Farm Collector


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


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


The 1911 edition of General Rules and Regulations
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

(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

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
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
(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

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


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

  • Published on May 1, 2003
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