Hammering Staybolts is a Time-honored Method of Testing for Broken Bolts: A Dull Thud Equals a Broken Bolt, a Clear Ring Means They're Okay - or Does it?

| May/June 2003

Hammering on staybolts has long been considered an accepted method of testing their integrity. But there are other methods of testing staybolts, and it seems that within the steam community there is at least some degree of uncertainty surrounding what constitutes an effective, conclusive test. In an attempt to take the mystery out testing staybolts, I looked to some of my old texts and to copies of old boiler regulations for insight.

Testing a cracked or broken staybolt with no pressure in the boiler. The impact of hammer 'A' will be transmitted directly to hammer 'B' the same as if the staybolt were sound.


In 1899 Charles McShane, writing in his book The Locomotive Up to Date, described three ways of testing stay-bolts in locomotive boilers. At the time, the accepted method for detecting broken staybolts was by sounding them with a hammer. To do this, the inspector first placed the boiler under a low hydrostatic pressure so the ends of any broken staybolts would be separated slightly. One inspector would then go inside the firebox and hold a hammer against the end of a staybolt while another inspector on the outside would tap the bolt on the outer end. The man on the inside could tell whether or not the bolt was broken by the way his hammer responded to the blows from the outside (see Figures 1 and 2).

McShane also described a second approach: 'Still another method, which is often adopted in doubtful cases, consists in the use of the teeth of one of the men in place of the sledge. In applying this method the inspector rests one end of a prick-punch or other similar object against the bolt and brings his front teeth to bear against the free end of the punch. When the staybolt is tapped at the other end this affords a very delicate and certain means of detecting a fracture if it exists.'

McShane advocated yet a third method of testing staybolts. This approach called for drilling a hole through the center of the outer end of each staybolt, running into the bolt far enough to run past the thickness of the boilerplate. If the hole intersected a crack or break in the staybolt water would escape through the drilled hole and be visible on the outside of the boiler (see Figure 3). Further, if a crack or break should develop at any time following the test steam would escape through the hole, providing a visible indication of staybolt failure.

McShane described an exercise that compared the effectiveness of testing staybolts by hammering against drilling. The exercise involved testing staybolts on 13 locomotive boilers, and McShane offered the following comments regarding one of the tests: 'I consider it the most severe comparative test of all, from the fact that three inspectors in turn did their level best to locate broken and partially broken staybolts by the hammer test, after being informed that the staybolts were to be drilled when they had completed their examination. They were given all the time they needed for a careful and accurate inspection. The result was that the hammer test located four broken staybolts and the drilling test discovered 46 others that were only partially broken.' McShane then summarized the results of the tests of all 13 locomotive boilers: 'A careful record of the broken and partially broken bolts detected in 13 engines shows that 440 were discovered by the hammer test and 619 additional ones by the drilling test.' There were an average of 81 broken or partly broken staybolts on each boiler, and only 34 of these were detected with a hammer.