bodged thread repair

I'm going over my newly-acquired Moto Guzzi T5 Polizia and removed the rear wheel to change the tyre. It was disappointing to find that the retaining ring for the cush drive was missing and it didn't take long to find out why. The ring is held by three M6x20 bolts - one is stripped, another broken 8mm below the surface. These have been a straightforward repair, but the third one is problematic:

The bolt is broken below the surface and an attempt has been made to drill it out, but this has not been done right and the drill has wandered into the aluminium. It has been Helicoiled at an angle, the coil being 1/3 in the remains of the steel bolt, and 2/3 in the aluminium. There is very little metal surrounding the area. My initial thought is to remove the helicoil and make a core drill to remove the remaining bolt, but then what to do.

Ideally it would need welding, but the hole is deep. I wondered if it could be filled with a spool gun, or MIG to get to the bottom of the hole. I don't know about TIG - it seems to be too small and too deep to get to the bottom of the hole effectively.

Does anyone have any suggestions?

Mick,

Ask on here **LINK** There a lot of pro welders on there and they know what they are talking about.

Replace the wheel with new or used one?

Bore out and fit a turned insert, weld insert in place by TIG?

How much is "very little metal surrounding the area"?

Could one of these work?

http://www.timesert.com/html/bigsert.html

If you post a picture, you will likely receive more focussed suggestions.

Edit:

Another option is 'Keensert', see:

https://uk.rs-online.com/web/p/thread-repair-inserts/6684705

(The full Keensert catalogue is downloadable as part of the technical data at the above link)

Edited By DC31k on 25/02/2023 14:25:56

An M6 x 1 Timesert won't help as the tapping drill size is the same as a helical so it can only go back in the same place.

Consider using high strength loctite to hold a special thin wall bush with an M6 x 1 thread down the middle. Last time I did such I was able to use 1/8" BSP for the bush thread. Being able to buy a solid blanking plug from the plumbing shop made it all much easier.

Sounds like there may not be enough room for a thread that large, almost 10 mm Ø, but M9 x1 and M8 x 1 exist. One time job standard taps and dies can be found relatively inexpensively on line.

Once you have the hole straightened up consider using a counterbore to take out the off angle part at the end so a suitable stub of aluminium alloy can be fitted with loctite to properly support the end of the bush. As you have hole depth to spare consider going deeper so a longer screw can be used. If you choose to do that make your bush just long enough to correct the off angled helicoiled portion then extend the thread into the parent metal guide by the thread in the bush.

Welding is a last resort. Not only is filling holes in old metal is tricky for all sorts of reasons it's also not a job that folk get much practice at. The professionals usually prefer to find another way or decline the job.

Clive

Edited By Clive Foster on 25/02/2023 15:28:06

Here's a picture;

Ive never felt comfortable 'bogging' jobs like this if If it were me I'd be buying another wheel

H

The 'big sert', produced by the Timesert people is designed to replace failed helicoils.

The thin wall bush is also produced commercially as a 'Keensert', available in at least two differing ODs (see RS link).

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The perspective of the photo is not clear. How far below the threaded hole is the spoke to the upper right of the photo? If it is lower, then you slice off the edge of the hole, so the repair area becomes an open channel, sit the wheel at 45 degrees and rebuild the entire boss. Many, many stripped out two stroke exhaust studs are repaired the same way.

An open channel allows you to remove everything that is not aluminium and to clean the hole to a surgical standard before welding. As an option, a car alloy wheel repair specialist should be able to do the welding for you.

You seem to have two good holes now, so how about using the retaining ring, or another specially made one, to act as a drill guide for the offset hole.
Obviously remove the dodgy Helicoil, and re-drill the hole, but using a spot drill and pecking at it.
Spot drills are far more rigid than jobbers, with very short flutes, so lend themselves to being guided by a close fitting bush. I some ways, provided there is an existing hole, the short flutes make them easier to steer, and more rigid, than a 2/3 flute slot end mill.
https://www.cromwell.co.uk/shop/cutting-tools/spot-drills/c/39151101
You might need ¼" spot drill, rather than 6mm to start with.

An M7 Helicoil uses a 7.2mm tapping drill, so maybe you could then use either a stepped 6/7mm stud, or an M7 bolt.

M8 uses 8.2 or 8.3mm drill, so there might still be enough meat in the boss to hold a larger stepped stud.
Some drill sizes HERE
https://www.thorintl.com/Drill-Sizes-HeliCoils-Metric.shtml

As above, there are other alternative inserts to Helicoils

Good luck
Bill
LeMans Mk3

Although the bodgerey is hideous … The amount of ‘meat’ remaining doesn’t look too bad.

I would first remove the Helicoil [inflicting the minimum possible damage to the wheel] and then measure carefully the minimum diameter which would clear away the damage and restore the correct location.

Then, and only then, decide what repair-scheme is appropriate.

As Bill says, a specially made replica of the hole-pattern on retaining ring would seem appropriate for this exercise.

MichaelG.

