Maintenance and Repairs

Removal of Broken Cylinder Head Studs






This page was contributed by Gavin Aris of Foretract Engineering


Broken cylinder head studs on marine engines are infrequent but when they do occur the broken threaded portion in the engine entablature can be very difficult to remove (the stud when fitted new is tightened onto a tapered “seat” in the entablature).


The most common place for a stud to fail is at the top of the lower thread (this is the weakest part of the stud which also has the most stress when in tension).


The methods shown here can also be used in many other marine applications.


Two stroke Engine

There is always a temptation for engineers to think that after a stud failure there are still 7 out of 8 studs holding the cylinder cover in place so it is business as usual. This is definitely not the case, as the extra loading on adjacent studs can lead to their failure also. As the cylinder head studs effectively “tie” the unit together, multiple stud failure can result in substantial damage to the piston, liner and entablature. Therefore a broken stud should be replaced as soon as possible.


Most failures are at a point where the stud is half way down the entablature hole, the top half of which is not threaded and is a counter bore.


Drilling and using a stud extractor is not suitable for removal of large studs used on marine two strokes (M50 upwards). Welding on nuts, etc to the top of the remaining stud can be successful, but is in no way guaranteed to get results due to the difficulty of welding down the hole.



Foretract Engineering are specialists in the removal of studs using the latest Metal Disintegration techniques. One of their machines is mounted onto the side of the engine using its magnetic mounting sometimes in conjunction with a specially made mounting plate. Once the machine is mounted on the engine, then the cutting head is lined up with the centre line of the hole containing the broken stud. An electrode of the relevant size (depending on stud diameter) is used to cut a square hole through the centre of the stud (usually a 1 ˝” or a 2” square).



Once a good square hole is cut in the top of the stud then a very positive  “drive” is achieved that can be used in conjunction with hydraulic wrenches, air impact wrenches or simply specially made extension bars that take the drive above the cylinder cover so that a long leverage bar can be used.


The advantages of using this method for removal of studs on two strokes with studs external to the jacket is that in many cases the affected unit requires no dismantling (sometimes the cylinder cover is required to be removed for access to studs under the exhaust trunking). The entire operation can be carried out during the turn around or waiting time at a port or anchorage. Typically time taken is approx 12 hrs and is guaranteed to be successful. No damage is ever caused to the hole threads as the square hole is always well away from the sides of the stud.



4 Stroke Engine


Most 4 strokes rely on the cylinder head studs maintain a good seal for the cooling water and very often only one broken stud can cause severe loss of coolant.

The method used by Foretract Engineering for removing cylinder head studs from medium and high speed 4 stroke engines is very similar to the method used on 2 strokes except that the head must nearly always be removed, a mounting plate is fitted in its place for the machine to be mounted over the liner.



The picture to the right shows a cylinder head stud on a MAK main engine on a RoRo ship. The cooling water leakage caused by the broken stud was so excessive that the engine had to be stopped and immediate repairs carried out. A ship repair company drilled an approx 1” hole near the centre of the stud and attempted to use an “Easy out” to unscrew it from the entablature. The Easy out snapped, so they then proceeded to weld a pipe down the hole onto the top of the stud this pipe was also welded to the entablature this made it impossible to screw out the broken stud. 


The situation faced by Foretract technicians when they arrived on board meant that the decision to use the collapsing method was taken. This method requires a precision square electrode to be used that is matched to the thread size and pitch. A square hole, which has an across corners size just under the “Core” size of the stud, is used. The cutting electrode is kept concentric with the hole and stud by means of an alignment bush (the hardness of the “easy out” material does not affect the “cut”).



Once the hole is completed by the metal disintegration machine special chisels and thread protection guards are used to break away the remaining four quadrants that are left in the hole. The small amount of damage at the top of the threads caused by welding was cleaned up by a combination of a precision carbide grinder and the correct “tap”.



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