Atlas ratio (a bit more under a Combat head) but use Hepolite Atlas pistons and try to get those without
the slot. It's not so critical with Atlas pistons because the combustion pressures are less and generally the
chap who fits Atlas pistons doesn't go wailing about at 7,000 rpm all day.
A final point about pistons early Commando pistons had a three piece oil control ring where the central
hart was U shaped in cross section. Later pistons, both slotted and combat. have a 3 piece oil ring of'
which the central part is circular in cross section, the two sides of the pressing are bent over until they
meet. The two types of ring arc not interchangeable as the depth of the groove for the oil ring is deeper in
(lie newer pistons. 1 !re old type rings are more difficult to fit in the bore than the new ones, and aren't so
good at oil control, so that's another reason for avoiding early-type pistons.
Rings last, on average, 25 - 30,000 miles. At this mileage the bores shouldn't be worn significantly and
new rings will restore your oil consumption If the bores are worn enough at this mileage it) need boring
then change your brand of oil (or, dare I say it, change your oil).
Greater ring wear will occur without air cleaners and also not having chrome top rings (often two cast
rings are supplied). In these cases oil consumption can get as high as 100 mpp when the handle is -well
wrung". Only tip l0 70,t)00 before new rings and 40,000 between rebores.
The 850 Mk 11A Roadster, complete with 'bean cans'.
STOP PRESS:
CLUTCH PLATES: If you change any early 750 clutch to phosphor bronze plates (although you'd do
better by changing the oil to Castrolite first. it's also cheaper) then you'll also need the hardened clutch
centre as well to help stop the bronze plates from notching in the centre. Part No. 063979 (Page 20).
GEARBOX 2ND GEAR: From Engine No. 306591 a new pair of 2nd gears were added 064639 and
064640 fully interchangeable with previous pairs 040418/040019 on no account mix the gears, pairs only.
The reason for the change? The factory said 'to provide a more graduated set of ratios' actually it was lo
up the ratios therefore lower the revs in 2nd gear to pass noise restrictions!
6
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