Engine Oil must be changed at intervals of 2,000 miles running. (After first 500 miles on new or overhauled engine.) To change proceed as follows :-

Drain oil tank by removing drain plug, below tank. Note that on MAC machines fitted with the fabric oil filter about half a pint of oil will be left in the filter chamber in the tank. Drain this oil off as described on page 43 and fill filter chamber with clean oil. Remove and clean suction filter plug (see later).

Loosen clip on top end of rubber hose between return union on crankcase and pipe on tank. Pull hose away from tank. Refill tank with fresh oil to within 1-in. from top of tank (i.e., 2½-in. from top face of filler neck). Start engine - catch dirty oil (which will be returned up return pipe hose) in small tin. When return begins to run clean stop engine and refit hose. Tighten hose clip.

Never carry out the job in dusty surroundings. Always see that oil is poured into tank from clean measures and containers.

PRIMING THE OIL PUMP.

Owing to the use of a spring-loaded ball valve between the oil tank and pump on all MAC models fitted with the fabric oil filter (that is all machines from Engine No. MAC15523 onwards), the pump will not feed oil into the engine if the main oil feed pipe has been disturbed and emptied of oil.

Before starting a new or overhauled engine, or if the oil pipe has been disturbed always fill the pipe with oil to prime the pump. This will be easier if the banjo union hollow bolt at the crankcase end of the pipe is first loosened to relieve air locks as the oil is injected at the top from a force feed oil can or a syringe. Tighten the bolt when the pipe is full and also tighten the union nut at the top.

Check the oil supply as described in the next section.

CHECKING THE OIL FEED.

Loosen the union nut securing the rocker oil feed pipe to the top of the timing cover.

Start the engine and run it slowly. After a few moments the oil should be forced out at the loose union in a steady stream showing that the pump is working. If the flow remains steady for not less than one full minute the union nut may be tightened and the machine put into commission.

If there is no flow stop the engine at once and reprime the oil pipe. The oil pump will not operate if there is any air leakage into the feed line between the ball valve and the pump. After long service the hose may be found to have swelled internally and the hose should therefore be checked for condition and renewed if it is in doubtful condition.

THE SUCTION FILTER.†

Remove filter plug every 2,000 miles by unscrewing from rear bottom corner of crankcase, using tubular sparking plug spanner. Clean plug in petrol. See that red fibre washer is in order. Refit plug and tighten up fully. (See Fig. 18.)

THE CYLINDER OIL FEED.†

(Not fitted after Engine No. MAC 15522.)

Every 2,000 miles remove bolt below cylinder base joint at rear on left side. Bolt is marked " jet " on head. Clean in petrol. See that small cross hole in the centre of bolt is clear. See that fibre washer below head is in order. Replace. Exercise great care in tightening so that bolt is not sheared off. (See Fig. 19.)

†These are not incorporated on early pre-war models