74 and later shock on 71 chassis

logician · 07:32 AM

I need to use some '74 and later rear shocks on a '71 chassis that still has the steel trailing arms. I'm guessing all that is required is a sleeve around the lower shock mounting bolt to match the larger diameter on the later shocks. Has anyone done this? Any suggestions where I could obtain that sleeve short notice?

-Juan

Scott Clarke · Scott Clarke's Garage

Juan-
I think that the pre and post '71 shocks are different lengths. I'm not sure the '74 shocks will work properly on your '71. Anyway, a bump with hopes of someone providing a more definite answer.
-Scott

911pcars · 09-19-2006, 05:06 PM

The shock length looks about the same according to the location of the lower shock mount on each control arm. The 72-73 steel control arms have the same geometry as the later arms (see below). The lower shock mount on 69-71 arms are in a slightly different location.

You still need to address the issue of the different size fastener on the lower shock mount.

Steel arm (72-73) vs aluminum (74-77)

Sherwood
for more info go here

Scott Clarke · Scott Clarke's Garage

Is the upper mount in the same location? I know that installing late arms on an early car results in the shocks fouling the area near the upper mount.
-Scott

RallyJon · 09-20-2006, 07:24 AM

You'll be bottoming out all over the place, with lots of droop travel and very little bump travel. I got the later shocks by mistake and it went together just fine, but didn't handle at all.

911pcars · 09-20-2006, 08:16 AM

"Is the upper mount in the same location? I know that installing late arms on an early car results in the shocks fouling the area near the upper mount."

Upper mount location is the same. However, the sheet metal "bell" around the upper shock mount (69-71) is narrower. Since late shocks are larger in diameter and the lower mount shifts slightly with al. arms, I removed the dust cover. I encountered add'l interference with the heat exchanger sheet metal at full suspension drop. Gentle persuasion on sheet metal with a BFH was my solution.

"You'll be bottoming out all over the place, with lots of droop travel and very little bump travel. I got the later shocks by mistake and it went together just fine, but didn't handle at all."

With what setup? Steel arms and post '74 shocks? You adapted the larger lower shock mount onto your steel arms?

Droop travel doesn't count much during vehicle operation unless you typically get the car airborne. If the late shocks are longer I can see a problem in bump range, but I don't know for sure. The distance between upper and lower shock mounting points between steel and al. control arms aren't that different and I suspect shock range is about the same.

Upgrading al. control arms onto early chassis is done quite frequently. Not so common is the task of adapting late (post '74) shocks on steel control arms.

Sherwood
http://members.rennlist.org/911pcars

RallyJon · 09-20-2006, 08:34 AM

Quote:

With what setup? Steel arms and post '74 shocks? You adapted the larger lower shock mount onto your steel arms?

Yes. When I got the correct replacement, the shock body was about an inch shorter and the overall length was almost two inches shorter.

logician · 09-20-2006, 08:46 AM

RallyJon, When you say the shock was shorter, presumably that is with the shock compressed and sitting on the bump stop. Did you compare bump stops? Also, what did you use for a sleeve to adapt to the 12mm bolt on the steel trailing arm to the larger 14mm diameter lower mount hole on the late shock? That's the part I'm looking for.

Sherwood, are the locations and heights of the shock towers the same?

-Juan

RallyJon · 09-20-2006, 08:51 AM

Here's a pic side by side. Note that these were made for the same given (lowered) ride height so the absolute length is not relevant, but the relative length illustrates the difference.

Early_S_Man · 09-20-2006, 09:41 AM

Guys,

For the 1972 models the angle of the shock was 'splayed' out a bit to clear the 915 transaxle, and the shock body was enlarged, too! It resulted in a slightly longer shock.

304065 · 09-20-2006, 10:23 AM

72 and later shocks do fit, but you have to remove the dust cover, and you may still get wear on the inside of the shock tower, as illustrated in the below photo.

Also, the shock body will grind against the heat exchanger. You can't see it in the photo, but the other side of the shock has about a 6" area where it's rubbed against the SSI. "Profiling" the SSI is necessary.

