BSA D14/4 BANTAM SUPREME D14/4 BANTAM SPORTS, Workshop Manual

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D14/4 ENGINE B6
FIG. B5.
PISTON RINGS
Examine the piston rings and note that they are
prevented from turning in their grooves by means of
pegs which locate in the piston ring grooves.
The outside face of each piston ring should posses a
smooth metallic surface and any signs of heat
discolouration indicates that the rings are in need of
replacement. The rings should also retain a certain
amount of "springiness" so that when released, the free
gap is considerably greater than the gap measured
when the ring is in the bore.
Each ring should be free in its groove but with
minimum side clearance. If the rings tend to stick in
the grooves, remove them and clean out all the carbon
from the groove and the inside face of the ring. A
broken piece of piston ring, ground as a chisel, will
provide a useful tool for removing carbon deposits
from the ring grooves. Care is necessary to permit
only a minimum amount of movement when removing
the rings as they are very brittle and can be broken
easily.
To check the piston ring gaps, place each ring in the
least worn part of the cylinder bore (usually at the
bottom) and locate it with the top of the piston to
ensure it is square in the bore. Measure the gap
between the ends of the ring with a feeler gauge. The
correct gap should be between ·009" (·2286 mm.) and
·013" (·3302 mm.) and although an increase of a few
thousandths of an inch is permissible, any large
increase to, say, ·025" indicates the need for
replacement rings.
See also that there is sufficient clearance between the
inner portion of the gap and the locating peg in the
groove. This can be checked by closing the ring in the
groove until the gap closes, proving that there is
clearance at the peg below. If the gap cannot be
closed, indicating that the steps are binding on the peg,
use a smooth file to ease the steps down.
It is advisable to check the gap of a new ring before
fitting, and if the gap is found to be less than 0·07"
(1778 mm.) the ends of the ring must be carefully filed
to the correct limit.
Protect the crankcase mouth with a piece of clean
rag and proceed to decarbonise the cylinder head and
barrel.
CYLINDER HEAD AND BARREL
Remove all carbon deposits from the cylinder head,
again bearing in mind that the aluminium is soft and
can easily be damaged if the decarbonising tool is
carelessly applied, and carefully wipe away all loose
particles.
As explained at the beginning of this section, most
of the carbon deposit likely to have accumulated in the
cylinder will be in the exhaust port and it is most
important that this is removed. Carefully scrape out
the carbon, taking care not to let the tool slip out of the
port and damage the surface of the bore. Examine the
transfer and inlet ports for the presence of carbon,
although this is unlikely to be excessive, and finally
wipe the ports and cylinder bore absolutely clean.
SMALL-END BEARING
The needle roller small-end bearing, because of its
obvious advantages over a plain bush, should not be
subjected to a great deal of wear. However, should it
be necessary to change the bearing, the old bearing
can be pushed out whilst at the same time, the new
bearing is pressed in with service tool No. 61-3791.
Check that the diameter of the gudgeon pin is as
quoted in General Data. If appreciable wear is
detected, the gudgeon pin will have to be renewed.