engine, Hooker has nice long tube headers.
- Edelbrock has the Pro Flo 14″ Air Cleaner. See the install
- ACDelco Rapid Fire Plugs seem nice a natural match for the GMT400.
- OPTIMA Batteries make aftermarket performance batteries.
- Pro Performance has an assortment of beautiful aluminum radiators. Here’s my install video of a stock radiator.
- Pro Performance has many suspension and brake parts for the 88-98 GM CK Chevy / GMC pickup.
- GearStar has GM performance transmissions.
- American Powertrain makes 5 and 6-speed manual transmissions.
- Sinister Performance does GM computer tuning.
GMT400 TBI Upgrade
Nobody is making parts for the TBI. Not surprisingly, the forums like GMT400.com are your best resource when considering a GMT400 TBI upgrade. There really aren’t TBI upgrades to be had if you do enough reading unless you’re going to upgrade to electronic fuel injection (EFI). I have the stock TBI setup in my truck and I have not had any issues. I think the best thing for an old TBI unit is TBI rebuild kit which is a project I intend to do.
Here are the common GMT400 TBI upgrades:
- TBI to EFI – Many aftermarket kits exist like the Edelbrock EFI
- TBI to Carburator – The forums are your best how-to resource
- Rebuild TBI unit (rebuild kits available at many places)
- Here’s a great article on recommended TBI mods.
Okay, so this is now the master list of GMT400 upgrades. I will be adding to this list as I learn more things. If you have something you’d like to add to this list, email me.