Stiffening Honda D-Series Engine Mounts With Poured Polyurethane
Introduction
This guide covers reinforcing the factory rubber engine mounts on Honda D-series cars by pouring a measured-hardness polyurethane casting compound into the mount cavities. The goal is to cut down engine movement and the slop, thunk, and sloppy handling that come from tired mounts, without paying for full aftermarket poly or solid mounts and without the harsh vibration that very hard fillers like 3M Window Weld produce. The key decision is durometer: Energy Suspension's inserts were later measured at about Shore 72A, and a bowling ball sits near 100A, so a Shore 60A compound was chosen as a firmer-but-livable middle ground that stays reasonable for daily driving. Poured filler bonds to the existing rubber and stiffens it in place, so the mount still looks stock and can be run on the street. It applies to the two side mounts on D-series-powered Civics and CRXs; the rear and upper mounts are typically left alone or filled with a much softer grade. Adapted from a community writeup by kommon_sense on D-Series.org (https://www.d-series.org/threads/diy-homemade-polyurethane-engine-mounts.15434/), including corrections from the follow-up discussion.
Reference
Tools Required
- 320-grit (or similar) sandpaper
- Duct tape
- Disposable leakproof container or tray
- Mixing container and stir stick
- Toothpick (for popping surface bubbles)
- Die grinder with wire-wheel attachment (for trimming)
- Oven or other controlled low heat source (for post-cure)
- Socket/wrench set for mount removal and installation
Parts Required
-
Two-part flexible polyurethane casting compound, Shore 60A (e.g. McMaster-Carr 8644K24)
-
A spare set of engine mounts (side mounts), new or good used
-
Optional release-friendly spacer material such as paraffin wax
Safety Warnings
- ⚠ Keep post-cure heat controlled and the burner off; the urethane melts above about 180F.
- ⚠ Never pour filler into a mount that is installed on the car - the engine settles into it as it cures and the mount ends up off-center.
- ⚠ Once the two parts are combined there is no reworking it, and it gels in roughly 15 minutes, so complete all masking and setup beforehand.
- ⚠ Mask openings very thoroughly; the compound can seep through several layers of tape and harden where you don't want it.
Step-by-Step Instructions
1 Choose the durometer and source the compound
Select a polyurethane casting compound by Shore hardness. Shore 60A is the recommended all-around choice: firm enough to control the engine but soft enough to daily drive, with vibration that is only noticeable at very low idle. If you want a plusher ride, step down to roughly 40-50A or leave the mounts only partially filled; for an all-out stiff setup, 80A or 94A is available but rides much harsher. McMaster-Carr stocks the two-part flexible urethanes as 8644K24 (60A), 8644K11 (80A), and 8644K18 (94A), plus a putty version, 8644K28. A separate McMaster 60A formula cures faster (about 12 hours at 120F) and is slightly stronger but sets up white rather than clear. Budget roughly under $30 for the compound; two side mounts consume only about half a batch, so splitting a container with a friend covers two cars.
2 Work with a spare set of mounts, not the ones on the car
Do not pour filler into mounts while they are still bolted in. With the engine's weight settling into the compound as it cures, the mount's effective center ends up biased low and the car has to sit undriveable for days. Instead buy a fresh set of side mounts (roughly under $40 each) or pull good used ones from a wrecker, and build them on the bench while you keep driving on the current mounts. Starting with sound mounts also means you are reinforcing intact rubber rather than trying to patch a failed one.
3 Scuff and mask each mount
Rough up the exposed rubber inside each mount cavity with sandpaper (around 320 grit is fine) so the poured urethane keys into it and bonds well. Then seal every opening thoroughly with duct tape to dam the cavity; the compound flows a little thicker than syrup and will find any gap. Set each taped mount upright in a disposable leakproof tray in case of seepage. Mask generously and use extra layers on wider openings, since harder grades in particular have been reported to weep straight through several layers of tape and set up as an unusable mess.
4 Add a clearance spacer before pouring (recommended)
Many chassis have a welded pad on the frame that acts as a bump stop for the mount, and a completely filled mount will not clear it, forcing you to grind material away later. To avoid that, set a spacer into the cavity after taping but before pouring so the cured block leaves room for the pad. Choose a material that releases cleanly: plain wood bonds to the urethane and is a pain to remove, so a block of paraffin wax (which can later be melted out) or another non-stick form is a better choice.
5 Mix and pour the urethane
The compound is a two-part system supplied at the correct mix ratio, so combine the two parts fully and stir for at least two minutes. Once mixed it is irreversible and gels in roughly 15 minutes, which is why all masking and prep must be done first. Pour slowly into each cavity to limit trapped air; break any small surface bubbles with a toothpick as they appear. Fill to the level you want (short of full if you are chasing a softer feel). Expect two side mounts to use only about half a typical batch.
6 Cure, then post-cure for strength
Let the mounts sit undisturbed for a full 48 hours at about 70F, warming the room if it is cold. The surface will feel hard within about 8 hours, but do not shortcut the two-day cure. For extra strength and durability, follow with a post-cure of roughly 4 hours near 150F. A gentle way to do this without exceeding limits is to heat a heavy covered pan, remove it from the burner, and let the mounts warm inside on residual heat. Keep the temperature under control: this urethane tops out around 180F and will begin to melt above that, since it is still a form of rubber.
7 Trim to clear the frame pad if needed
If you did not use a clearance spacer and the finished mount fouls the welded frame pad, remove the interference with a die grinder fitted with a wire-wheel attachment. Taking about 1/4 to 3/8 inch off the top face of the poured block is typically enough. Test-fit against the chassis and remove only as much as required for the mount to seat properly.
8 Install and evaluate
Bolt the reinforced side mounts back in and torque the fasteners normally. On the road the engine feels planted, steering response sharpens, and the previous slop and hard-turn thunk are gone. Expect a little extra buzz and vibration at very low idle (below roughly 600 rpm); it fades above about 1000 rpm and generally settles further after a week of driving. If any residual harshness bothers you, softening options include drilling a few holes in the cured block or, on a rebuild, using a softer grade.
Pro Tips
- 💡 60A is the sweet spot for a street car; go 40-50A or leave the mounts partly unfilled if you want it plusher, and reserve 80-94A for track use.
- 💡 Some builders found both filled side mounts together too stiff and ran only the driver's side, leaving the passenger side stock.
- 💡 Pour slowly and pop any surface bubbles with a toothpick to keep the block solid.
- 💡 Filling only the two side mounts leaves the rear and uppers stock; a very soft grade (around 30A) can reinforce those without making them hard.
- 💡 The alternate McMaster 60A formula cures in about 12 hours at 120F and costs a little less, but sets up white instead of clear.
- 💡 A firmer 94A fill in the rear mount is effective at controlling front-to-back engine movement under hard launches.
Was this guide helpful?