Juan, your real issue is that the '71 arms use a 12mm bolt whereas the later Bilsteins are 14mm. Get yourself a 12mm ID, 14mm OD bushing from a shock supplier and you will be all set. Or you can modify the steel arms to receive 14mm bolts, which is what I would so insofar as the mount is in single shear (and it's easy to weld steel).

logician · 09-20-2006, 10:28 AM

Warren, So the top of the shocks were moved out in the later years? If anything, that would make the shock more vertical and shorter, not longer.

RallyJon's shocks are non-stock and so his experience doesn't really say anything about the factory shock lengths. So far Sherwood's explanation that the early and late shocks are the same length is sounding the best. Anyone have some OEM shocks that they could measure?

-Juan

logician · 09-20-2006, 10:34 AM

John,

Thanks for the pointer to for that bushing.

One question about rubbing. Assuming that heat exchanger clearance is not an issue (for race cars) then does spacing the bottom of the shock toward the middle of the car increase or decrease the rubbing at the shock tower? Are you using aluminum trailing arms with the shock mounts cut back to increase clearance for the heat exchangers?

-Juan

RallyJon · 10:57 AM

Quote:

So far Sherwood's explanation that the early and late shocks are the same length is sounding the best.

Ah, the internet at it's best. If you wait long enough, you'll get enough answers that you can choose one you like.

The 72- Bilsteins are 574mm long extended and the -71s are 555mm. Since the 72- ones are "Sports" and the -71s are "HDs" one reason the difference is only 19mm might be that the Sports are designed to be run at a lower ride height (but on the later arms of course).

The custom shocks I had done are identical in every way except that the short ones are for -71 arms and the long ones are for 72- arms. They are truly an apples-to-apples comparison despite your dismissal.

Early_S_Man · 09-20-2006, 11:31 AM

Juan,

No, the top mount stayed essentially in the same place for '72 ... it was the lower eyelet that moved outwards, hence my usage of splayed -- spread further apart. The horizontal leg of the triangle was elongated, and so was the hypotenuse!

logician · 09-21-2006, 12:16 PM

Quote:

Originally posted by RallyJon
Ah, the internet at it's best. If you wait long enough, you'll get enough answers that you can choose one you like.

Well the trick is picking the right answer out of all the choices.

Seriously, thanks for all the input. Just trying to sort it out.

-Juan

911pcars · 09-21-2006, 02:59 PM

"The custom shocks I had done are identical in every way except that the short ones are for -71 arms and the long ones are for 72- arms. They are truly an apples-to-apples comparison despite your dismissal."

Rallyjon,
If so, the effect of installing al. arms in an early chassis is net zero - a wash. It's a slightly longer shock, but the lower mount is further away (longer hypotenuse per John Walker). And if "sport" shocks are slightly shorter, all the better with a lower ride height (the factory ride height is too high).

One of the tricks to minimizing shock-to-heat exchanger interference is to trim the aluminum shock mount boss down to the steel-threaded insert.

Sherwood

RallyJon · 09-21-2006, 03:40 PM

Quote:

If so, the effect of installing al. arms in an early chassis is net zero - a wash.

Sure, if you use the later shocks. Choose shocks to go with whichever banana arms you're using.

Danimal16 · 09-23-2006, 07:05 AM

Sherwood,

I have read your article and followed these posts. I was wondering if you could please give me an idea on how much the lower shock mount boss should be trimmed? I have a 71t which I am mounting the earlier aluminum arms on and looking to switch to the later shocks, the sports. Any thoughts or clarifications would be appreciated.

Thanks in advance.

DanB

Quote:

Originally posted by 911pcars
"The custom shocks I had done are identical in every way except that the short ones are for -71 arms and the long ones are for 72- arms. They are truly an apples-to-apples comparison despite your dismissal."

Rallyjon,
If so, the effect of installing al. arms in an early chassis is net zero - a wash. It's a slightly longer shock, but the lower mount is further away (longer hypotenuse per John Walker). And if "sport" shocks are slightly shorter, all the better with a lower ride height (the factory ride height is too high).

One of the tricks to minimizing shock-to-heat exchanger interference is to trim the aluminum shock mount boss down to the steel-threaded insert.

Sherwood

911pcars · 09-23-2006, 10:27 AM

Dan,
I included this illustration in the article. According to my sources, you can remove about 1 in. from the aluminum arm before the threaded insert begins.

Hope this helps,
Sherwood