Rear View Mirror Tension - Stripped Threads

[Longtooth]

My interior rear view mirror was getting loser, so I took it apart to tighten the tension and found that the threads on the goose-neck tapped hole were nearly completely stripped.

I'm thinking a helicoil insert to fix, but since goose-neck material is pot-metal, I'm worried that drilling out the hole for a helicoil might fracture/crack the end of the goose-neck surrounding the tapped hole in the process.  Any other quality methods of fixing this problem permanently?

UPDATE #1:
Before checking with a machine shop to determine viability inserting a helicoil, I was curious about how & why the tapped hole threads became completely stripped... and why the mirror tension failure occurred after over three years of having had no problem with it.

The reason the tapped threads are stripped turns out to be related to the tension created by the tension spring under compression... and the compression required to get at least 3 of the total of 6 threads engaged is approx. 50% of the spring length.  I haven't yet measured the force under 50% spring length compression, but I can't compress it to that level by hand!, so the force required to get 3 threads engaged will be relatively larger than I'd thought.  The uncompressed spring length is 1 cm.  The tension bolt's tip has a clearance to the 1st thread before compressing the screw of approx. 0.34 cm... and another approx. 0.21 cm to get 3 threads fully engaged... 0.55 cm total compression required for 3 threads engaged.  To engage all 6 threads requires another approx. 0.21 cm... so a total spring compression of approx. 3/4's of the spring's length.  That would require full spring compression... all turns bottomed out on each other... .which is extremely excessive in terms of force.  

Moreover, since the spring needs to be compressed by approx. 1/3rd it's length just to get the 1st threads to engage, it's very possible under that compression force to easily & unknowingly cross thread the bolt into the tapped hole.  This is likely what occurred at some time prior to my ownership.  When I disassembed & reassembled things 3 years ago I was apparently able to get a couple of cross threaded threads engaged/wedged with sufficient tension / compression to hold the mirror in position.  With cross threaded conditions however the force on the remaining remnants of threads eventually pulled the tapped metal (pot metal) threaded / wedged points loose, hence my problem with no longer being able to hold the mirror in position.

What I can't understand though is why the design uses such a large spring.... with length that requires approx. 1/3rd spring compression just to begin engaging the bolt threads with the tapped hole threads.  

UPDATE #2
On more careful examination, it turns out that clearance hole in the mirror bracket isn't a clearance hole afterall... it's actually all stripped threads!!!!

This make far more sense from design point of view, since screw would engage the tapped threads long before the spring get's compressed at all.  As to cause of totally stripped out threads in the 1st place, all I can surmise is that the threads were stripped out by prior owner and that I'd been able to engage only 1 or 2 remaining threads at the bottom when I'd re-assembled the mirror 3 years ago... which required excessive spring compression, hence tension on the 1 or 2 hole threads I'd been able to engage... and therefore that the force under that compression exceeded the ability of the metal to withstand that force over time.... therefore eventually giving out and leaving me with the dangling mirror a few weeks ago.

The precision machinist said he can install a helicoil without damaging the bracket.  The helicoil doesn't have to go the bottom
of the existing tapped hole since the screw wouldn't be able to get to that depth anyway due to the amount of compression that would be required on the spring.  It's obvious from the length of the stripped threads (now) that the most the threads had ever previously been engaged was by no more than ~ 0.35 cm beyond the uncompressed spring position... or maximum of approx. 1/3rd spring compression.

UPDATE #3
Bracket (gooseneck) returned from machinest today... no muss, no fuss 5 x 0.8 metric helicoil installed.  No marks/indents/scratches on chrome or cup surface due to installation.  Will reassemble and report on tension adjustment capability tomorrow. Helicoil & installation cost $60 cash (Precision machine shop rates in our area are $85 - $100)... a lot cheaper (I think) than a used bracket plus rechroming even if I could have found one quickly.

UPDATE #4 - Final
Tension is now adjustable... and right. Re-assembled mirror & installed.  Thought I'd lost the little flat-spring that's needed for the day/nite mirror flipper... found it on garage floor right where I'd thought it must have dropped, but needed a flood-lamp to find it after spending an unsuccessful 1/2 hour on my hands & knees.  Went on our 1st Sunday Drive today to check out the mirror's ability to stay precisely where I adjusted it.  It stayed absolutely put.

See attached picture of the relation of the screw to the tapped hole in the bracket with uncompressed spring. Dimensions are my measurements or calculated... accuracy only to +/- 0.02 cm (0.2 mm) though.  The tapped hole in the bracket should have threads all the way to the bottom of the spherical cup --- unlike my stripped thread bracket.
Download Attachment: Mirror Tensioner.JPG
99.51 KB

[psmith]

I have a loose mirror too.  I've tried several times to remove it from the windshield frame, and even though I know how it's mounted, I can't budge it.  How hard, and in what direction do you yank on it to remove it?

ps are you going on the drive on Sunday?

[Longtooth]

Pete... yes as to Sunday Drive.

I'd attach a picture of the pins & compression spring in the top of the goose-neck bracket, but my bracket's at the machine shop.... so sorry 'bout that.

I give bracket (at highest point nearest it's base) a sharp yank  toward left side and down, in parallel with window (yank right to left).  Reason is that the two retaining pins with the compression spring between them is oriented side-side, so that a right to left with downward yank only has to free up (detach) one of the two compression pins from their indent.  Yanking front-rear requires both pins to release (compress spring) at same time, making force applied double that required for a side-side release.

Compression pins have a spherical surface where they meet the spherical indent in the mounting piece (which is the part attached directly to the windshield frame with 2 screws).

Also, in side-side yanking action, the moment on the single pin (either left or right side, depending on which direction side-side you pull/yank) is greater than when using front-rear yanking motion.... the downside of course is that this also puts a greater moment on the goose-neck bracket and it's thinner (side-side) than in the front-rear direction.  That's why I apply force (hand held) as high up near the mounting pins (top of goose-neck) as I can.... since the moment on the goose-neck is reduced for the same force applied to the compression spring (pins).

Reason I do right to left yank is because the left pin is slightly scarred so it's retentive force due to friction on the scarred end/tip is greater than the right pin which is smoother on my mirror holder.

When I see you this Sunday (if you're participating), bring a can of WD-40 (or liquid wrench) with the little plastic squirt tube (straw) that usually comes with it ... we'll inject some lube into the space between the bracket and mounting piece.... it'll help loosen up the compression spring and the friction between the pins and their indents. Don't forget to bring a rag to soak up the excess lube/spray though.

Also, possibly a hard plastic wedge with tip thin enough to potentially force between the bracket and the mounting piece.... a screw driver works too, but damage to chrome is always the potential issue. Ideally MB would have designed a slot into the sides of the mounting bracket so that just such a method could have been used to apply the force and direction required.... alas, they didn't though.

[psmith]

LT, Thanks for the detailed description.  I have seen a diagram of the attachment setup, so I know what I'm up against, but I just haven't been able to get it loose.  I haven't tried it the way you describe, but it makes a lot of sense.  I'll shoot a little WD on it Saturday night and maybe we can get if off on Sunday (after the drive )