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. These points will not arise of genuine
B.S.A. spares are fitted as the gaps on these are already correct when the rings are
sent out, but if for any reason genuine B.S.A. spares are not obtainable, these points
must receive careful attention. First place the ring in the cylinder bore in a position
where it is clear of the ports making certain that it is square by pressing the skirt of
the piston against it or a suitable bar of material of the correct diameter, 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 dead
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 dead 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 TOWARDS THE REAR ON
D1
AND THE FRONT ON
D5
ENGINES.
Then put a piece of clean rag over the piston and crankcase mouth and turn your
attention to the cylinder barrel and head.
Cylinder Head and Ports
Remove all carbon deposit from the cylinder head, bearing in mind again that the
aluminium is soft and easily damaged if the decarbonising tool is carelessly applied,
and carefully wipe clean to ensure the removal of all loose particles. Most of the
carbon deposit likely to have accumulated in the cylinder will be in the exhaust port,
and cleanliness at this point is most important as explained above. Scrape this out
carefully, taking care not to let the tool slip into the bore and damage the surface of
the latter. Examine the transfer and inlet ports for the presence of carbon, although
this is unlikely to be heavy, and finally wipe the ports and the cylinder bore
absolutely clean.
Big-end Bearing
While the cylinder is off, it is as well to test the big-end bearing for wear. This is
done by taking hold of the connecting rod stem and pulling it upwards until the crank
is at the top dead centre. Then holding it in this position try gently but firmly to pull
and push the connecting rod in the direction of its travel in order to feel whether there
is any play. If the big-end is in a sound condition there should be no play in this
direction although it may be possible to rock the rod sideways, i.e. at right angles to
the axis of the machine. If vertical play is perceptible in the big-end and you do not
feel qualified to decide whether the amount in evidence is permissible or not, you
should seek expert advice. This point is not likely to give trouble, however, provided
that the machine has been carefully used and adequately lubricated, for the big-end
bearing is of ample dimensions for the work it has to do.
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