Plimogz

Ok. First-off, I wish I'd bought 9mm Shimano steel bolts. Those seem like the solution for me. Sadly though, I'm stuck with some less-than-ideal options until I next order some parts. Namely:

The chainring is a stainless Surly job, the Vernier calliper puts its thickness at 2.30 mm. the spider is 3.80mm thick where it's drilled for bolts.

I bought some 6mm steel bolts. Unfortunately, these seem a little short. In fact, the nuts are only 4mm deep. Which I feel isn't optimal. Optimally, these bolts should have been 8mm long.

So I went out and bought some longer bolts, but upon closer inspection, these are light alloy affairs. Which I can't help but feel must be weaker than the steel variety.

Question is, what's a good rule of thumb for chainring bolt length? I'd hate to taco this chainring due to bolt failure.

FBinNY 

In most designs it's important that the nut portion is long enough to reach just short of all the way through the chainring (s) and support spider. It's the nut that actually provides support and keeps the ring from shifting so it has to engage all the parts, but at the same time end sub flush so the screw portion can pack everything tight (analogous to threadless headset stack height).

If the nut is properly long, then the screw can be much shorter, say only 5-6mm since only 4-5 thread engagement is needed to achieve full strength.

The above applies to steel chainring nuts, but some alloy nuts aren't strong enough and require a screw long enough to reach all the way through and engage up to the flange on the nut in order to achieve full strenth.

Plimogz

Ok. Thank you for your reply.

In any case, I went ahead and installed the short steel bolts (destroyed one Race Face alloy bolt by over-tightening. Don't really know wtf happened, I was using a torx screwdriver and wouldn't have thought that its grip would allow for that much torque). From what you wrote, I think that I might get away with these shortish nuts on account of running just the one chainring. Fingers crossed.

With the nut inserted through the chainring, approximately 1.7mm protrude into the spider. I tried it out tonight; didn't fail yet -- I really hope it doesn't; don't think I could stomach scrapping/buying yet another stainless steel chainring before it's seen any serious use. What a waste that last one was.

At first I was more worried about the bolts not engaging all the nuts' threads. Now I'm thinking that the likeliest cause of failure might actually be the nuts' poor engagement within the spider.

I really wish these bolts were 2mm longer...

Oh well, at least I've now learned to appreciate good, steel chainring bolts...

Thanks again

fietsbob

Alloy chainrings are typically counter bored so the head of the chainring ring nut/bolt,
is flush with the surface of the chainring..
so thickness at the inner faces of the bolts is lessened.

Jeff Wills

The OP said the chainring he had is a stainless steel Surly.