REMOVAL OF BROKEN CYLINDER COVER STUDS FROM TWO AND FOUR STROKE
MARINE ENGINES, USING SPECIALISTS
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”.