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I have an aluminium clamp on a bike where the thread has stripped. The steel bolt is okay, but the clamp has very little movement - I can't get a helicoil or drill through one side of the clamp without drilling out the metal under the shoulders of the bolt.

Fortunately the hole is not blind - I can see up the bottom of it to the leading face of the bolt.

So is it feasible to drill and install the helicoil from the "wrong" side?

I think so, but I've been wrong before. One possibility is braking off the tang will leave a rough "leading" edge that may not engage the bolt smoothly, and would be hard to file.


Own work Top view, bolt on left showing normal position, helicoil on right side showing how I want to fit it

Own work Side view showing edge of clamp inside for bolt's shoulder to rest on. As you can see there's not a lot of metal there, so removing any is bad.

I can't really put a washer on the outside, the top view shows the sides are not parallel. Would have to make a pair of "diagonal" washers, more like a bushing cut diagonally.

Neither the drill nor the tool nor the helicoil itself will fit through the bolt-side hole.

I cannot buy a replacement clamp at any price - they just don't exist. Getting a whole clamp made would be uneconomical.

Own work Here's the place the clamp goes on the bike - commonally available "normal" clamps just won't fit. The bike is 20 years old too so no chance of finding new parts for it. Yes there's a big crack too - thats being hard soldered by a pro next week.

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    To be clear this is about a bicycle part that needs a M6 thread. but its a mechanic type question and installing helicoils is outside the experience of everyone in the bicycles chat. So I figure its on-topic here because of prior questions about thread-replacements. – Criggie Feb 5 '18 at 4:25
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    Another possibilty is finding an old one on an old bike being scrapped - asking at a bike shop or two may be fruitful. – Solar Mike Feb 5 '18 at 6:35
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    Is the problem that the seattube is a weird diameter and clamps to fit it don't exist, or that standard clamps won't match the aero profile of the tube? I'd go with @myself's suggestion, and failing that try a shim to the next available size and live with the profile not matching. – armb Feb 5 '18 at 10:20
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    You might be able to re-tap all the way through, put the helicoil in far enough that the tang is in the gap, then grind it off with a cutting disk. – armb Feb 5 '18 at 10:29
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    Nice bike - its worth saving if you can. – Pete Feb 5 '18 at 20:57
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You could partially enlarge the hole (where the tread stripped). Place a round steel nut into the enlarged part. You could also fix the nut with glue

enter image description here

  • I'd worry about creating a stress riser and having the whole thing snap. I thought of going the other way and making two thick bushings as washers, but cut it to the slope of the side so that a much longer bolt is flat. – Criggie Feb 5 '18 at 8:48
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    @Criggie the bushings method would be the most material-gentle method, yes. About the stress riser: You already have one on the other side (where the allen head rests). So it should not make a big difference (if you make an exact copy) ? – Martin Feb 5 '18 at 8:53
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    This is the method I'd use. I was going to write my own post of the same thing before I realized the read almost exactly to what I'd written. I don't think I'd worry too much about a stress riser here. If the nut used to back it was trimmed down a little bit to fit the whole, then either using Loctite or superglue, affix it into the hole, I don't think there'd be any issue. Far better than what you have now, that's for sure. – Pᴀᴜʟsᴛᴇʀ2 Feb 5 '18 at 13:36
  • Ideally you would make a hexagonal hole for the nut to fit in. But that could be either tedious and hard (filing by hand, into a blind recess), or expensive (having it machined). – armb Feb 6 '18 at 7:38
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Well, I did it, and it was one of the most straightforward fixes possible. Here's what I did...

  1. So I bought a cheap M6 1.0 kit for $27 NZD. Trademe https://trademe.tmcdn.co.nz/photoserver/plusw/555457348.jpg
  2. Clamp was secured in a drill vise so it wouldn't move. I used two soft-jaws, this photo just shows how it looked. Own work
  3. Drilled! I checked the max angle required on the drill-press handle so it wouldn't start drilling the other side of the clamp. Note-Alloy drills like butter, in that I had to put minimal pressure on the drill. Got to be careful to not overdrill the depth. Own work
  4. Tapping - again dead easy. The aluminium was soft enough I could do two or three turns before backing out the spoil. Was super careful to get it straight. Own work
  5. Here's the spoil/swarf from tapping. Surprisingly little material. Own work
  6. Threading in the helicoil from the wrong side. Just after the photo but before turning, I put blue locktite on the coil. The tang is on the inside so we do have to break it off later. Own work
  7. Coil is threadded all the way in. Since its a spring, it will wind up around the install tool so you need to keep tension on the tool when moving hands between turns. You can see the last turn is not fully in the right thread. The bolt doesn't go that far through so it shouldn't be a problem. Own work
  8. Done! You can see the tang is broken off. I had to use the punch and a hammer to tap it off. I haven't tensioned the bolt because the threadlock takes ~24 hours to cure. Own work

Completed!

