machinists - is this possible?

Hopefully some machinist will read this who also has enough knowledge of cars that this post will make sense.

I own a 1988 Porsche 944. The front struts are unitized; that is, the caps that hold the inserts into the housing are crimped on. I can only replace them as a complete unit. They're made by Sachs/Boge, which is not a big plus in my book.

Earlier cars used a threaded cap at the top of the strut tube to hold the insert in. Those struts are rebuildable by unscrewing the cap and simply replacing the insert. Bilstein makes inserts for these struts, and new Bilstein inserts are significantly cheaper than the new Sachs/Boge assemblies. (win/win, better dampers *and* cheaper too!)

From what I've been told, the two assemblies are not directly interchangeable due to differences in the way they mount to the steering knuckle (there were some suspension geometry changes between the '86 and '87 model years, when the non-rebuildable struts were introduced.)

So here's why I'm posting to this newsgroup and not an automotive group

- I have a pair of '87-88 style struts that are shot. I paid $20 to have them shipped to me so I could play with them (so if I screw them up, I'm out the cost of a semi-expensive dinner.) I have my suspicions that if I grind the crimped caps off of them that I will find that they are otherwise identical to the earlier struts in terms of the tube and insert. If that is the case, would it be possible to simply have a machinist cut threads in the top of the strut housing tubes? I'm not sure what the implications are of trying to turn something this big and unbalanced in a lathe, is that a problem?

If you can tell me that this *should* work, I'm going to try to find a set of junk early struts as well, and will gleefully start cutting stuff up :)

thanks,

nate

Reply to
Nate Nagel
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Nate Nagel wrote in news: snipped-for-privacy@news3.newsguy.com:

I don't see why it wouldn't work, provided there is enough wall thickness in the tube to support a thread strong enough to hold the cartridge. Worst case, weld on additional material around the top, external thread it and make a cap for them. (provided there is enough room for that solution)

Reply to
Anthony

Be carefull; cutting threads into a stressed tube, that you don't weaken the assembly. Not the place to have something break on you.

Reply to
MadDogR75

Reply to
JR North

I had similar struts on a Toyota and wanted "high performance" shocks. they cut the top off the struts, threaded them and inserted an after-market shock.

If your struts are the same the bottom attaches to the suspension and the shaft mounts in a spherical bushing on the car. The strut rotates around the shaft when the wheels turn. If this is the design then there is little force on the top of the strut - the load is between the shaft and the attachment to the suspension member. The only load on the top of the strut tube is rebound or when the wheel is off the ground which should be negligible.

Bruce in Bangkok (brucepaigeatgmaildotcom)

Reply to
Bruce

yup, you're envisioning it correctly... the main question in my mind is does the unbalanced weight of the attachment to the suspension (it's welded to the side of the tube) cause an issue when chucking it up in a lathe? Or is there any other gotcha that I'm not seeing?

nate

Reply to
Nate Nagel

I'm going back a few years but as I remember the strut looked like a tube, say 18" - 2 feet long with a U shaped, two bolt, bracket on the bottom. I took the car into the shop one afternoon and picked it up the following morning. They had cut the top off the tube, threaded the tube (I believe on the inside), installed the shock and screwed the retainer in the strut. After I had the modification done I drove the car for, probably, 3 years with no problems.

Bruce in Bangkok (brucepaigeatgmaildotcom)

Reply to
Bruce

Ah... mine are a little different. Rather than having a U-shaped piece of metal at the bottom, there's two heavy tabs welded to the side of the tube to mount to the spindle carrier. Two holes in each tab, the upper of the two is slotted to allow for camber adjustment. Other than that you're envisioning what I have perfectly. I just didn't know if chucking that up in a lathe would cause a problem, as most of the lathe work I've done is with stock that is more or less balanced. Yes, I know just enough about machining to be dangerous :)

This may all be academic as I'm supposed to be getting a pair of strut assemblies off of a wrecked car with something like 500 miles on them, but I'd still like to investigate this idea for the future.

here's a pic of a similar strut:

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nate

Reply to
N8N

O.K. If you cut the small mounting ears between the spring mount and the bracket that attaches to the suspension it will look pretty much the same as my Toyota struts.

No, being out of balance isn't going to interfere with machining it. Job Shops frequently machine things that are out of balance -- just slow the machine down till it stops shaking.

