Stripped thread on chain ring

[RodWatts]

Campag Chorus four arm crank
Anyone got ideas on how to fix a chainring bolt where the thread has stripped on the crank hole on a four arm spider.
Would ‘Chemical Metal’ work?
Thanks
Rod

[Mick F]

Double or triple?

I have a triple Chorus on Mercian, and the crank hole threads for the inner ring is rather weak. Not stripped, but not far off. Either poor design or poor manufacturing - or both. The middle and outer have their own crank hole and that's fine.
(This means that there are two crank holes with a triple.)

What I've done, is to fit the inner ring semi-permanently. I can strip off the outer and middle rings to deep clean it whilst leaving the inner in place. I used a strong Loctite - sorry, can't remember which it was as I was lent the bottle by a friend.

If you have a double, it would seem that you could do the same as me, but you'll be unable to strip the chainset at all.

Other than that, you're looking at an engineering solution drilling out the damaged treads and fitting maybe a Helicoil insert or fitting a threaded bush.

[Mick F]

PS.
Just remembered.
The worn crank hole is only worn/damaged on the outer threads. The threads further in are fine.
I bought a long chainring bolt and cut it to exactly the right length and screwed it in, using the Loctite as an insurance.

Perhaps your threads are like mine?

[Brucey]

the officially sanctioned repair for this is to

a) try retapping/lengthening the thread in the crank and if that fails then
b) fit a helicoil

IIRC the usual thread size is M8x 0.75mm. Not sure if that applies in this case or even if helicoil kits in that size are readily available or not though.

Chemical metal might work; worst case is that if it doesn't, you are back to the same options as above.

cheers

[Mick F]

PPS:
Just realised that my story isn't completely complete. Anyone who has a Chorus triple - or even a Record triple will see where I wasn't completely correct.

Of course, the inner ring on a triple Chorus is screwed into five crank holes as the inner has its own spider as part of the crank forging. It was one of these, not the one in line with the crank. Viewed from the inside, it is the one top right.

Whatever ..........the issue is the same, with the same methods of fixing it.

[RodWatts]

Should have said it’s a triple.
Can you recall where you sourced the longer crank bolt.
On a quick search I couldn’t find any thing except by buying a complete 5 bolt set.
There does appear to be plenty of good thread further down the hole.
Thanks
Rod

[Brucey]

some chainsets use longer bolts of the (supposedly) correct type, eg TA vega/carmina ones here

https://www.sjscycles.co.uk/chainsets-spares/

however I don't think they available (from this supplier) individually.

cheers

[Mick F]

Should have said it’s a triple.
Can you recall where you sourced the longer crank bolt.

Hi Rod, I'll get back to this after a think.

New Chorus Triple chainset fitted 10th June 2007 as a 9sp system.
Rebuilt with Chorus 10sp Ergos 1st May 2008.
I keep some records.

As for when and where the longer bolts were fitted, I don't know. No records but it was probably after my LEJOGandBACK in
Summer 2008 as I had the bolt issues with the Chorus triple and bought a Comp Triple on this forum for the journey, then fixed my Chorus triple and then sold the Comp. Again, on this forum. Wish I'd kept the Comp Triple.

I didn't bookmark the site where I bought the bolts, so it probably was eBay at a guess.
As I say, I'll have a think.

[Mick F]

It could have been SJS.
They sell a variety of chainring fixing stuff, but it was a while back when I bought mine.
Unless I undo one, I don't know the length I have. As I had issues, I'm unwilling to unscrew one to check.

[RodWatts]

Thanks Mick/ Bruce

At least I know they’re out there somewhere...
Nothing like a nice long ‘interweb’ search!

Rod

[Mick F]

Found my thread!
My memory is now refreshed.
Have a read.
viewtopic.php?f=5&t=13918

The link to Chainreaction where I bought the bolts is dead now, but there must be others out there somewhere.

[NATURAL ANKLING]

Hi,
Bought a crank second hand and two threads stripped a third was very suspect.
IIRC I tried using chemi metal but it was a bit of a bodge method, no need to go into it because it was flawed.

Chemi Metal is tappable, buy you probably need a decent amount of thickness, it would probably work if you tap a bigger hole, then fill with chemi and when set drill and tap, small fine thread so not sure about the strength, certainly probably fail easer than the solid metal crank for sure.

Anyway after getting a tap, I decided to make a plug from a larger steel bolt, drill and tap crank M12 x 1.25 IIRC, thread lock the bolt into crank, cut flush with crank face.
Then through the sprocket assembled to crank, used as a drill jig start hole wth 8+ mm drill (possibly larger to just fit through sprocket hole 8.5 something, closer the better) then after spotting, finish drilling hole with tapping drill into plug and tap 8x0.75.
Or its a insert as suggested, both methods require some metal working skill.

I did it with a pistol drill in a vice but not recommended, you need a pillar drill with a method to hold crank so bolt face is perpendicular with bolt axis. Not easy to set up.
This method of a bolt as a plug means that only 0.75 mm wall thickness between the bolt plug thread and your chainring bolt thread!
And that's in theory, in practice the threads will over lap (eccentric) because of hand drilling even with a pillar drill.
Only a milling machine would guarantee a reasonable concentricity, using a eye scope to align on original hole, but a small unsupported thread in steel of even 25% will still be as strong and stronger than original alu thread. I made three plugs and one was a bit lop sided after tapping but on a five bolt ring, even four will be fine on a steel sprocket.
Insert method (helicoil whatever is as strong...stronger than the original in the case of aluminium crank)

Longer bolt, good idea, drill carefully a deeper hole without breaking through, tapping size, then you will need a plug tap to get max thread depth, measure full insertion of new bolt, and make sure the gap between bolt head and crank face is less than the sprocket thickness before assembling the sprocket, before tightening up else it might crack the crank.

Any tapping into aluminium will require some tapping fluid, or risk ripping thread! and a weaker solution.

Good luck.

[RodWatts]

Natural Ankling

Wow. I wish I had the equipment and skills to do all that!
I do have a club friend who probably could / would..

In the mean time I’ve found some longer bolts (thanks to good old SJS!)

Thanks all for your input!
Rod W