Converting '71 to Aluminum Trailing Arms

[304065]

The '71 uses a 12mm bolt to mount the shock to the trailing arm.

The Aluminum arms use a 14mm bolt.

What did you do about the shock bushing? Did you have the bushing changed? Use an aftermarket bushing?

[Grady Clay]

John,

I had the opposite issue. I put RSR Bilsteins on my early SWB. The shock took a 14 mm and the arm a 12 mm. I made an adaptor for the spherical bearing at the bottom of the shock.

You can either use the later shocks or mod the bottom of your existing shock. I would think you could install spherical bearings.

Best,
Grady

[jpnovak]

I bolted the later shocks and arms to my car. The rear of the shock slightly rubs the shock tower hole. btw, I don't have any rubbing issues with the HEs the way some have described.

[jluetjen]

Ditto. I just used shocks from a later car.

[Bill Verburg]

The upper shock position was also changed around '72

[304065]

I talked to Bilstein, they recommended that I ream the steel insert out to fit, they do it occasionally.

[Grady Clay]

If you have any issues with slight rubbing in the shock tower, you can use the plastic cover from some cheep shocks. Just make sure there is the substantial washer under the rubber mounts. If you feel the need, media blast it and paint it Bilstein Green. I like to set-up with only the top part of an old steel cover in place. This allows you to trim the rubber bump stops when possible and use a zip-tie around the shaft to gauge the maximum compression.

On a seriously lowered 911 you should have modified shafts. You must have good suspension travel for proper handling. 911s that ride on the bump stops handle terribly.

Remember, the rubber upper support rings are part of the mass-spring-damper system. You can stiffen them up slightly by trimming the length of the sleeve that goes over the threaded part of the shaft. On early cars this requires reinforcing the rear shock tower like RSR (BA 2nd Ed p.201.)

Make sure the suspension can move through its entire range of travel without any interference. This is easy to do when the torsion bar isn’t installed but all the other components are. Remember to check the CV axial play at each point.

The bottom shock bolt needs a safety washer to prevent any failure of the attachment from allowing the shock to come off the bolt. There should always be clearance between the shock and the trailing arm and the safety washer. Remember to allow clearance for lateral deformation of the rear suspension and chassis.

Best,
Grady

[304065]

Thanks Grady. I'll check for interference when I trial fit everything up.

I remember Carroll Smith's discussion about "Silasto Bump Rubbers," he goes in to pretty good detail there. I also know that if the rear suspension hits the bump stop and stays there, it's spin time. . . The shocks for the '72 have something like an inch and a quarter more travel, according to Bilstein. My car is very low, so the shorter rod should help.

I hear you about the safety washers, I'll make sure there's something appropriate on there. I was thinking I should probably put a smaller washer over the bushing, then the larger one with an OD that's larger than the shock's OD, to keep it from sliding off if the bushing tears, but spaced out enough so that the large washer doesn't grind against the shock.

[randywebb]

"Silasto Bump Rubbers,"

also take a look at the analysis of suspension options for the 996 in Pano 2-3 years ago. A 2 part article done by an anaesthesiologist (who said those guys were wasting their educations...), another doc and an engineer or mechanic. They did a great job of actually measuring things.