In my case the shocks were (I don;t remember the name) American made and designed for a Toyota. I can't remember whether they came with the retaining nut but I do remember when I opened the box thinking "They won't fit". Anyway, I took them to a suspension and wheel shop who didn't seem a bit perturbed. Just said, "you'll have to come back tomorrow". When I came back the car was ready and as I previously said I drove it for three years with no problems.

Have at it!

Bruce in Bangkok (brucepaigeatgmaildotcom)

Reply to
Bruce

Not as long as the attachment clears the ways when turning. Threading can be (and often is) done at very low speeds. I often thread at about 60 RPM, to facilitate stopping at the end of the thread. The more likely gotcha would be whether or not the tube walls are thick enough to take threads. If not, it might be possible to make a threaded bushing or collar that could be welded or silverbrazed in place.

Reply to
Don Foreman

Thanks guys for the replies. Like I said I know just enough to be dangerous. I guess I will now actively pursue trying to find a set of junk early-style struts to see if the idea is physically workable.

nate

Reply to
N8N

GM had a similar design on one of the early FWD cars. On those you used an oversized pipe cutter to cut the top off the old tubes. The replacement insert kit also had a pair of new oversized tubes with the seals in them. They slid over the outside of the old tubes and clamped in place with a WIDE band clamp.

Reply to
Steve W.

Yeah, that part seems okay given the slow speed of threading.

But is it time for someone to point out that cut threads tend to be weaker than the rolled threads found on many manufactured items? Not saying it is a problem, just that all threads/threading methods are not created equal in terms of strength, stress concentrations, material properties, etc.

Another way of phrasing it: what happens if the threads strip or the assembly breaks in the vicinity of the threads when the vehicle is travelling at speed? Seems like the severity (or not - I can't fully picture the assembly)) of that should be thought through.

Reply to
cs_posting

snip

just to add a little more smoke to this fire, on the porsche, at least, up through the 86 model year, the struts had replaceable cores. the top, as nate mentioned, screwed in to hold the core inside. However, this is a McPherson strut, so the spring is around the outside and the spring is compressed to install it - the top end of the strut shaft has a holder for the top of the spring, a ball bearing (for rotation), and a bracket that is held into the fender well with 4 bolts. The fitting that holds the inside of the core in place also has to hold several hundred pounds of force to keep the spring compressed (at least it does any time the weight of the car isn't on the wheel) so I would be very distrustful of a solution that couldn't easily hold that with a 10 to 1 safety margin. I know this information because I have several of these spanning the model years in question.

Reply to
William Noble

I believe that the threads on the struts that are factory threaded are cut; either that or someone has a hugeass thread roller, because the tubes are about 2" in diameter.

You make a good point however and I will definitely ensure that new and old struts appear to be identical or at least similar in strength before actually trying this idea on the road.

nate

Reply to
Nate Nagel

Got any junk early struts you want to sell me? :)

nate

Reply to
Nate Nagel

When the strut is fully extended there is very little pressure on the relatively large fine threaded strut cap. A 50% thread on that 3/8" depth at 2" plus diameter would have a safety factor of something better than 10.(likely SIGNIFICANTLY better). On the road there is virtually no tension on that thread.

Reply to
clare at snyder.on.ca

snip------------------

the tubing is about .010 thick if memory serves, so thread depth is probably nothing like 3/8 of an inch. you are right that UNDER NORMAL DRIVING on the road there is no vertical load on the thread, however, there is a vertical load if you jack up the car, or if you drive hard enough for the suspension to top out, which could happen with a pothole or somethng. Also, as I recall, porsche switched designs for a good reason, having to do with heat transfer as I recall - the cartrige type design did not allow as rapid heat transfer under hard driving conditions.

Reply to
William Noble

Don sez: ". . .Threading can be (and often is) done at very low speeds. I often thread at

For critical threads or to test the threading set-up, I sometimes hand crank my lathe spindle. I made an expanding arbor and crank to fit the back end of the lathe spindle. The expanding arbor was fashioned from a tailpipe expander obtained from HF.

Bob Swinney

Reply to
Robert Swinney

I suspect he meant length of threaded region. .010 is very thin metal, Bill. I don't have a Porsche so I won't say you're wrong, but I rather doubt that any steel that thin is used in their construction.

That could indeed be an issue. Dampers produce a lot of heat when exercised vigorously.

Reply to
Don Foreman

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