Thwaites SPRITE NIMBUS, Manual

Page: 13 / 54

SERVICING 40M-3 GEARBOX - SPRITE/NIMBUS MODELS
GENERAL:
1. Cleanliness is important. Remove all loose dirt and see that all bolt heads are clear
of cement, etc. outside the gearbox before any inner part is exposed. Drain the oil
from the gearbox.
2. Use a mandrel press, if available, for pressing out bearings, shafts, etc., otherwise
use hardwood or non-ferrous metal drifts.
3. Wash all parts in clean paraffin or petrol and lightly oil before reassembly. If new
ballraces are being fitted, do not remove wrappings until they are needed for
assembly.
4. Work on a clean bench.
GEARBOX WITH PLAIN INPUT AND OUTPUT:
1. Undo the four bolts (item 10), securing the front extension cover (item 8) and
remove bolts. Undo and remove the top cover and gear lever assembly complete.
Using a suitable lever, move the selector forks (item 79) and (item 52) so that two
speeds are engaged simultaneously. The gear train should now be locked, and will
not rotate.
2. Untab the bolt (item 60) which secures the output coupling and remove the bolt.
Slack off the six bolts (item 34) and (35) securing the end cover (item 29) and
remove it, also remove the output coupling (item 33) and spacer (item 30).
REMOVE SELECTOR FORKS AND MECHANISM:
1. Undo and remove the bolt (item 80) and retaining plate (item 75) on the back of the
gearbox. Drive forward the upper selector shaft (item 55), through the fork and the
front of the gearbox. Inside the fork is a ball and spring. Unless care is taken the ball
may fall out into the body of the gearbox. As soon as the shaft has left the fork, close
the ends of the hole in the fork with the fingers, and remove complete from the
gearbox. Now undo the split pins in the clevis pins (item 48), remove the clevis pins
and baulk plate, and then remove the lower shaft and fork in the manner described
above.
REMOVAL OF GEARSET:
1. Undo and remove the bolts (item 10) holding the front extension housing. Remove
the extension housing and primary shaft assembly complete. It may be necessary to
use a lever between the head of the primary shaft and the mainshaft to do this. Take
care not to damage the gear teeth in so doing.
2. The mainshaft is now pressed out through the front of the gearbox. The gears and
spacers on the mainshaft are removed from the back end of the mainshaft, one by
one, after it has cleared its supporting ballrace in the back of the gearbox.
3. Remove the circlip (item 58) securing the mainshaft ballrace, and press out the race.
Remove the circlip (item 69) and the circlips (item 68) from the end of the layshaft.
Now reverse the box, and using a drift which is nearly the same diameter as the
bronze bearing (item 76) drive the layshaft and bearing back until the bearing is
clear of the gearbox. If too small a drift is used, the end of the bronze bush will be
collapsed and the bush will be spoiled.
4. Place two plates about 1/8" thick between the ballrace and the gearbox and press the
layshaft back into the gearbox, thus removing the ballrace. Alternatively a suitable
pulley extractor may be used. Now remove the spacer and reverse layshaft gear via
the ballrace bore in the case, and the remainder of the layshaft assembly can be
removed from inside the gearcase via the top opening.
2-2