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?

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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.

McShane further noted: 'One of the most striking things about staybolt failures is that the break almost invariably occurs at the outside end of the hole that is, at the end which is away from the fire sheet.' He followed this with the following advice about drilling staybolts: 'As the staybolt invariably breaks just inside of the outer sheet, the hole must be drilled into the outer or visible end of the staybolt.'

Testing a broken staybolt with enough pressure in the boiler to separate the broken ends. The impact of hammer 'A' will not be transmitted to hammer 'B', indicating a broken staybolt.


The 1911 edition of General Rules and Regulations Prescribed by the Board of Supervising Inspectors of the Steamboat Inspection Service (now the U.S. Coast Guard) stipulated: 'All screw staybolts shall be drilled at the ends with a 1/8-inch hole to at least a depth of 1/2-inch beyond the inside surface of the sheet.' (By 1931 the diameter of the hole had been increased to 3/16-inch.)

This would suggest that, had traction engine builders sold boilers for marine use, it would have been necessary for them to drill the staybolts, even if they had not done so on their traction engines. In 1905, the Aultman & Taylor Company was an approved supplier of marine boilers.


In 1924, the American Society of Mechanical Engineers (ASME) Boiler Code stated, 'The outside end of solid staybolts, 8 inches and less in length, shall be drilled with a hole at least 3/16-inch in diameter to a depth extending at least 1/2-inch beyond the inside of the plate, or hollow staybolts may be used. On boilers having a grate area not exceeding 15 square feet the drilling of staybolts is optional.' By 1943 ASME eliminated the exclusion for boilers with grate areas of less than 15 square feet. For comparison, a 32/110 HP J.I. Case traction engine had only 12.06 square feet of grate area. Consequently, a traction engine boiler built to 1924 ASME standards would not have to have drilled staybolts. By the time the exclusion was eliminated, there were very few, if any, traction engines being built.


Appendix 3 of the National Board of Boiler and Pressure Vessel Inspectors Inspection Code (1998) specifies that all staybolts shorter than 8 inches in length shall have telltale holes of 3/16- to 7/32-inch diameter and at least 1-1/4 inches deep. This applies to both threaded staybolts and staybolts installed using full penetration welds. Even though this is an inspection code, there is no mention of any hammer test for broken staybolts.

The National Board Inspection Code includes this provision: 'Telltale holes shall be reopened after driving.' This seems to indicate that staybolts can be drilled prior to being installed, as long as the holes are drilled out after the end of the bolt is riveted over. The Inspection Code also states, 'Bolts are to be driven in such a manner as to expand radially the bolt body and threads into the sheet prior to forming the head.' This would undoubtedly close up the 'telltale' hole and require it to be drilled out after the staybolt was headed. This would also apply to hollow staybolts mentioned in the ASME code.


The following provisions regarding staybolts are extracted from the current federal railroad regulations for steam locomotives. You will note that these regulations require both telltale holes and hammer testing of locomotive staybolts.

'Sec. 230.38 Telltale Holes.

(a)  Staybolts less than 8 inches long. All staybolts shorter than 8 inches, except flexible bolts, shall have telltale holes 3/16- to 7/32-inch diameter and at least 1-1/4 inches deep in the outer end.

(b) Reduced body staybolts. On reduced body staybolts, the telltale hole shall extend beyond the fillet and into the reduced section of the staybolt. Staybolts may have through telltale holes.

(c)  Telltale holes kept open. All telltale holes ... must be kept open at all times.

(I have seen locomotives where mud-dauber wasps have filled every telltale hole. According to one source, these nests are sometimes capable of withstanding the pressure of the steam.)

Sec. 230.39 Broken Staybolts.

(a) Maximum allowable number of broken staybolts. No boiler shall be allowed to remain in service with two broken staybolts located within 24 inches of each other, as measured inside the firebox or combustion chamber on a straight line. No boiler shall be allowed to remain in service with more than four broken staybolts inside the entire firebox and combustion chamber combined.

Sec. 230.40 Time and Method of Staybolt Testing.

(a) Time of hammer testing. (1) General: All staybolts shall be hammer tested at every 31-ser-vice-day inspection, except as provided in paragraph (a)(2) of this section. All staybolts also shall be hammer tested under hydrostatic pressure any time hydrostatic pressure above the MAWP [maximum allowable working pressure] specified on the boiler specification form is applied to the boiler.

(b) Method of hammer testing. If staybolts are tested while the boiler contains water, the hydrostatic pressure must be not less than 95 percent of the MAWP. The steam locomotive owner and/or operator shall tap each bolt with a hammer and determine broken bolts from the sound of the vibration of the sheet. Whenever staybolts are tested while the boiler is not under pressure, such as during the 31-service-day inspection, the stay bolt test must be made with all the water drained from the boiler.'


The Canadian boiler regulations provide specific information regarding the locations where drilled staybolts must be used on portable and traction engines. I assume that these requirements are based on experience, indicating that these are the areas where broken staybolts are most likely to be found.

