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944 Spring Plate Bearing Installation    

Upgrading to Polybronze performance

Put the car up on jackstands, or a lift.   At a minimum you need both rear wheels up in the air, but the job is easiest with all 4 wheels up.   Remove both rear wheels.

Remove 2 bolts attaching spring plates to the trailing arm. Remove the sway bar drop links.

Popular wisdom says disconnect the brake line flex hoses, remove the exhaust, and drop the torsion bar tube.   I found this wasn’t required.   It is possible to tilt the torsion tube down, one side at a time, and gain the required clearance.

Do remove the spring clips that hold the brake line flex hoses to the trailing arms and the tub.   Pulling these clips off affords a couple extra inches of slack on the brake lines.

Pull the trailing arm clear of the spring plate.    You will need to remove the bolt that secures the trailing arm to the torsion bar tube.   On the driver side you will need to remove one side of the axle to avoid interference with the muffler.  Your goal is to have the spring plates hang free without interference from the trailing arms.   Lifting the trailing arm with a jack will help. 

You want the spring plate to hang free so you can create a reference line marking its angle relative to the torsion bar tube.  This mark will be extremely valuable when you re-assemble to avoid repeated attempts at re-indexing the torsion bars.

 Draw a pencil line as shown: 

Again, be sure the trailing arm is not interfering with the angle of the spring plate.   If you plan to re-install the same torsion bars, your goal will be to re-install the spring plates with the same angle.  If you are installing stiffer torsion bars, you will need to install the spring plates a bit higher than the reference angle.  The amount will depend on the stiffness of the torsion bars, but at least you have a staring point to reference.

Remove the rear bolt that secures the torsion bar tube to the chassis.  

Remove 4 bolts surrounding spring plate, 1 bolt from forward bushing, and 1 bolt from the rear mount. 

You will need to pry the forward bushing down to free it from the bracket.

You should now be able to tip down the torsion bar tube, one side at a time.  Be sure you are not  putting excessive stress on the brake flex hoses.

Remove the spring plate cover.  Remove the rear section of the rubber rocker valence.

Pull the spring plate from the torsion tube and remove.  The torsion bar sometimes sticks in the spring plate.  Usually it can be freed by rocking it back and forth while pulling.

With the rocker valence removed and the torsion bar tube tilted down you should be able to fully remove the spring plate and the torsion bar.

With the spring plates on the bench, the deterioration of the factory rubber bushings is obvious.   The factory rubber cold flows over time from the weight of the vehicle such that even low mileage bushings deform after 15 years or so.  In this case the car is 18 years old with about 80k miles, fairly fresh as 944s go.

These pictures tell the tale; the digital readout is partly obscured but one side of the bushing is 16mm the other is 20.22mm.   It is supposed to be round.   These measures are taken on the bench, but with the weight of the vehicle in the difference would be even greater. 

Next step is to remove the bushings.  The bushings are vulcanized to the spring plates and they are very secure.  Removal is rumored to be a very difficult job, and if done incorrectly it is a difficult job.   I’ve done this many times for 911s and have developed a technique that is easy.   Follow my instructions here and you will have it done in no time.

Secure the spring plate in a bench vice.   Use a propane torch to heat the metal tube of the plate from the inside.   Your goal is to get the metal hot enough to melt the contacting rubber.   Heat it for several minutes.

While it is still hot, work a flat blade screwdriver between the rubber and the metal tube.  Work all the way around to separate the rubber.   Use a razor knife to separate the rubber from the flat part of the plate.  The bushing can now be easily pried off.

This will remove 90%+ of the rubber in one big donut.  Next use a razor knife to cut off remaining large chunks.   Cut away the thin layer that covers the tube portion of the spring plate.   Small bits of rubber will remain.   These can be removed easily with some medium sandpaper.   

Be sure to get the tube portion of the spring plate very clean, any residual rubber here would interfere with the fit of the new bushings.

Next I recommend having the plates re-plated.   Get them plated along with the hardware with yellow zinc dichromate.  Shop around and you can get this done for $40-$50, well worth it.   You may have some other parts you’d like to throw in the batch too. 

Not only do the plates look like new, the acid bath used in the process will clean off any remaining rubber bits.   Don’t rely too much on this for rubber removal though, give the plater clean parts.

This really gives the project a finished appearance.   Do it once, do it right and be proud.

Next step is to install the Polybronze bearings. Polybronze is actually a bronze bearing that rides on a steel race fitted to the spring plate.  The bronze on steel provides a precisely machined low-friction bearing action that never squeaks and can be re-lubricated.

This picture is of the prototype set that has been adapted from 911 parts.  Four of these are used.  Production parts will look the same, except the grease grooves will be sized properly for the part.

The bronze is surrounded by a polyurethane jacket.  The polyurethane is not part of the friction surface, its purpose is to accommodate irregularities in the factory mounts. Because the bronze core is rigid, the poly jacket can compress slightly for irregularities without pinching the bronze friction surface.

