I recently helped a buddy throw a bbc front motor plate onto his Chevelle, and it reminded me just how much of a difference these things make when you're pushing serious horsepower. If you've ever looked under the hood of a serious drag car or a high-end pro-street build, you've probably seen that thick slab of aluminum sandwiched between the engine and the accessories. It looks cool, sure, but it's doing a whole lot more than just looking pretty.
When you're dealing with the sheer torque of a Big Block Chevy, those factory-style rubber or even polyurethane side mounts start to show their weaknesses pretty fast. The engine wants to twist, and that energy that should be going to the rear tires is instead being wasted on flexing the frame or tearing up mount bushings. That's where a solid motor plate comes into play.
Why Switch From Side Mounts?
The most obvious reason people move to a bbc front motor plate is for strength. Think about the physics for a second. A standard motor mount attaches to a relatively small area on the side of the block. Under heavy load, all that force is concentrated on those few bolts. A motor plate, however, spreads that load across the entire front of the engine block and ties it directly into the frame rails.
Beyond just the strength, it also cleans up the engine bay significantly. By getting rid of the bulky side mounts, you suddenly have a ton of room for headers. If you've ever tried to snake a set of 2 1/8-inch primary tubes around a factory motor mount, you know exactly what kind of nightmare that is. With a plate, that area is wide open, making header installation and spark plug changes way less of a headache.
Another thing people don't always consider is engine height and setback. When you're using a motor plate, you aren't beholden to where the factory put the mounting pads on the frame. If you need to drop the engine an inch to clear a low-profile hood, or push it back two inches for better weight distribution, the plate makes that possible. You just trim the plate to fit your specific chassis width and weld your tabs where you want them.
Picking the Right Plate Thickness
Most of the plates you'll find on the market are made from 6061-T6 aluminum. You'll usually see them in two common thicknesses: 1/4-inch (.250") and 3/8-inch (.375"). Choosing between them usually depends on how much power you're making and how much weight you're trying to save.
For a lot of street-strip guys, the 1/4-inch bbc front motor plate is plenty. It's lighter and easier to work with, and it's strong enough for most naturally aspirated builds. But, if you're running a big blower or a massive turbo setup that puts incredible stress on the front of the crank, moving up to the 3/8-inch plate is a smart move. It adds a bit of weight, but the added rigidity is worth the peace of mind. You don't want the front of your engine "walking" around when you're under full boost.
The Installation Process
Installing a bbc front motor plate isn't exactly a "bolt-on and go" afternoon project. It requires some patience, a good level, and probably a fair amount of measuring. Since these plates are usually sold as "universal" or "trim-to-fit," you're going to be doing some cutting.
Squaring the Engine
The most critical part of the whole job is making sure the engine is square in the car. I like to start by getting the car up on jack stands and making sure the chassis itself is level. Then, you drop the engine in (usually supported by a floor jack or a hoist) and start measuring from the frame rails to the crankshaft centerline.
If the engine is crooked, your driveshaft angles are going to be a mess, and you'll end up with vibrations that'll shake your teeth out. Take your time here. Use a plumb bob if you have to. Once you have the engine exactly where you want it—centered and at the right height—you can mark where the plate needs to be trimmed to meet the frame.
Trimming and Notching
Most guys use a band saw or a jigsaw with a fine-tooth metal blade to trim the outer edges of the plate. Aluminum cuts pretty easily, but it can gum up a blade fast, so a little bit of lubricant or even some WD-40 helps keep things moving smoothly.
Don't forget to check for clearance around your steering box or rack-and-pinion. Sometimes the plate needs a little "clearancing" (which is just a fancy way of saying you need to cut a notch in it) to make room for the steering shaft. Just make sure you aren't removing so much material that you compromise the structural integrity of the plate.
Dealing With Water Pumps and Accessories
One of the "fun" surprises people run into with a bbc front motor plate is that it changes the spacing for everything on the front of the engine. Since the plate is sandwiched between the block and the water pump, your water pump is now sitting 1/4-inch or 3/8-inch further forward than it used to be.
This means your pulleys won't align anymore. You'll either need to buy spacers for the crank pulley and the alternator, or you'll need to get a dedicated accessory drive kit designed for use with a motor plate. Also, don't forget that you'll need two sets of water pump gaskets—one for the block-to-plate surface and one for the plate-to-pump surface. Most guys also use a little bit of RTV just to make sure there are no annoying drips later on.
Don't Forget the Mid-Plate
I can't stress this enough: if you're running a front motor plate, you must run a mid-plate as well. The mid-plate sits between the engine block and the transmission bellhousing.
Think about it this way: if you only support the engine at the very front (the plate) and the very back (the transmission mount), the whole middle section of the drivetrain is basically a long lever. Under torque, the engine and trans will want to flex at the bellhousing. This is a great way to crack a transmission case or snap a bellhousing clean off. A mid-plate supports the back of the engine, creating a solid "bridge" that keeps everything in line.
Maintaining Grounding
Since aluminum isn't as good of a conductor as the steel frame, and you might be using painted or powder-coated tabs, you need to be really careful about your electrical grounds. I've seen guys pull their hair out trying to figure out a "mysterious" starter issue, only to realize the engine wasn't properly grounded to the chassis because the motor plate was acting as an insulator.
Always run a heavy-duty ground strap directly from the engine block to the frame. Don't rely on the motor plate to carry that current. It's a simple step that saves a lot of frustration down the road.
Is It Worth It for a Street Car?
This is a question I get a lot. If you're just cruising to the local car show and occasionally hitting the gas at a green light, a bbc front motor plate might be overkill. It does transfer a lot more vibration and noise into the cabin since there's no rubber to soak up the engine's harmonics. You're going to feel the engine hum through the steering wheel and the floorboards.
However, if you're building a high-performance street machine that sees some track time, it's a fantastic upgrade. It simplifies the engine bay, provides a rock-solid mounting solution, and honestly, it just looks "pro." There's something about the look of a big block held in place by a massive plate that says you mean business.
At the end of the day, installing a motor plate is one of those jobs that rewards the "measure twice, cut once" philosophy. It's a bit of a chore to get everything lined up and trimmed perfectly, but once it's in, you'll never have to worry about a torn motor mount again. Just keep an eye on your pulley alignment and make sure you've got those grounds sorted, and you'll be good to go.