The cheapest new wheel I can find is £1391.97. I can't find a used one that matches mine - the brake offset is different on the ones I've enquired about, and they're all in Europe. I've been bitten buying used parts from EU breakers and trying to return scrap. Also, the screws coming loose and breaking when they hit the webbing in the drive box is not uncommon and I don't want to risk buying something that still needs repairing. Buying in the UK would be a better option to avoid the prospect of return postage and import costs. To get the bike on the road I'd prefer a good solid repair for the time being and if a wheel becomes available buy a replacement, if necessary.

The threaded hole extends into the spoke web, but the Helicoiled depth is fairly shallow.

You're not getting a timesert onto that there's not enough meat. If it were mine I would make a hardened drill bushing and put a 6mm slot drill down it until all of the bolt was drilled out, then I would simply use a longer bolt to pick up the remaining thread.

To use high tensile material, could you topen the hole to 7 x 1, and then machine a long M8 bolt / stud to make a stepped stud with a M7 thread for the wheel and a 6mm to retain the ring?

Howard

I thought of a stepped stud. Maybe even an M8, or as previously mentioned, an M7 Helicoil. I can't see how much of the remaining M6 stud is left, but getting that out cleanly first and taking a look how much of the original hole is left is perhaps the best path. I was even thinking of removing the Helicoil and filling that hole with a solid slug of threaded alloy bar, coated with JB Weld and made a very tight fit, then treating the job as though it had never been Helicoiled. Because that hole is much shallower then the original, and it runs sharply away from the stud, it may be that the re-threaded original hole would not extend too far into the JB welded slug. Whilst the original bolt is M6x20, the hole is 22mm deep - easily enough to get at least a 1.5D insert into good metal and maybe even a D2.

I don't think there's much force on the retaining ring that the bolts hold - they went from 6 screws to 3 in later years and the ring is fairly thin gauge sheet steel.

When I was working I would specify a "Tappex" insert when a thread in an aluminium part needed reclaiming. These inserts could be used when a Helicoil insert had been stripped or otherwise damaged. The drilling size for them was close to the outside diameter of the Helicoil tap. The Tappex inserts were also self tapping so were simple to use.

The type that I favoured was the "H Type" either the standard insert or the thin wall version, the slotted end type could close up and nip the thread which I did not like. For aluminium I always specified the Austenitic Stainless Steel material as the expansion was close to that of the aluminium.

**LINK**

It will of course depend on what the hole is like when you manage to get the bits out and the remaining wall thickness. I hope that this is some help, the wheel is too expensive not to have a look at every option.

You can get a sample of the insert from the company, I used to have some in the draw at work when we were investigating them for use.

Neil

Just looking through the posts again, I think that I would feel very uncomfortable about welding an alloy that I did not know the exact composition and heat treatment. Get it wrong and you could end up with cracks where you do not want them.

Someone may correct me, but I seem to remember that the highest stresses occur in the hub area around the fasteners of flywheels, I assume that a motor bike wheel would be similar.

Neil

There appears to be a wheel on eBay, which you've probably seen, but it's in Germany
https://www.ebay.co.uk/itm/384071838430

If you've another bike on the road and fancy a ride out, I should be able to supply both M6 & ¼" spotting drills up North in Buxton; beverages can be supplied.

I'd have to check, but I think I also have an M8 coarse Helicoil kit as well, if the resultant hole is too big to tap conventionally

Bill

+1 on this. A slot drill with a hardened guide bush is far less likely to wander than even a stub drill. You are trying to drill out an offset hole in two different materials, one hard and one soft, that's a big ask for a drill bit.

I can't really tell from the photo but you might be able to make your own oversize threaded insert and drill and tap to suit after removing the bolt if there is not enough thread to use deeper in the original bolt hole.

Or alternatively, if the existing helicoil is securely in place, is there any reason you can't use it as is? If it is not quite straight, you might have to enlarge the hole in the ring that it retains and maybe even machine a tapered washer to suit? A bit bodgey for sure, but may be an alternative if you absolutely don't have enough "meat" there for inserts etc and the threaded hole has not extra threads deeper inside. Better to "bodge" than destroy an expensive wheel if there is not enough metal there to play around with, and it does look very thin there. Not familiar with Guzzi wheels but the bolts are not transmitting drive, just holding the retaining ring for the cush rubbers, are they not?

Edited By Hopper on 26/02/2023 02:55:40

I'm taking a fresh look at this today to see how I can locate a hardened bush to get a slot drill to at least square-off the bolt remains.

I saw the wheel in Germany, though it looks in poorer condition overall than my own (apart from the damaged threads). There's an issue with the VAT I can't figure out. The price is shown with differential VAT applied at the point of sale, according to German rules. To import it, I'm unable to clarify if the seller has to recalculate the item less differential VAT (which of course gives away their buy-in price) and I import the wheel VAT-free at the point of entry and pay UK VAT, customs and handling fee prior to receipt. I can see this adding up to around £250 total, which is a lot to take a gamble on. If it's returned ,I then have to reclaim the costs (less fees) and argue to over the postage.

Time honored method is to make a drill bush out of silver steel and harden in oil or water. But such bushes are available commercially readymade for use in industry.