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    Great job with the follow-up! – Zaid Feb 6 '18 at 6:43
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    Nice one. A suggestion: a bit of plywood/MDF/plastic in the open slot will indicate when you've got through the first half of the clamp, and protect the second half from the drill moving suddenly. It should be a loose fit but not so loose it can rotate, – Chris H Feb 6 '18 at 13:07
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    @ChrisH yes that would have been a good idea. I simply eyeballed the angle of the drill press handle. The press used to have an adjustable drop limiter and gauge, but it was awkward so was removed years ago. This is one of those times it would have been handy. – Criggie Feb 6 '18 at 22:47
  • @Zaid thanks - the answer is just as important as the question, and the process of getting from Q to A is generally relevant too. – Criggie Feb 6 '18 at 22:50
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You comment that this is M6. Presumably M6×1. For a helicoil you'd need to run a 6.3 mm drill down there and then you'd be tapping to something like M7×1.

M7 screws are rare as hens teeth but they do exist and the tapping drill is 6 mm, which is what you'd have if you cleaned the existing M6 thread. The ones pictured (meant for stem bolts)appear to have a smaller head in proportion to the thread than metric screws usually have. They may well be near enough the same head diameter as a standard M6. Apparently Bontrager use M7 saddle post bolts (reported by another supplier), with an 11mm head diameter -- you might need to open out the head hole a touch, but you might have enough room as the clearance holes are often a bit oversized in clamps.

Taps are easily available (just be sure to get M7×1 and not something even more specialised). A set of taper and bottoming taps would be good, but you could get away with just a taper tap.

These are UK links, but the taps and screws are available on eBay with worldwide shipping.

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You could take it to an engineering company and get them to fill the hole using a welder, then re-drill and tap to the original size if your helicoil doesn't work.

  • Excellent thought - buit the clamp is aluminium alloy of some sort. I'm not sure exactly what alloy it is either. – Criggie Feb 5 '18 at 8:49
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Can you live with ugly but strong?

You could put a longer bolt through if there were wedge-shaped shims on either side. You could cut and shape these just from some bar stock by hand - no precision is needed, just a hole big enough to fit the bolt and the key thing is getting the triangle angle correct so the bolt head and nut have parallel sides.

You could even trial it by cutting them from timber, which has the advantage of being easier to shape by sanding, but may not be hard enough for the degree you need to tighten the bolts.

The advantage of this approach is that it is totally non-destructive so you can keep trying!

  • Excellent idea. That was one of the backup plans. – Criggie Feb 6 '18 at 3:50
  • Another possibility is to fill the hole with liquid metal, drill and re-tap. Its surprisingly strong. – Sir Swears-a-lot Feb 15 '18 at 8:27
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Your last paragraph sums up the problem exactly. The tang is usually either left or broken off at the very bottom of the hole and causes no problem as it is not normally reached by the bolt.

If you can drill it and tap it ready for the helicoil, you may be better off just putting a larger bolt in.

If you install the helicoil from the correct side - could you add a washer or one of the bolts with a wider head to cover the larger hole?

Another possibility may be to use a longer bolt and put a nut over the top of the clamp, - ugly though...

A picture or two would make the possible options much easier to consider.

Edit, so now after seeing the pictures I would drill the old threaded hole a bit larger and then make a blank to fit in that hole tightly (welding it up is a good suggestion) and cement it in place, then re-drill and tap it - cutting the outer profile to fit.

  • I checked - the helicoil doesn't fit through the hole at all. So either I would have to bend the clamp apart, or do it from the wrong side as I ended up doing. – Criggie Feb 6 '18 at 5:40
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From what I read, it seems that primary problem in installing the helicoil from the wrong side is that the tang would be left on the entry side of the helicoil.

How about just installing the helicoil backwards? Depending on the insertion tool this may require some improvising to get it installed, but aluminum is so soft you could probably thread it in even with just pliers.

There are also threaded inserts that do not have a tang, instead having slots for a screwdriver, for example e-z lok.

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    The big problem with screwing it in backwards is how the helicoil works. Normally, as you twist the helicoil, you actually shrink the Helicoil just a little bit, kind of like a spring, where it is drawn tighter around itself making it have less of a diameter, and thus making it easier to get into its hole. This also helps it to stay in place better after the twist is done putting it in place, because it resumes its regular shape which holds it. Screwing it the other way (tang last) would cause it to expand at first, making it very hard (if not impossible) to insert. – Pᴀᴜʟsᴛᴇʀ2 Feb 5 '18 at 19:19
  • I tried this and it simply wouldn't go in after about 2 turns. When installing it properly (from the wrong side) the springy coil tightens around the tool making it go better. – Criggie Feb 6 '18 at 5:41
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Get a quick-release skewer. Fit (grind) an angle on a thick aluminum washer so that the skewer applies correctly. Solder fitted washers to both sides of your clamp with argon welder. That'll be neat and safe solution.

Please don't rerun your damaged thread with larger diameter. I've been thru exactly the same situation personally. You don't want to share experience of your seat drop or fall off when you're busy pedalling or soaring down the slope, believe me!

But, if you will, aluminum alloys generally would take the backwards thread application. Just do a step by step, rotate 10-15 degrees max, then back, then repeat. Apply light oil.

  • I've tried replacing a bolt seatclamp with a QR skewer on my "spare" road bike and it just can't clamp down as hard as a bolt can. But its worth a try if I end up ruining this clamp with the helicoil. – Criggie Feb 6 '18 at 2:44

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