Getting your obs chevy ls swap motor mounts lined up correctly is arguably the most important part of the entire build. If you've spent any time on the forums or scrolling through social media lately, you know the OBS (Old Body Style) Chevy—those 1988 to 1998 C/K trucks—is the undisputed king of the custom truck world right now. They have that perfect boxy look, they're still relatively easy to find, and they're surprisingly comfortable to drive. However, the original small block or V6 engines they came with are usually a bit tired by now.
The LS engine is the obvious solution for more power and reliability, but it isn't a direct "bolt-in" affair in the way a 350 to a 305 swap used to be. The block design is different, the mounting points on the engine are moved, and the frame on these trucks can be a bit picky about where things sit. That's why picking the right mounts is about more than just finding something that fits the holes; it's about making sure your entire drivetrain works as a cohesive unit.
The Struggle with Fixed vs. Adjustable Mounts
When you start shopping for obs chevy ls swap motor mounts, you're going to run into two main camps: fixed position plates and adjustable "slider" mounts. This is usually where the big debates happen in the garage.
Fixed mounts are great because they take the guesswork out of the equation. You bolt them to the engine, bolt the engine to the frame, and you're done. The problem is that not every OBS truck is built exactly the same, and not every LS swap uses the same transmission. If you're using a 4L60E, your spacing might be different than if you're trying to jam a T56 manual in there. Fixed mounts can sometimes leave you with no room at the firewall or, worse, leave your engine sitting too far forward, making the cooling fans a nightmare to install.
Adjustable slider mounts, on the other hand, are a lifesaver for the "while I'm at it" builder. These mounts allow you to slide the engine forward or backward, usually by about an inch or two. This is huge when you're trying to clear the AC compressor against the frame or making sure the oil pan doesn't hit the crossmember. I've seen plenty of guys get halfway through a swap only to realize their distributor-less ignition coils are hitting the firewall. With sliders, you just nudge the whole thing forward an inch, and suddenly you have all the room in the world.
Why Placement Affects Everything Else
It's easy to think of the mounts as just brackets, but they really dictate the geometry of the entire truck. If the engine is sitting too high, you might have trouble getting the hood to close, especially if you're using a tall Vortec truck intake manifold. If it's too low, your oil pan becomes a target for every speed bump and pothole in town.
Oil Pan Clearance Concerns
The stock truck oil pan that comes on most 5.3L or 6.0L engines is notoriously deep. On an OBS Chevy, that pan likes to hang down way below the front crossmember. If your obs chevy ls swap motor mounts don't lift the engine just right—or if you don't use the right aftermarket pan—you're going to have a bad time. Most guys end up switching to a Muscle Car pan or a Holley 302-1 style pan to keep things tucked up. Your choice of mounts needs to account for this clearance. You don't want to find out after the engine is torqued down that your oil pan is resting on the frame.
Transmission Tunnel and Firewall
The OBS trucks have a decent amount of room, but it's not infinite. If you push the engine back too far to get better weight distribution, you might find that you have to start "massaging" the transmission tunnel with a large hammer. Or, if you're running a big-body transmission like a 4L80E, the fit gets tight real fast. Good mounts let you find that "Goldilocks" zone where the engine is far enough back to clear the radiator but far enough forward to keep the transmission from rubbing the floorboards.
Managing the Driveline Angle
One thing that doesn't get talked about enough is driveline angle. You can't just throw the engine in there at a weird tilt and expect it to work. If the tail of the transmission is pointing too far down or the engine is tilted too far back because of the mounts, you're going to deal with some nasty vibrations.
In a perfect world, your engine and transmission should be tilted back at about a 3-degree angle. This aligns with the rear differential to ensure the U-joints live a long, happy life. Cheap, poorly designed obs chevy ls swap motor mounts can sometimes throw this angle off, making it impossible to get the truck smooth at highway speeds. It's worth spending a little more on a reputable brand that has actually tested their geometry rather than just buying the cheapest plates you find on an auction site.
Polyurethane vs. Rubber Bushings
Another thing to consider is what's actually cushioning the engine. Most swap mounts use either the factory style rubber clamshells or a new polyurethane bushing.
- Rubber: These are great for a daily driver. They soak up the vibrations and keep the cabin quiet. If you're just building a cruiser to take to the hardware store or car shows, stick with rubber. Your mirrors won't shake, and your teeth won't rattle.
- Polyurethane: These are much stiffer. They're excellent for performance because they don't allow the engine to twist as much under load. This means more power gets to the wheels faster. The downside? You're going to feel the engine a lot more. If you have a big cam with a lot of chop, a poly mount will make the whole truck dance at a red light.
Most guys doing an LS swap in an OBS truck tend to go with poly because it's more durable and stands up to the heat and torque of the LS better than 30-year-old rubber. Just know what you're signing up for in terms of "road feel."
Installation Tips for a Painless Swap
If you're doing this on your driveway, there are a few tricks to make the obs chevy ls swap motor mounts work with you instead of against you. First, don't tighten anything down until every bolt is started. I know it's tempting to cinch down the frame brackets as soon as they're in, but you need that little bit of "wiggle room" to get the long through-bolts into the engine mounts.
Also, keep a floor jack under the oil pan (with a block of wood to protect it) while you're lining things up. Sometimes the engine needs to be tilted just a hair to get the holes to line up. And please, for the love of all things mechanical, check your header clearance before you finish. Some swap headers don't play nice with certain mounts, and it's a lot easier to swap a bracket now than it is to pull a header back out once the steering shaft is in the way.
Final Thoughts on Choosing Your Setup
At the end of the day, your obs chevy ls swap motor mounts are the foundation of your build. It's the literal link between your frame and your power. While it's tempting to save a few bucks here, this is one area where quality really pays off in the long run. Whether you go with adjustable sliders for the flexibility or fixed plates for the simplicity, just make sure you're thinking about the "big picture"—oil pan clearance, driveline angles, and how much vibration you're willing to live with.
Once you get those mounts bolted in and that LS drops into place for the final time, everything else—the wiring, the plumbing, the tuning—starts to feel a lot more manageable. There's nothing quite like seeing a modern engine sitting perfectly in a classic OBS engine bay, looking like it belonged there from the factory. Get the mounts right, and the rest will follow.