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.