15. Examination and renovation: engine cases and covers
Small cracks or holes in aluminium castings may be repaired with an
epoxy resin adhesive, such as Araldite, as a temporary expedient.
Permanent repairs can be effected only by welding and a specialist will
be able to advise on the availability of a proposed repair.
Damaged threads can be economically reclaimed by using a diamond
section wire insert of the Helicoil type, which is easily fitted after
drilling and re-tapping the affected thread. Most motorcycle dealers and
small engineering firms offer a service of this kind.
Sheared studs or screws can usually be removed with screw
extractors, which consist of tapered left-hand thread screws of very
hard steel. These are inserted by screwing anticlockwise into a
pre-drilled hole in the stud. If any problem arises which seems to be
beyond your scope it is worthwhile consulting a professional engineering
firm before condemning an otherwise sound casing; many such firms
advertise in the motorcycle papers.
If gasket or other mating surfaces are marked or damaged in any way
they can be reclaimed by rubbing them on a sheet of fine abrasive paper
laid on an absolutely flat surface such as a sheet of plate glass. Use a
gentle figure-of-eight pattern, maintaining light but even pressure on
the casting. Note that if large amounts of material are to be removed,
advice should be sought as to the viability of re-using the casting in
question; the internal clearances are minimal in many cases between
rotating or moving components and the castings. Stop work as soon as the
entire mating surface is polished by the action of the paper.
Large surfaces such as the cylinder head or block gasket surface
will have to be skimmed on a surface plate if warped. This is a task for
a light engineering business only; be careful to warn them to remove only
the minimum amount of metal necessary to true up the face. If excessive
warpage is found, seek expert advice.
Note that the mating surface may become distorted outwards around
the mounting screw holes, usually because these have been grossly over tightened. If such is the case, use a large drill bit or countersink
to very lightly skim the raised lip from around the screw hole, then
clean up the whole surface as described above.
Finally, check that all screw or bolt tapped holes are clean down to
the bottom of each hole; serious damage can be caused by forcing a screw
or bolt down a dirty thread and against an incorrect stop caused by the
presence of dirt oil. swarf or blobs of old jointing compound. At the
very least the component concerned will be incorrectly fastened, at
worst the casting could be cracked. The simplest way of cleaning such
holes is to use a length of welding rod or similar to check that the
hole is clean all the way to the bottom and to dig out any embedded
foreign matter, then to give each hole a squirt of contact cleaner or
similar solvent applied from an aerosol via the long plastic nozzle
usually supplied. Be careful to wear suitable eye protection while doing
this; the amount of dirt and debris that can be ejected from each hole