If you're tired of your engine flopping around like a fish out of water every time you shift, making some diy polyurethane motor mounts is one of the most cost-effective ways to stiffen things up. It's a classic weekend project for anyone who wants better throttle response and a more "connected" feel to the car without spending hundreds on fancy aftermarket racing mounts. Let's be real, factory rubber mounts are designed for comfort, not performance, and they eventually turn into a mushy mess that lets your engine move way more than it should.
By filling or pouring your own mounts, you get to control exactly how stiff they are. You can go for something street-friendly or something so stiff it'll rattle the change right out of your cup holder. It's a bit messy and takes some patience, but the results are usually worth the effort.
Why Bother With Polyurethane?
Most cars come off the assembly line with hollow or very soft rubber mounts. Manufacturers do this because they want to isolate the cabin from engine vibrations. It makes the car feel smooth and quiet, which is great for a daily driver. But if you're actually into driving, that softness translates to "engine slop." When you hit the gas, the engine twists before the power actually makes it to the wheels. This causes wheel hop, vague shifting, and just a general feeling of disconnectedness.
Polyurethane is the middle ground. It's much stiffer than rubber but still has a tiny bit of give compared to solid metal mounts. Solid mounts are great for dedicated track cars, but they'll literally vibrate your dashboard apart on the highway. With diy polyurethane motor mounts, you can choose the hardness (measured in Shore A) that fits your specific needs.
Plus, there's the cost factor. A set of performance mounts for some cars can be $300 or more. A DIY kit or a tub of liquid polyurethane usually costs about $30 to $50. If you're on a budget, this is a no-brainer.
Choosing Your Hardness: The Shore Scale
Before you go buying materials, you need to understand Shore hardness. This is how we measure how squishy or solid the polyurethane is. If you get this wrong, you're going to hate your car every time you start it up.
- 60A (Soft): This is roughly equivalent to a brand-new, high-quality rubber mount. It's great for a daily driver where you want things to stay quiet but you want the mounts to last longer than the stock ones.
- 80A (Medium): This is the "sweet spot" for most enthusiasts. It's noticeably stiffer, improves throttle response, and reduces engine movement, but it won't make your teeth rattle at a red light. You'll feel a bit more vibration, but it's manageable.
- 94A+ (Hard): This is for track-focused cars or people who don't care about comfort at all. At this level, you're basically making a semi-solid mount. Expect your mirrors to vibrate and every engine noise to be amplified in the cabin.
What You'll Need to Get Started
You don't need a massive workshop for this, but you do need a few specific items. Don't try to wing it with whatever is under your kitchen sink.
- Liquid Polyurethane: You usually buy this as a two-part kit (Part A and Part B).
- Old Motor Mounts: You can use the ones currently in your car, but it's often easier to grab a spare set from a junkyard so your car isn't on jack stands for three days while the poly cures.
- A Torch or Saw: To get the old rubber out if you're doing a full pour.
- Duct Tape: To seal the mount so the liquid doesn't leak out.
- Degreaser and Sandpaper: Polyurethane won't stick to grease or smooth metal.
- Mixing Cups and Stir Sticks: Use stuff you can throw away.
- Gloves: This stuff is incredibly sticky. If you get it on your hands, you'll be wearing it for a week.
Preparing the Mounts
The success of your diy polyurethane motor mounts depends entirely on how well you prep the metal. If there's even a hint of oil or old rubber dust left inside, the poly will eventually delaminate and peel away, leaving you with a failed mount and a big mess.
If you're filling a mount that's still mostly intact, you just need to clean out the hollow voids. But if the mount is totally shot, you'll want to remove the old rubber entirely. This is usually the most annoying part of the job. You can use a hole saw to cut out the bulk of it, or use a propane torch to melt the bond between the rubber and the metal sleeve. Just a heads up: burning rubber smells terrible and produces a lot of black smoke, so do this outside and away from anything flammable.
Once the rubber is out, use a wire brush or some 80-grit sandpaper to scuff up the inside of the metal housing and the outside of the center pin. This gives the polyurethane something to "bite" into. Finally, hit everything with a heavy-duty degreaser or brake cleaner to make sure it's surgically clean.
The Pouring Process
Now comes the fun part. You need to create a "mold" out of your mount. This is where duct tape becomes your best friend. Wrap the bottom and sides of the mount tightly. You want to make sure there are no gaps where the liquid can seep out. Some people use modeling clay to seal the edges, which works pretty well too.
Make sure the center pin is perfectly centered. If it's crooked when the poly cures, your engine won't line up correctly when you go to bolt it back in. You can usually use some scrap wood or even more tape to hold the center pin in place while you work.
When you mix the two parts of the polyurethane, follow the instructions on the package to the letter. Most are a 1:1 or 2:1 ratio by weight or volume. Stir it slowly. If you whip it like you're making a cake, you'll introduce millions of tiny air bubbles that will weaken the final mount.
Pour the mixture into the mount slowly. Try to pour into one spot and let the liquid flow naturally into the corners to avoid trapping air pockets. Once it's full, give the mount a few gentle taps on the workbench to help any remaining bubbles rise to the surface.
The Waiting Game (Curing)
This is the hardest part: waiting. Most liquid polyurethanes take about 24 to 48 hours to cure fully. Some might "feel" hard after six hours, but don't be tempted to install them yet. If the center hasn't fully chemically bonded and hardened, the weight of the engine will just squish the liquid center right out.
Keep the mounts in a warm, dry place. If it's too cold, the chemical reaction slows down significantly. I usually give it a full two days just to be safe. Once cured, peel off the tape, trim any "flash" or excess poly with a utility knife, and you're ready to go.
Dealing With NVH (Noise, Vibration, Harshness)
I'd be lying if I said there were no downsides to diy polyurethane motor mounts. When you stiffen up the connection between the engine and the chassis, you're going to experience more NVH.
At idle, you might feel a slight hum through the steering wheel. If you went with a high Shore rating, your rearview mirror might get a little blurry. For most people who care about performance, this is a small price to pay for the improved shifting and lack of engine movement. However, if you're building a luxury cruiser, you might want to stick with the softest poly you can find or just buy new OEM rubber mounts.
The good news is that polyurethane mounts often "break in" slightly after the first few hundred miles. They'll stay stiff, but the initial harshness usually mellows out a bit as the material settles.
Final Thoughts
Making diy polyurethane motor mounts is a rite of passage for many car enthusiasts. It's a project that requires more elbow grease than technical skill, and it provides a tangible difference in how the car drives. You'll notice it every time you let off the clutch or blip the throttle.
Just remember: prep is everything. Take your time cleaning the old mounts, seal your tape "molds" properly, and don't rush the cure time. If you do it right, these mounts will likely outlast the car itself. Plus, there's a certain satisfaction in knowing you saved a couple hundred bucks by doing it yourself in your garage.