The product has been available for 911s for about a year and is extremely popular.  The product has none of the troublesome fitment issues and squeaking that have made regular polyurethane bushings infamous. 

The races are fitted to the spring plates.  Springs plates vary slightly in diameter (one reason regular polyurethane bushings are so hard fit properly).  To accommodate the variation, shims are fit between the race and spring plate to make it good and tight.  The shims are included with the product.  The race provides a precisely machined surface for the bronze bearing to ride upon.

Above pic shows the spring plates with both races installed and bearings slipped on.

Press the bronze bearings into the spring plate covers using bench vice or press.  Liquid soap is used to lubricate the polyurethane to ease insertion.

Once pressed in, drill a hole for the grease fitting.  A small pilot hole is first drilled fully through one side of the bearing.  A large hole is then drilled and tapped in the aluminum cover plate to accept the grease fitting.

Pic above shows the grease fitting and installed in the assembly. 

And the back side of the assembly.

 Next install bearing in the torsion tube.

Clean up the ID of the torsion tube using sandpaper and scraping tools.  The ID tends to have hard residue from the rubber bushings, this must be removed to get a good fit.

Lube up the bronze bearing with liquid soap and insert it into the cleaned up torsion tube.  Then use the spring plate cover plate and a block of ½ wood to press the inner bearing into place.  Tighten the 4 cover plate bolts in sequence to push the bearing in.

Next drill the torsion tube for the grease fitting.  Again, using a small pilot hole through the bearing then a shallow larger hole tapped for the grease fitting.

Start by assembling the spring plates.  The large eccentric bolt provides a height adjustment capability.  Position it in the center of its range.  This will give you +- .75 inches of range to make final adjustments.

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Next install the spring plates

Apply a good quality grease to the bearing surfaces. Try to fill the grease grooves.

I recommend doing a dry run, installing the spring plates without torsion bars to ensure the plates are moving freely.   Pull the trailing arm back so it does not interfere with the spring plate.  Slip the spring plate into place, install the spring plate cover and torque it down.  

The spring plates should move freely without binding or friction.  If they are tight, inspect the bearings to be sure they are full seated.   With the plates moving freely, remove the spring plates covers and plates.   

Grease up the torsion bars.  It's a good idea to grease the entire bar for corrosion protection and the splines for easy installation.  Slip the bar into the torsion bar tube.

Slip the spring plate over the torsion bar.  Carefully compare the scribed line to the edge of the spring plate.  You will note that indexing the torsion bar one spline makes a large change in the angle of the spring plate.   Not to worry. 

The torsion bar has a different spline count on each end.   This makes it possible to make very fine adjustments in spring plate angle by rotating both splined ends simultaneously.  I'll leave it as a homework assignment for anyone who wants to compute the affect of rotating each end.   I find it easier to simply pull the torsion bar out, rotate the inside a spline or two, then do another test fit. Repeat until you get the desired angle.   It usually only takes a few attempts to dial it in.

If you are re-installing the same torsion bars and want to maintain the same ride height your job is relatively easy; make the lines parallel.  

If you are installing stiffer torsion bars and / or are adjusting ride height, you need to install the spring plate at an angle different than scribed line.  Stiffer torsion bars will droop less under the weight of the car.  The stiffer the torsion bar, the less droop.  You will need to take a guess at how much to compensate for the new torsion bar stiffness and / or ride height.   Fortunately the spring plates offer some adjustment for ride height, so you just need to get close.  Careful here, too far off and you will be pulling this apart again to re-index the torsion bars.

Install the spring plate cover.  Attach spring plate to trailing arm using two bolts.

You may need to lift the trailing arm with a jack to get the rearmost  lower spring plate cover bolt and spacer into place.

Install the inner trailing arm link.  The fit is tight, tap it into place using a mallet.

Next attach the front bushings to the tub.  Use a jack to raise entire torsion tube assembly up into position.  It is helpful to have two jacks, one on each side.     

Once the bushing lines up, slip the bolt in.  It probably wont go through the backside yet, you'll come back to this.

While you are raising the assembly up, make sure it aligns properly with the little hooks on the torque tube.  A little pushing and prodding should be all it takes.

Attach the rear mounting bolts.

With the rear mounting bolts in place, the front bushing bolt hole should now line up better and the bolts should be fully installed.

Now it's just cleanup.  Reinstall the sway bar, lower shock bolts, driver side axle, and spring clips for the brake lines.   Go through and check all bolts for proper torque.  Give a couple squirts of grease to each of the grease nipples for good measure.

All put back together, it looks like this:

To finish off the job you'll need to fine-tune ride height using the spring plate height adjusters.  This is a straight forward process of loosening the bolts and turning the big eccentric nut.  You will need the special 36mm wrench made for this job.  Then get a good four wheel alignment and corner balance.

Then go out and drive.  Appreciate your car's new-found handling precision.

Suspension

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