Reinforcing Diff Mount and Subframe Bushings with Polyurethane

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Wednesday, Mar 10 2010 7:59:08 AM
Mountain Time (GMT-7)

For BMW E28 Series

Summary: This repair writeup outlines how to fill your diff mount and or subframe bushings with polyurethane.

If you'd like to read more about the benefits, side effects and other opinions about poly-filled bushings and diff mounts, I suggest searching online BMW forums. Here is a thread that was quite popular that may shed some light on the debate.

Tips: Take your time when taping, it will keep the poly from seeping and give you a better end result.

RELATED
See my tutorial on subframe bushing installation here.

general info

Tools Required:		Optional Tools Recommended:
Flathead screwdriver to open can. Masking tape.		Newspaper or covering for work area. Harsh environment gloves
Time Required:		Parts Required:
30 minutes alone, two separate sessions.		2 BMW subframe bushings, BMW part #33 31 1 130 488 BMW M535i diff mount, BMW part #? Devcon Flexane Liquid 80, 1 pound. Purchase from McMaster-Carr, search for item #8644K11, $26.66

		A large floor jack will be helpful.

gallery (click any pic to enlarge)

BMW E28 M535i diff mount, before poly-filling. This thing will nearly double in weight when filled.	You can see why it needs to be taped - when looking at the side, the polyurethane would just leak out.	Here's the bottom of the diff mount, which will also need to be taped. Especially the bolt hole.
A decent angle shot so you can see the recesses.	And here are stock BMW E28 subframe bushings. Notice all that room to fill with poly!!! :) The one on the left shows you the bottom, the one on the right is the top view.	Here's a side view. Since both ends have hollow areas, they will need to be filled in two pouring sessions: one pourring session for the bottom ends and the diff mount, and a second pouring session for the top ends of the bushings.
1) Clean the diff mount and the bushings with acetone or a special cleaning agent for prepping rubber for polyurethane. Devcon makes such an agent.	2) Step two is simple: just tape the sides and bottom of the diff mount. The subframe bushings require no taping, but you might at least want to tape the bolt holes so that you don't get poly in there.	3) After taping, mix up HALF of the polyurethane. The Devcon kit includes everything you'll need to mix it up. I measured out HALF of the poly and HALF of the curing agent, so I could have enough later for a correctly measured mix on a second pour.
4) Seal up the polyurethane and the curing agent TIGHTLY so they will not spoil until you get to them tomorrow.	5) Then start filling!!! Be slow and cautious. The poly is VERY thick and slow-pouring when mixed.	Here's a close up of the diff mount when filled. I quit filling once it got to this point, I didn't want to have it cover the hole and have to cut that out later.
6) Fill both of the subframe bushings on one end. Either end, it doesn't matter. You'll do the other sides 24 hours later	7) After filling one side of each bushing, and the diff mount, let them set for 24 hours in an 70-80 degree indoor temperature room.	8) 24 hours later, mix up the other half of the poly and pour it into the other ends of the bushings. Let them cure 4-7 days for FULL cure before installation. I removed the tape after 48 hours.

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