the truth?: rear trail-arm upgrades

[jpnovak]

As my project moves forward I have to start considering the future. One of the plans for my 71 coupe is to retrofit some aluminum rear trailing arms from the donor 78 SC. I have read that there is some machining required so that the new arms work in the old tub. It seems that the mounting points for the newer arms are appox. 1 in. inward of the old steel arms. Here is what I have discovered.

1. Easy fix- modify heat exchangers for clearance of the shock with new arms. this will obviously leave the shock in the wrong geometry by having the bottom of the shock. I would think this may lead to other problems.

2. More difficult but correct fix - mill/remove about 1" of the mounting point so that correct shock geometry is restored and clearance for HE's gained.

So here are my questions.
Has anyone done this conversion and what did it entail?

Which route did you choose?

How did the upper shock mounts and/or suspension change to accomodate the different mounting point?

Thanks for all the help.

Jamie

[911pcars]

Jamie,
This is a common upgrade.

You're correct in that the lower shock mount now sits more inboard (BTW, you'll need to upgrade to the later shock design as the lower mount is different). The 12 mm shock bolt threads into an insert in the al. arm while the early shocks use a smaller bolt and nut arrangement.

The thread insert begins approx. 1" inboard of the mounting surface. Remove this material to get a little closer to the correct shock geometry. I did it with a rotary cutoff wheel, then cleaned it up with a mill file. Others have done this on a 3-axis milling machine, but I don't think this amount of precision is that necessary.

The upper shock mount is the same, but the area is larger on the later 911s (73?) and it may be a little tighter fit depending on the shock brand you choose. I removed the dust shields on the Koni Sport shocks for add'l upper shock area clearance. The heat exchanger sheet metal also requires a little "persuasive modification" when the suspension is at a full jounce position.

Remember. The camber and toe adjusters and the sway bar mounts are different as well.

See Excellence magazine, April, '96. There's a few errors but basic information is in the article.

Regards,
Sherwood Lee
http://members.rennlist.org/911pcars
www.seinesystems.com

[jluetjen]

I've done the same upgrade on my '69. By the way, get the earlierst aluminum arms that you can. I put '76's on my car and then I needed to change the axles because of a slight interference between the older outside CV and the aluminum arm. Of course I couldn't put the new CV's on the old axle. And then I needed to replace the mounting flanges on the inside of the rear axles with 915 flanges so that they would fit the '76 inner CV. Finally I had to have the bearing surface on the flanged turned down by 1 mm at a local machine shop to make them fit in my 901 gearbox. I think I could have avoided all of that if I had gotton arms from a '74 or a '75 (I think). But it's water under the bridge now...

As long as you get non-turbo arms, you do not need to worry about changing the inner mounting point. They are the same. Finally, prepare yourself - you will most likely have to drop the engine. It's really not that bad. I've seen lots of people say how they cut the bolts and stuff to get them out. I tried and I think it will take longer and bruse more knuckles doing that. At the end of the day, I think it's easier to just drop the engine. You can do the valves at the same time.

[jpnovak]

Sherwood,

Thanks for the additional information. Sounds like the mod is not too difficult. I will just remove the outer aluminum until I reach the steel insert. The cutoff wheel and file sound easy enough. I do have access to a mill but I would imagine that complete and total disassembly would be required to mount it on the table.

I don't have to worry about compatiblity of the CV's or shocks. They are all coming off the donor car. The entire rear susp. and drivetrain are going into the 71. Can't wait to get it running.

Thanks for your help.

Jamie