So errr this happened….
https://photos.app.goo.gl/e8XFueMzWcC4MufJ7

Unmarked arms, no obvious faults, no play in the taper. BANG, cracked in half as I took off from the lights.

Leaves me with a problem. This is a 9speed Campag Mirage Triple crankset from ~2005ish. I really don’t want to replace the entire groupset, it would be cheaper to buy a new bike in some respects. What’s your opinion – replace this with a used one (can’t find any direct replacements on ebay) or replace the crankset with a 10 speed compatible triple (hard to find, expensive). Bear in mind I paid £250 (used) for the whole bike about 10 years ago, a £150 crankset seems like a poor choice.

That looks like it’s full of plaster!

What chain rings were you running? There’s no difference between any 7/8/9 (10 or 11 if such things exist) speed triple crank set and the next beyond the obvious bcd and axle standards so…
assuming you can match the bcd you can use the existing rings, if you can’t match the bcd any triple which fits your bb should work fine if you can get correctly sized rings.
(Hollow tech type mtb axles are longer than road so if that’s what you’re on (i gather from taper you’re not) you’d need a mtb bottom bracket in order to fit the spacers as roadthreads are usually too short)

There is a marginal difference in ring thickness (and supposedly spacing) from a 9 to 10/11 speed chain ring but it shouldn’t have any noticable effect on your 9 speed drive train (if you went the other way round you may struggle to get the 11speed chain to sit nicely on your 9 speed rings)

A double would also work with some tweaking of front mech limits subject to the obvious issues with ring choice.

Bear in mind campag square taper is ISO, most others are JIS, so you’ll need another BB if you replace with anything other than campag.

Bear in mind I paid £250 (used) for the whole bike about 10 years ago, a £150 crankset seems like a poor choice

The relevant figure is the value of the bike to you now, not what you paid.

IF you want a Campag replacement (you’ve not been clear) then you will hve limited and expensive choice. There are surely loads of other brands and chainsets that will work fine, cheaply.

In fact you’ll be able to get a generic LH crank for £20 that will work.

Re the taper, I wonder if it’s a JIS on the bike as it’s not inserted very deeply.

Out of interest, what is the difference between ISO and JIS, is it the angle of the tapers, or just the length of the taper? Or Both?

It looks like it’s failed from the square taper, which isn’t unheard of but I would wonder whether they were assembled correctly – I only discovered relatively recently that square tapers shouldn’t be greased or you can just tighten until you rip the hole open like that even without pedaling.
I believe Campag is the only company that actually gives instructions on this, too, incidentally.

Anyway to fix your problem as others have said generic or second hand LH crank.

This wasn’t a fun day. At least mine wasn’t full of cement.

Ohhh man there’s more info here than I realised – I’d forgotten you lot are the oracle.

Yep, it does look oddly full of plaster but it seems to actually be cast alu, I’ll grab some close-ups for interest’s sake!

Hmm I hadn’t considered going 9>10/11 actually isn’t a problem on ring thickness, but I wondered if spacing was a problem – I know my shifters are semi-indexed, so probably can work around it (the index clicks are smaller than 1 ring space but not tiny.

I didn’t realise their taper was different, I knew they were rhombic rather than square. Definitely knew they shouldn’t be greased but also not sure how that’s greased – the first owner had it for 2 years and said he’d done nothing to it, I’ve had it for the rest of the time and never pulled the arms off – I guess maybe it was assembled that way. It does look a little like it’s burst, rather than snapped. I guess a casting flaw at that location could generate a fatigue failure start point, and I’m no small fry.

barrykellett – that’s an amazing failure! Again, weird to see so many brittle failures though – I’d never have assumed these would fail in a brittle mode!

I knew they were rhombic rather than square

They are? I doubt that very much. PITA to machine with what benefit?

I thought they were both square, with ISO being shorter and smaller at the end.

Its not really rhombic, they just call it that, it just means they’re 45 degree rotated in the arm.

Were it me, I think I’d go for a second hand off ebay (e.g. item 254070118036) (not mine) – unless the colour needs to match? As noted above you’ll be going the ‘right’ way for 9/10/11 compatibility, so pick one and go for it!

iirc JIS and ISO differ in taper angle and length, meaning the end is thinner so the cranks go on further – I can’t remember which way round it is, but the net result is that it’ll work either way, but might change your chainline. Or the inner ring will hit the chainstay (guess how I know…).

https://www.sheldonbrown.com/bbtaper.html

re ISO vs JIS

taper angle is the same, but the cross-section is different, so if you mix it’ll either slide on too far, or not enough.

i might have one of those cranks!!
I’ll check over the weekend.
Free if I do.

Edit- mine is silver though

Oooh thanks! Let me know and I’ll cover postage and beer! Nothing a little black paint couldn’t solve. Only minor issues would be these seem to be 170mm long, not sure how common that is.