BSA D1 125 c.c. Bantam Instruction Manual page 14

But if for any reason the big-end bearing has deteriorated as the result of neglect
or abuse, it should be replaced and unless you have the necessary experience and
facilities for this class of work it is preferable to have it done by an expert repairer.
For the benefit of an experienced owner who may wish to dismantle his engine/gear
unit, a horizontal section of this is given in Fig. 3, but it is emphasized that no attempt
should be made to carry out work of this nature unless it becomes absolutely
necessary.
Reassembly
Before attempting to replace the cylinder barrel over the piston, smear the latter
generously with engine oil and then place it over the piston, carefully manipulating
the rings into the end of the bore and seeing that they enter freely without the
application of force. As soon as the cylinder barrel is home, replace the cylinder head
and put the washers and nuts on the four holding down bolts. Tighten the nuts in
diagonal order so as to avoid distortion.
F . 3. Horizontal section of D1 power unit.
IG
Examine the sparking plug and replace if sound, and finally reconnect the
exhaust pipe and the carburetter.
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