this guide in full with pictures
|How to remove an engine stud.
An engine stud is often an alternative to a bolt. It is a straight
length of steel rod that has threads at both ends. On JAWA CZ engines the
most common studs are the 4 for each cylinder barrel and head assembly and
the two for the carburettor. See parts shown in red on diagrams below:
In the motorcycle engine, the
stud is screwed into the crankcase components during manufacture and it is
seldom necessary to replace it. However, over time the threads can become
damaged or the rod bends. Sometimes it
helps the dismantling process if one or more studs are removed. Stud removal
is an easy process explained below.
To remove any stud you need
access to the exposed threaded section that section not screwed into the
crankcase. In the case of cylinder studs it is normally essential to remove
the cylinder head NOT the barrel which can often remain in place.
See the Babetta moped engine picture below:
The reason it is necessary to
dismantle is to allow you access to fit 2 (two) nuts to the threads on the
stud. Sometimes this can be done without dismantling for example with carb
studs but on cylinder heads the cooling fins get in the way for the next
process. With carb studs they are not usually long enough to allow the
fitting of 2 nuts unless you can use thinner nuts. (Thin Half nuts are
You need to lock the nuts
against each other and onto the stud. I do this by hand at first using my
fingers to place the 2 nuts fully onto the stud, then turn the lower nut
clockwise and the top nut anticlockwise until they are tight against each
other. I then use two spanners to complete the tightening process. You have
now made the stud into a bolt. Then using one spanner turn the top nut
anticlockwise and the stud should start to turn and unscrew from the
crankcase. If you did not lock the nuts together hard enough the top nut
will simply unscrew and you will have to repeat the locking process and try
again. Remove the stud. Take the replacement stud repeat the process of
locking on the 2 nuts and then screw it into place. I avoid using Loctite
type products! Unlock the nuts from each other by reversing the locking
process and you stud is ready for use. Sometimes the nut has rusted
to the stud and the normal process of removing the nut will turn the stud
and allow its removal.
Broken studs: If
the break leaves sufficient stud to be gripped by a tool like a molegrip
then clamp on and unscrew. If however the stud breaks close to the crankcase
you have 2 options take the part to a professional or have a go
yourself. When a camshaft holder bolt sheared on the cylinder head of my
Suzuki I opted to remove the head and have the broken piece removed from it
by an engineering firm as the risk of
damage to an expensive component was high.
To remove a broken stud or bolt
you need a quality drill bit, drill and a stud remover of the appropriate
size. You drill STRAIGHT down the centre of the broken stud but NOT into the
component it is fitted to. Before starting you have had to flatten the
broken end and punch mark the centre. The stud remover is like a tap tool
but it threads in anticlockwise so as you tighten the remover the remainder
of the stud releases. It is easy to go off centre when drilling or to break
the stud remover and then you do have problems as they are hardened metal!
Engineers have an alternative process called spark erosion that can
effectively burn out the broken piece. You now know how to fix your studs.
P.S. replace studs with studs NOT fully threaded bar.
Note: JAWA CZ studs should
not be used for the belly button, nose etc!
By Pete Edwards
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