bodged thread repair

i feel your pain Mick. As a fellow Guzzi owner, many of the 'helpers' suggestions can easily fall by the wayside as they don't know how cr*p some of the alloy castings are. You should be able to screw the helicoil out with a tapered blade but then, you need to make an accurate jig to ensure true drilling of hole. A decent welder should be able to plug weld the hole and then redrill (Thus need for an accurate jig, with location from other holes) and tap for either original size or helicoil.

Fond out about the 'Quality' of Italian alloy when having to fit hellies in my Le Mans crankcase to bell housing bolts. Screwloc was originally used by factory to make up for casting defect(s).

Regards Ian.

My neighbour asked me to do a very similar repair on a bodged Italian Lambretta scooter wheel hub.
The only way on that one was to get the old helicoil out and have it TIG welded to fill the hole.

I managed to get it re-drilled and tapped for the stud by using a wheel rim as a drilling jig.

Phil

The Helicoil removed easily so my next step is to remove the remains of the bolt. I need to make up a really rigid jig because of the angle formed when the Helicoil drill veered off. I don't have a milling machine, but my pillar drill is fairly rigid and so long as everything is bolted down firmly it should be OK. I just need to make a threaded adapter for the slot drill this afternoon.

I hit a few unexpected problems getting the remains of the original bolt out. The other broken one drilled out easily, but I found that the screw in this location was hardened. Despite using a firmly clamped 5/16x3" plate with a hardened, press-fitted bush, the slot drill had a really difficult time and created a much larger hole than its 6mm diameter. However, the resulting hole is straight and centred in the boss, but tapers from 6.8mm at the top to 6.1mm from just over halfway down. I have probably 12mm of depth that would take a 6mm Dx2 insert with full thread engagement.

The 6mm insert kit I have uses a 6.3mm tapping drill. I'm thinking that I either go out to a 7mm insert and get another kit, or use the 6mm insert and ream the upper tapered section parallel to 7mm to take a stepped set screw, This would have a plain 7mm section to act as a dowel, whilst the rest of it would be 6mm. This may be preferable to drilling out the hole for a 7mm insert and losing more material.

At least the hole diameter that runs into the spoke web hasn't been affected too much and I can avoid any welding.

A standard M6 Timesert has an 8mm Ø countersunk section at the top going down a couple of mm or so. This will take out the top end of your hole taper and give a clean entry to the screw. There may be a small portion of the insert not completely supported where the taper runs out but that should be no problem.

I've successfully done similar repairs in "abused" holes using Timeserts installed as per book. I used Loctite for additional back up strength but whether that was necessary is a matter of opinion. Worked for me on jobs rather more heavily loaded than your job appears to be too.

There is no doubt that the all one piece bush style construction of a Timesert deals much better with abused holes than the simpler, spring style, helical construction. I'm convinced that the solid ring at the top is a major contributor to this as it ensures a clean entry to the screw or bolt. Pretty much every problem I've seen with helicoiled holes has been due to the screw or bolt not entering cleanly and subsequently being forced.

Bad point about Timeserts is the very high cost. Although I have to wonder if there are quality / performance issues withe the economy range helicoil clone kits which may mean they are best restricted to less ambitious duties.

Clive

Edited By Clive Foster on 27/02/2023 13:42:56

I've notice that the cheap helicoil clones don't have a well-formed section to the insert and don't give the correct internal thread profile or engagement. I use the Australian Recoil kits and have never had any problems. Timeserts look good, but the basic 6mm kit is over £190 in the UK.

Baerfix are certainly cheaper, though I've never used them myself, but a downside is the 9.3mm drill for an M6 thread.
https://uni-thread.com/tr_baerfix_metric_kits_detail.php?01581-6

Bill

Given the corrosion caused by the ali/water/stainless mix would it be better to use stainless studs and nylocks? The Loctight would keep the water out.

Probably a daft idea, but 1/4 UNC would be just a tad bigger than M6, and a slightly bigger tapping hole than is strictly necessary wouldn't weaken it significantly

£160 is indeed outrageous for a 6 mm Timesert kit that can be got for around $90 in the USA.

Found an Amazon listing at £133 but you'd need to get the longer inserts on top as that only has the standard shorter ones. Another £30!

There is an apparent Timesert clone branded Sennmonn which are also one piece with rim and proper thread lead in. Around £80 on Amazon for M6 x 1 kit. Stainless steel inserts make them attractive but they are simple bushes which, unlike Timeserts, aren't expanded into place. Need to use high strength Loctite to retain them which ought to be fine.

