Nuts, Bolts, and Threads used in the Engine Room - Tap, Die, and Tapping Operation

Nuts, Bolts, and Threads used in the Engine Room - Tap, Die, and Tapping Operation
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Taps and tapping operation

Hand taps are used in the engine room to repair damaged internal threads and to make new ones after drilling and reaming operations. Taps are identified by the threads they will machine and form, for example M12 x1.5, where M stands for Metric threads (the type of threads used), 12 means 12 millimeter major diameter, and 1.5 stands for the pitch, which in this case is 1.5 mm. Also they are identified as number 1, 2, and 3 or also as Taper, second, and plug or bottoming.

After a hole has been drilled, the number one or taper tap is inserted in the hole, with a tap holder. In taper tap the last five or six threads are ground out to enable the entry of the tap in the hole easily. Care must be taken that the tap is parallel to the hole, this first step is crucial as it will determine whether at the end you get a correctly threaded hole or if you have to do it again after re-drilling (at a larger size). Tapping must be done in stages, a quarter turn clock wise tapping motion, must be followed by similar motion in anti-clock wise direction, to allow for the cut metal bits to come out. Lubrication with water or cutting oil will make the machining easy and protect the taps.

After tapping with the first tap has been done, the hole is cleaned with compressed air, and then the second tap in inserted and tapping is done in the same way. Frequent to and fro movements must be done, any hard obstruction or too much force required must be avoided and the cause investigated. After the tapping has been done a similar sized bolt must be inserted into the hole and the quality of the threads checked.

In holes which are blind, the second tap would not create threads at the end, for this purpose the third tap or the plug or the bottoming one must be used.

Taps and Dies

Dies and die stocks

As taps are used for machining internal threads, threading dies are used for machining external threads and for repairing them. The die operation is used to create external threads on cylindrical items like bolts, studs, and pipes. The die is rotated in the same way is done in the tapping operation. Both cutting and releasing strokes are continued alternatively till the desired length of the rod is threaded. The complete depth of the cut is not obtained in one operation and the jaws should be brought together and the operations repeated. Care should be taken that the die stock is always held at right angles to the axis of the work. Oil or other lubricant should be used for easing the cutting effort.



Screw Pitch Gauge or Thread Gauge

They are used to check the threads and the pitch of threads used in different items. An unknown thread can be identified by the help of this.

Thread Gauge

Thread Gauge

Procedure to remove a stud

The studs on any machinery need to be replaced or removed, but it often confuses the junior engineers. The procedure is very simple; take two nuts, first screw the first nut on the stud, then screw the other one till it is touching the first nut. Now take two spanners, hold on the lower nut and tighten the top one very tightly. After they have become tight open with the spanner on the lower nut. The entire stud along with the two nuts will come out. The new stud has to be put in with similar procedure, however during tightening the upper nut must be held with the spanner and tightened.


Stud bolt

Procedure to remove a broken stud

Sometimes the stud breaks at the surface and there are not sufficient threads to use nuts for the removal. In this case the following methods can be used to remove the studs: Welding a nut on the stud: It the simplest method, if the place permit the welding and hot work, then a nut can be welded on the broken stud and after cooling the nut can be opened and the stud will come along with it. Using a stud extractor: In this a small hole is drilled at the center of the stud after a facing operation and marking the spot to be drilled by a center punch. Then a stud extractor is hammered into the hole and the extractor is turned anti-clock wise. The stud will come out with the extractor. Holding the broken stud on a bench vice and a clockwise turn will free the extractor for further use. Drilling out the stud: in this method after the facing and the marking with a center punch, the hole is re-drilled to the bottom. The stud is destroyed and then a re-threading operation must be done to repair the threads or make new ones. Removing the stud with a center punch: this method is used where the stud is not very firmly stuck. The stud can be nudged out by hitting at the periphery with a center punch after a liberal dose of a liquid penetrating lubricant.

Stud removal by a stud extractor

stud removal

Care of nuts and bolts on board a ship

On board a ship with salt water air everywhere and a lot of humidity, the nuts, bolts, and the threaded bars are to be properly stored. Generally they must be kept separate and not mixed up as in times of need, this can be frustrating and also inventory control would be impossible. The nuts and bolts would quickly rust in a store room where humid air is available. They could be lubricated or kept in air tight containers. A bolt which has rusted has to be properly cleaned by buffing. However this is only possible for a light degree of rust, any heavy corrosion would render the nuts and bolts unfit for use. Threaded bars kept outside could get damaged due to falling items, so a good idea would be to cover them with a split rubber hose. Also covering with a plastic sheet would suffice. A correct inventory taken every month end would help the second engineer to make a requisition (authorized by the Chief Engineer) to the owners/managers.

Nuts and Bolts

Nuts and Bolts


The modern sea going ship is of welded construction and houses machinery held together by a number of fasteners, all threaded. Almost every day a marine engineer has to come across threads, in one form or the other. The marine engineer must be aware of the different types of threads used on board, the machining operations to repair a damaged thread or to manufacture a new one. The care of threads in the form of nuts and bolts, their identification, and their inventory management is of utmost importance. In this brief article it has been tried to explain the importance of nuts, bolts, and threads in the everyday life of a marine engineer. It is our endeavor that all sea going junior engineers and senior marine engineers would benefit from this article.


  1. A course in workshop technology, by B.S. Raghuwanshi. A Dhanpat Rai & Sons Publications.
  2. Production Technology, by R.K. Jain. A Khanna Publication.
  3. Materials and processes in manufacturing, by E. Paul DeGarmo, J.T. Black, and Ronald A. Kosher. A Prentice Hall India Publication.

Image credits

  1. Stud bolt:
  2. Thread gauge:
  3. Taps and dies:
  4. Nuts and bolts:
  5. Iso meteric thread M20:
  6. Castle nut:[email protected]/128463532
  7. Wing nut:
  8. Iso metric thread:
  9. foundation bolt:
  10. j bolt u clamp eye bolt:
  11. screw:[email protected]/427383721/
  12. thread cutting on lathe:
  13. stud removal:
  14. die:

This post is part of the series: Nuts, Bolts, and threads used in Engine Room

These articles discusses the different types of nuts and bolts and threads used in the engine room of a merchant ship. The various ways of threading and tapping done on board, the care and maintainance of nuts and bolts. How to remove a stud, stud extractor, remove a broken stud, thread gauge.

  1. Nuts, Bolts, and Threads used in the Engine Room - Part 1
  2. Nuts, Bolts, and threads used in Engine Room - Part 2