Re: The Rest Of The Story

49 CFR, Part 230 states that a boiler may not remain in service with more than 4 broken staybolts anywhere in the firebox. No two broken staybolts can be closer than 24 inches of each other as measured on a straight line. Any broken bolts must be replaced immediately if the above restrictions are exceeded or in any case at the next 31 service day inspection.

This is more restrictive than the old rule, but even in that case you would never be allowed to have 4 adjacent bolts broken. If each bolt is supporting 16 square inches at 200 psi, that is a load of 3200#. A 7/8" diameter unthreaded stay has .601 square inches of cross-sectional area which loads the bolt to 5325 psi tensile stress, well below the maximum allowed of 7500 psi. One broken bolt loads the adjacent 4 bolts to about 4000# or 6656 psi. Two broken bolts loads the adjacent 6 bolts to roughly 4200# or 6988 psi. Four bolts broken in an adjacent square Will load the adjacent 8 bolts to around 4800# or 7987 psi, exceeding the allowable 7500 psi limit.

Often times staybolts are loaded much closer to the allowable 7500 psi by design which gives less leeway for broken bolts. While newer codes allowed higher stress limits on staybolts, because steam locomotives invariably break bolts at inconvenient times when they cannot immediately be replaced, the lower stress limit allows for not overloading adjacent bolts until the broken one could be replaced. Pennsy "L" rules required the replacement of the four adjacent bolts to a broken one as a matter of course.