Realistically £160 (if you get via Amazon) for a Timesert kit isn't too stupid if it gets you a known good repair that just works. Any sort of welding and other mechanical work to give clean hole for a helical or a filled hole to drill and tap yourself is liable to end up a similar cost. At least if you go the timesert route you have the kit for other jobs. Or, given the known outrageous price, odds are getting around half your money back via E-Bay, Facebook Marketplace et al seem pretty good.

The Sennmonn kit seems a pretty good option as its effectively the same as making your own bushes but you get all the tooling too.

Clive

Edited By Clive Foster on 27/02/2023 16:12:30

The £133 kit looks to be shipped from the USA. If so, then it could attract additional import charges. I'll take a look at the other options - the main thing is not to have to remove any more metal.

I like the idea of 1/4UNC, but as a Helicoil insert. The tapping drill is 6.7mm, so would be a decent fit, though I don't know about the reduced core area.

according to interweb 1/4 UNC has bigger core diameter than M6.

Thanks, I took a look at my 1/4" UNC tap and compared it with an M6 screw and found the same. I also have some 1/4"x24 taps and dies, but finding inserts for that size is more difficult.

After a lot more pondering I've concluded that there's enough metal in the boss to tap it out to M8 down to where the spoke web comes to, which should be amply strong enough with a stepped stud machined up from a stainless allen bolt. The maximum major diameter given by Helicoil for an M7 insert tap (which would have been my other choice) is 8.422mm, so an M8 tap would be slightly better. Looking at the retaining ring and how the wheel hub fits into the drive box it looks like the ring is only really there to prevent the axial movement of the cush drive, which is very limited anyhow. Three M6 bolts suggest that can't be that much force on the ring.

I note that Tracey Tools sell Helicoil type thread insert kits in UNC sizes. Their prices exclude VAT and delivery, that might suit your needs.

Neil

I went ahead and tapped out the hole to M8 for a depth of 15mm and this has worked out well with a stepped stud machined from a stainless allen bolt. I think when this is Loctited in with high-strength retainer there should be no further problems.

Posting here has been a great help in considering different options and increased my knowledge of different insert types, so thanks to everyone who has replied.

Good afternoon all.

Good job well done by the sound of it.

I see this sort of thing on a regular basis in my workshop. My preference would have been to mill the hole out with a 7mm slot drill, & make a bush with an M8x1 thread.

It's amusing when some oaf (usually the customer & his mate) have had a go at 'drilling' it out. I have been known to ask if they were wearing a blindfold during the attempt.

On the subject of thread inserts. Both Helicoil & Timeserts have their place. . A Helicoil can be cut down to size. I've put as little as 4 twists in shallow holes. A Timesert cannot be cut down, nor can they be fitted in blind holes unless they're deep, because the tool that expands the insert has to pass right through it in order to secure the insert.

A Helicoil can also jump a gap, like in a single pinch bolt on a clutch lever perch.

However, Timeserts are stronger. So for stripped sump plugs, & wheel spindle threads in fork lowers etc I use Timeserts.

As one poster said, the Australian Recoil inserts are the way to go. They're excellent!

Cheers.

Stewart.

Edited By Bdog507 on 01/03/2023 16:38:42

There was a fairly well done comparison video some while back that found the opposite to be true.

What's your technique for making such a thin walled bush? Looking up the major and minor diameters gives a minimum wall thickness of 0.5mm. I'd like to experiment with making up some inserts like this for future use.

It would be interesting to see that. I'll have to see if I can find it.

I've used both for the last gawd knows how many years, & have found the opposite. With either they need to be properly installed, i.e. Timeserts need to be fully expanded in order to work correctly. If they're not they often unscrew instead of the fastener within them, & they can also pull out of the thread they're screwed into.

I have yet to have either fail out of the thousands I've installed, but that's what torque wrenches are for. I've seen many badly installed ones fail however.

II still reckon on using helicoils for 95% of occasions. Not only are they more versatile, they're a damn sight cheaper too.

Cheers.

Stewart

Very good taps and dies basically. I usually make such things out of EN8 it being a little stronger than normal mild steel.

All done in the lathe. I put the outer thread on first with the die expanded a little & then drill the tapping hole slowly. Then I use a spiral point machine tap by hand & carefully. Good SP taps cut the thread more progressively than even a taper hand tap. Then if need be I close the die up & carefully take a little more off the external thread.

It's a faff, & I reckon on getting one bush out of two tries.

I used to have some hand tap sets where each tap cut the thread more deeply. Obviously one had to pass all three taps through in order to get a full thread. These were excellent for such delicate tapping. I can't remember what make they were, nor do I know if such things are still available.

Cheers.

Stewart.