does anyone know the pros and cons of single versus double bolts for limd atachment ,I think only three boyers use the double bolt type ,anyone have an idea why.

I think doubles are overkill and look odd. I have used singles on all my bows and see no cons. If the receiver is a good machined receiver then it hold the limb in place when strung. Most singles also have a post that aligns the limb.

I've built about 50 3 piece takedowns and all of them had single bolts. I've never had a failure due too limb bolt. I've seen a handle break at the thread insert and with 2 bolts you double that chance of failure.