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
--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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)
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Be carefull; cutting threads into a stressed tube, that you don't weaken the assembly. Not the place to have something break on you.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
They are not the same. The crimped style do not use inserts; the shock piston/oil/gas are enclosed in the strut tube which forms the outer wall of the 'shock'. This does not mean you can't gut them, and install an insert with proper machining of the tube, But the tube structure may not support this scheme. JR Dweller in the cellar
Nate Nagel wrote:

--
--------------------------------------------------------------
Home Page: http://www.seanet.com/~jasonrnorth
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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)
--
Posted via a free Usenet account from http://www.teranews.com


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Bruce wrote:

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
--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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)
--
Posted via a free Usenet account from http://www.teranews.com


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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:
http://market.autopartsfair.com/porsche-engine_parts/item-o6999_60860lvh.html
nate
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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)
--
Posted via a free Usenet account from http://www.teranews.com


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
cs snipped-for-privacy@hotmail.com wrote:

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
--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

    What happens when you're using a collet drawbar in the spindle?
    Enjoy,         DoN.
--
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Don asks, "What happens when you're using a collet drawbar in the spindle?"
I made a 2nd expandable arbor and crank to fit the hollow Royal drawbar (spindle).
Bob Swinney
| Voice (all times): (703) 938-4564

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
    [ ... arbor with crank for spindle ... ]

    And what happens when you need to turn the spindle in reverse? In my case, that would work too, because it is a lever closing drawbar, with a lock to keep it from rotating relative to the spindle, but for handwheel style closers, I would expect problems. (An example of when you would use this is when using a tap or a die in a tailstock or carriage holder and need to back it off or out.) But -- I also have the releasing tap holders for the bed turret, so it allows the tap to free-wheel until I can stop the spindle and reverse it. Yes, I do tap under power with this setup.
    Enjoy,         DoN.
--
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Don asks:
"> And what happens when you need to turn the spindle in reverse?

Don,
I have a Royal hand closing drawbar with a round handwheel on the outside end. My expandable arbor has a 10 in. crank. The expander is cranked into position with the lathe "in gear" usu. in the slowest threading speed. The crank is turned until it expands enough to begin turning the spindle against the heavy drag of my lathe (13 x 40). Then, with the lathe taken out of gear the spindle can be easily cranked by hand. In my machine, the lead screw remains engaged when "out of gear." Hand cranking makes for a very handy and sensitive setup for slow operations in the lathe.
I find it to be a good "cutoff starter" as well, particularly when working with larger stock. The ideal cut-off speed for larger pieces is usu. much slower than the min. 105 rpm of my machine. Hand cranking can take a lot of fear out of cutting off.
If ever I encounter the situation where there is enough resistance to unscrew the expandable arbor, I should expect to lend some other assistance such as barring the spindle backwards via a wrench in the camlock holes or etc. With a screw-on back plate I would still expect to lend some extra backing effort if the need arose.
Bob Swinney
| Voice (all times): (703) 938-4564

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
says...

If it comes to that, have a look at this piece I made to hold a depth stop in my lever type collet closer tube. You could just as easily clamp a crank in it.
http://www.suscom-maine.net/~nsimmons/news/ColletStop.pdf
A socket set screw in the 1/4-20 hole expands the body in the closer tube, a socket head cap screw in the 10-32 clamps on the stop rod (or crank).
Ned Simmons
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Nate Nagel wrote:

Threading is usually done at modest RPMs, so balance is not a real issue. What you need is a lathe that has a short enough spindle so the suspension bracket can hang out the rear of the lathe spindle while the top end of the strut is in the chuck jaws. Otherwise, you need a lathe with a HUGE spindle through hole to clear the bracket. I don't know how big the bracket is, most small-shop lathes have fairly small through holes, between 3/4 and 1.5", say. My 15" Sheldon has a 2.25" through hole, and the spindle is about 29" long from the back to the tips of the chuck jaws.
Jon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.