Piston
Place the cylinder head and barrel on one side on a bench and examine the piston.
It should not be necessary to remove this from the connecting rod, but if it should be
desired to do this for any reason, first remove the circlip from one end of the gudgeon
pin using a pair of pointed nose pliers or some suitable instrument to lever the circlip
out. Then, with the aid of a suitable extractor, withdraw the gudgeon pin. If it is too
tight to move, it can be released by warming the piston by means of a rag soaked in
hot water and wrung out. Application of this rag will cause the aluminium alloy of
the piston to expand more than the steel gudgeon pin, thus releasing the latter which
can then be freely removed. Be careful not to damage the needle rollers comprising
the gudgeon pin bearing in the connecting rod.
Scrape off any carbon which has accumulated on the crown of the piston, taking
care not to damage the relatively soft surface of the metal itself, and after removing
all the carbon, wipe clean with an oily rag.
Piston Rings
Now examine the piston rings noting that these are located in their grooves by
means of pegs which engage in the piston ring gaps. If in good condition the rings
will be found to present a uniformly smooth metallic surface over their entire
peripheries, and if they are in this condition and obviously have a certain amount of
“springiness” as evidenced by the fact that their free gap is considerably greater than
the closed gap when in the bore, they should not be disturbed. If, on the other hand,
the rings show signs of heat as evidenced by brown or more highly discoloured
patches, they should be replaced by new rings, and in this case particular attention
should be paid to the fit of the ends of the rings on their locating pegs in the piston
ring grooves, and they should also be checked in the bore to ensure that they have an
adequate gap. First place the ring in the cylinder bore in a position where it is clear
of the ports making certain that it is parallel with the bore by inserting the piston with
its skirt against the ring. Examine the gap which should be not less than specified
(see Technical Data, page 5). Having satisfied yourself on this point, place the ring
in its groove on the piston and make certain that it is free without perceptible up and
down play. If it is not free and the groove itself is clean, rub the ring down on a piece
of fine emery cloth laid on a flat surface, using a rotary motion of the arm to ensure
uniform pressure on the ring. As soon as ring is found to be free in its groove, wipe it
absolutely clean and fit it into position.
Check also that there is sufficient clearance between the inner portion of the gap
and the locating peg in the groove. Do this by closing the ring in its groove by finger
pressure until there is no gap, thus showing that there is clearance at the peg
underneath. If the gap will not close, indicating that the steps are binding on the peg,
ease the steps gently with a smooth file. If the piston has been removed from the
connecting rod refit it, first putting a smear of oil on the gudgeon pin, not forgetting a
new circlip to replace the one which was removed.
The piston must be replaced in its original position—i.e., with piston ring gaps
Then put a piece of clean rag over the piston and crankcase mouth and turn your
