Lotus Elan

Thread Repair - Rear Shock Absorber Tubes

PostPost by: Tmac897 » Thu Apr 16, 2020 5:48 pm

Has anyone had to repair the threads on the rear shock absorber tubes? I purchased new shocks, and removed the old, but the new retaining nut on one of the tubes wants to cross thread as I try to reinstall the new shock.

I'm thinking of usung a thread file, but I don't know the pitch of the thread. If someone knows that inforation (thread pitch) or has done this repair in the past, any information would be helpful.

Hope all are well in these difficult times.

Best regards,
Tony
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PostPost by: 2cams70 » Fri Apr 17, 2020 2:32 am

Sorry can't help you with the thread specification but no doubt others can. I would suggest however that you use a die nut type thread restorer if it's a male thread (note: different to a die for cutting threads) in preference to a thread file. It will restore the threads more easily and with greater accuracy and because it's not designed to cut threads will only remove the absolute minimum of material. Quite a few for sale on Ebay once you know the size.

Note that sometimes the Nyloc nuts supplied with replacement shock absorbers are poor quality and have insufficient thread engagement length. The nut may look superficially ok but when you examine more closely you might find only a couple of turns of engagement with most of the height of the fastener made up of the nylon insert rather than thread.

In such situations my preference is to use two plain nuts one tightened on top of the other to provide locking effect. You could use a single nut with some Loctite of course but the problem with this is that it's difficult to stop the piston shaft spinning due to the thread friction caused by the Loctite when the time comes to undo.
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PostPost by: StressCraxx » Fri Apr 17, 2020 5:27 am

Since the tubes are a single point cut female thread, suggest you obtain a thread pitch gage from a tool supplier.
Then obtain a threading tap of the same pitch to clean or restore the threads. The steel of the tube is very soft.

The alternative would be to take the tube to a machine shop and have them re-cut the first threads.
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PostPost by: 2cams70 » Fri Apr 17, 2020 5:46 am

If they are female threads on the Lotus I suggest you use these from Lang tools in lieu of a threading tap as they are made slightly undersize and will therefore remove no more metal than necessary to clean up the thread. They are also extremely useful for cleaning muck out of threaded holes etc.

It is possible to improvise of course by using a standard bolt with some slots cut into the sides but trust me after purchasing a set of these you won't look back. They sell them on Ebay and the cost is quite reasonable.

http://langtools.com/rethreader-tools/

On second thoughts it seems to be the inside of the strut tubes you are talking about rather than the shock absorber piston rod in which case you are unlikely to easily obtain either a thread restorer or tap of large enough diameter to suit in which case yes - a thread file is probably your best option. The files usually have multiple pitches so you can select whichever suits best
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PostPost by: alan.barker » Fri Apr 17, 2020 7:14 am

Maybe the problem with the Thread inside the Strut Tube is there is still some of the Dimple/Centre Punch that was used to lock the original Special Nut
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PostPost by: nmauduit » Fri Apr 17, 2020 8:42 am

I did a search using "lotus elan shock tube thread pitch" and it returned

https://lotuselan.net/forums/viewtopic.php?f=16&t=11733&start=

"1.895" major diameter 24 tpi 60 deg thread was what I had some
replacement tubes made to a few years back. "

can't easily check with lockdown and my thread gauges away, but a quick caliper measure of 8 threads look compatible with 24 TPI

P1050523.JPG and
shock tube thread 24 TPI
S4SE 36/8198
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PostPost by: 512BB » Fri Apr 17, 2020 12:01 pm

Should the threads on the tube turn out to be nacked Tony, don't fret. I was working on one of my cars last summer, rear wheel bearing on one side had started to rumble on corners and wanted to tweak the setting on standard Koni inserts, which means removing and stripping down the rear suspension to do the bearings and to make the adjustment to the damper.

Got one side off the car, undid the retaining collar and removed the insert . Drained out the oil, cleaned out the tube, reset the Koni to 1 1/2 turns from base, it was set at 1, but after 45k miles I wanted a bit firmer damping, and replaced the damper in the tube. Filled with oil and refitted the collar. Started to tighten the collar and just as it was getting near to tight.................BANG. Gave me quite a start, what the f..k was that. Checked a few things but couldn't see anything obvious. Continued to tighten the collar and BANG again. Oh dear, it was obvious this time what was happening, threads gone in the tube and 4 days before European trip.

Sod it, what now, I didn't have time to replace a new tube in the hub, very difficult job anyway. I didn't have a spare hub assembly either. So after a bit of thinking, deceided to tighten the collar down to just before the threads let go, and spot welded the collar in 4 places to the tube. What the heck, the tube was nacked anyway, and the Koni inserts will go another 50k miles before they need touching again :D smiles all round. Trip went off great, no probs.

Any by the time those 50k miles comes round, I will have a new assembly ready to go. How about that for a bodge!

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PostPost by: alan.barker » Fri Apr 17, 2020 12:14 pm

But i thought the Collar Nut is in Alloy and the Tube Steel :?
The normal way is to put a Dimple in the Tube with a Centre Punch :?
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PostPost by: RichardHawkins » Fri Apr 17, 2020 8:17 pm

Tony,

I have just measured the old bits that I have kept. The original nut that I removed measures 1.904” OD and was some sort of non ferrous alloy. The replacements were steel and measure 1.899” OD. I measure the pitch at 24 threads per inch.

I was not happy with the fit of either the original nuts or the replacements. I made nuts from brass to fit the tubes. I chose brass in the hope that I could get the buggers apart in the future!

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PostPost by: RedBarnMan » Fri Apr 17, 2020 8:56 pm

Any local machine shop should be able to take a bit of 2" diameter steel and cut the 24tpi thread to the right spec. On a decent lathe it's only a few minutes work. A couple of slots milled across the thread or a few hacksaw cut grooves would turn it into a tap. I've done this before on vintage car parts and it worked well for me.
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PostPost by: 2cams70 » Sat Apr 18, 2020 12:19 am

FYI that strut cartridge retaining nut looks very much like a standard Ford Anglia / Cortina / Escort one. I believe the rear struts on Elans were adapted from Ford Anglia 105E front struts. The spring seats and tubes look very similar.
They should not be too hard to obtain in that case and you could therefore just modify one by cutting slots in it for thread cleaning.
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PostPost by: Tmac897 » Wed Apr 29, 2020 2:41 pm

The threads are indeed 24 pitch. The machine shop I have used for this restoration didn’t have anything wide enough to recut the threads. I did some looking around the Internet for other ideas. Based on that, I ordered a rotary thread file suitable for internal threads. It came in a kit with two sizes (diameters) and a small 1/4” collet for about $25 bucks from Amazon. The thread file is universal pitch. It’s made for a rotary tool, like a Dremel. But I don’t have one of those, so I just used my trusty Hitachi cordless drill.

I was aware of the dimple, but it seemed less worrisome than the thread damage from a fall off my work bench, and from the apparently overzealous use of an air gun with a punch tool to do the original nut removal. Since I was careful with the initial tries to rethread the new collar nut on, I didn’t cross thread anything too badly. I also cleaned up the threads on the nut with the rotary file. That job would have gone more smoothly with an external thread file of the correct pitch, but I didn’t have one of those, either. In any case, the threads on the nut weren’t too bad.

Using a drill is a little cumbersome. You really need two hands to control it properly, and the rpms are much lower than recommended. But working carefully and somewhat more patiently than I usually attack these tasks, I am happy to say that I was able to get the nut on properly.

Thanks for all your suggestions. Now I can finally reassemble the rear suspension, and have a rolling chassis once again!
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