'In vertical boilers where the height of the firebox measured above the center of the rivets in the foundation ring O.G. [ogee] exceeds 36 inches, and in locomotive type boilers where the firebox exceeds any one of the following dimensions, viz. 48 inches in height, 50 inches in length or 36 inches in width inside, the outside ends of solid screwed stays 8 inches or less in length, in the side or end exceeding the above dimensions, shall be drilled with a hole at least 3/16-inch in diameter to a depth extending at least 1/2-inch beyond the inside of the plates; or alternatively hollow staybolts may be used. In wet-bottom portable and traction boilers the two lower horizontal rows in the side sheets immediately above the lower curve of the firebox and the outer longitudinal rows on the bottom and in all boilers of locomotive type, including portable and traction boilers, the upper row of staybolts in the throat sheet next to the waist seam if less than 8 inches long shall be so drilled. Solid staybolts over 8 inches long and flexible staybolts of either the jointed or ball-and-socket type need not be drilled. All screwed staybolts used in water legs of water-tube boilers must be drilled at each end.'

According to the J.I. Case Company's 1914 catalog, the firebox on a 110 HP Case traction engine was 49-1/2 inches long, 35-1/4 inches wide and 36 inches above the grate. The firebox on the Case 110 HP engine fell just under the Canadian maximums.


In 1911 D.A. Low included illustrations of three types of drilled staybolts in his book, Pocket-Book for Mechanical Engineers. Low mentions these illustrations in reference to the Board of Trade rules and Lloyds rules but does not cite the specific requirements. I am including his information because of a novel practice he mentions.

One of his illustrations depicts staybolts similar to those shown illustrated in this article, while the other two illustrations show staybolts with larger holes. The one known as Yarrow's staybolt had a hole through its entire length and the other, known as Park's staybolt, had a hole extending to the same depth as shown in Figure 3. Low included the following comment regarding these staybolts: 'The ends of screwed stays may be expanded after they are in place by driving a drift into holes drilled in them, as in Park's and Yarrow's stays.' Wouldn't it be convenient to be able to repair a seeping staybolt in a manner similar to repairing a seeping tube? I doubt if Mr. Low had traction and portable boilers in mind.


Only two of my old texts on traction engines mention the possibility of a staybolt breaking. The Thresher's Guide (1910) simply acknowledges that added strain can be put on the boilerplate if one of the staybolts should break. In Instructions for Traction and Stationary Engineers, William Boss recognizes that staybolts can break and goes on to explain how to test for broken ones: 'They should be tested occasionally to make sure they are not broken. This is usually done by holding a heavy hammer on one end and striking the other end with another hammer. If it sounds solid and does not spring, it is all right. If not, it should be removed and another staybolt put in.'

The photo at right shows the crown sheet from the Case 110 that exploded in Medina, Ohio, in 2001. Of note are the points where the crown sheet separated from the staybolts. It is possible previous welds on the crown sheet failed, not the staybolts themselves.


The Medina County Sheriff's report on the July 29, 2001, explosion of Cliff Kovacic's Case 110 stated, 'One crown sheet stay appeared to have been broken prior to this explosion.' I have looked closely at the photos in the Sheriff's report and cannot find any indication of a staybolt failure that resembles the ones shown in Figures 1-3. Judging from the photos, I suspect that the failure occurred where the crown sheet had pulled away from a staybolt some time in the past and had subsequently been repaired by welding. I surmise that what was seen in the Sheriff's investigation might have been the failure of this weld. (See Photos 34 and 39 in section seven of the Sheriff's report.)

The report of a broken staybolt on the crownsheet of the Medina boiler, reports of the deteriorated condition of the crown sheet and the testimony that the engine had been plowing a few weeks earlier with 200 psi on the boiler combine to demonstrate the adequacy of the factors of safety that were used when this boiler was built. The boiler was designed for 160 psi when new and the safety valve, at the time of the explosion, was stamped for 125 psi.


Note that in the locomotive regulations the operator shall, '... tap each bolt with a hammer and determine broken bolts from the sound of the vibration of the sheet.' Such wording suggests variations in the way the hammer test is performed. One engine owner recently told me that when a boiler shop hammer-tested the staybolts in his boiler, the repairmen used a hammer so large, and hit the staybolts so hard, the owner was concerned that even if no staybolts were broken prior to the test, there might be some afterward. In this instance the mechanic might have been depending on' ... the sound of the vibration of the sheet,' rather than the reaction of a second hammer inside of the firebox.

Flexing of thinner boiler plates may explain reduced incident of staybolt breakage in agricultural versus locomotive boilers. Adapted From The Locomotive Up To Date, 1899.

I don't know to what extent broken staybolts are a problem in traction and portable engines today, and I have not heard stories from modern times of serious incidents resulting from broken staybolts. It is my understanding that fireboxes of locomotives are made of thicker steel than that used in fireboxes of traction and portable boilers. McShane suggests that the thinner steel on the inside of fireboxes is more flexible than the thicker steel on the outside and that this lack of flexibility in the thicker steel causes the staybolt to be flexed more, thus becoming more likely to break adjacent to the heavier plates. This may account for broken staybolts being more of a problem on locomotive boilers than on traction and portable boilers. The reduced thickness of the steel on portable and traction boilers may also help explain why we see seepage develop at the threads rather than breakage of the staybolt. Also, traction and portable boilers are not subjected to the constant and severe vibration and movement experienced in locomotives.

As we continue to operate our antique boilers, it makes sense for us to be aware of the various techniques we can use to test our staybolts. In doing so, perhaps we can reduce the chances of having to learn another lesson from someone's tragic experience.

Bruce E. Babcock is a regular contributor to Steam Traction. Contact him at: 11155 Stout Road, Amanda, OH 43102, or e-mail: babcock2@gte. net