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10th August 1920, Page 29
10th August 1920
Page 29
Page 29, 10th August 1920 — For DRIVERS, MEC ANICS & FOREMEN.
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Which of the following most accurately describes the problem?

TEN SHILLINGS is paid to the sender of any let r which we publish on this page, and an EXTRA FIVE SHILLINGS to the sender of the one which we select las being the best each week. All notes are edited befize being published. Mention your employer's name, in con.fidene, as evidence of good faith. Address, D., M. and F., "The Commercial Motor," 7-15, Rosebery Avenue, London, B.C. I.

Lamps Alight.

On Saturday, August 14th, light your lamps at 8.53 in London, 9.24 in Edinburgh, 9.13 in Newcastle, 9.13 in Liverpool, 9.2 in Birmingham, 9.3 in Bristol, and 9.56 in Dublin.

Boring Connecting Rod Big-ends.

The sender of the following communication has been awarded the extra payment of 5s. this week.

[2124] " H.M." (West Bromwich) writes :—"One of the most important jobs which can fall to the lot of the repair man is that of reboring the big-ends of engine connecting rods. Not only is it necessary that they should be bored out to the right size exactly, and also that the finish of the bored hole should be of the very best, but the accuracy of the, alignment of the hole, which must be exactly parallel with that in the small end, is most important, while it is very

essential that the distance from centre to centre of the two holes be the same for every'rod of.a set. In the manufacturer's works all these operations are performed to jig, but it is of course quite out of the question for the repair man to have a separate jig made for every type and make of rod that comes to him for repair. The cost of the jigs alone would be tremendous, apart from the possibility of many of them being used only once. I have, however, devised a universal jig for the purpose, and it has undoubtedly saved its cost many times over, on .ccount of the time saved in setting a set of rods up.

"The fixture which I use is fairly clearly shown on the accompanying sketch [which we have had. redrawn.—Ers]. A is a plain flat face-plate, which may be screwed -on to. the nose of any ordinary lathe which has a gap that is wide enough to swing it. It must be quite large enough in diameter to take the largest and longest rod which it is anticipated will have to be bored. At convenient intervals round and near its circumference are drilled and

reamered in. holes, for the • reception of the pivot pins, to which I will refer again in a moment. At convenient paints, one on each side of the centre of the plate, must be drilled two other in. holes, as shown. These are for the reception of the blocks marked G, which are to be made as illustrated by one of the smaller sketches. Their purpose is that of adjusting and holding the bia-end of the rod in the exact centre of the plate. The pivot pins must be made a good fit in the holes in the plate, as well as in the gudgeon pin holes of the rods. They are screwed up in place against a collar, which may be of a thickness equal to half the difference between the lengths of the big-end bearings and the gudgeon pin bearings, as they are shown in the sketch, and in that form allow the end of the big-end bearing to rest against the plate. If it be desired to have the back of the rod clear of the plate, which may be more convenient in ease the boring tool is required to go right out to that side, then the collar on the pivot, pin should be made correspondingly deeper."

.Autovac Troubles.

[2125] " (Norwich) writes :—" A short

time ago there appeared in the 'P.M. and F.' pages several 'letters from drivers who had had trouble with Autovaks. As I have had many years' experience of this class of fitting, it occurred to me that a few hints and tips might be found useful.

" Surplus of petrol is an occasional trouble on vehicles fitted with the Autovac. It is generally due to one of the following causes : The float may be sticking or punctured, the-needle valve may be out of line with its setting, or otherwise not properly performing its function, the operating springs may be stretched or broken or the disc valve may not be working. All but the last of these 'can easily be diagnosed by anyone used to motorcars or lorries, for they are familiar troubles in conneetioff with carburetters. The last and least easily recognized is, unfortunately, the most frequent. It is generally caused by-the elbow of the supply pipe being screwed too far on the nipple in the Autovac tank, thus interfering with the proper working of the valve. The remedy is to take out the elbow, file the bottom of it, and at the -same time open the three claws a little with a penknife so as ta, make quite sure the valve is free.

"As regards the other faults mentioned, to diecover and to remedy them entails dismantling the Autovac. When doing this, care is necessary to ensure theipsrts not being mishandled in the process. 'Commence by removing all pipes, then, with a good screwdriver, take out the screws on the top of the tank, taking care when removing the lid that the jointing is kept intact. All the mechanism lifts out with the top, so that the various parts can then ,be tested. Springs should be tried by gently raising and lowering the float. If both are in good order, the toggles should drop over the centre very sharply. When replacing the top, take care that the small air valye hole is left free. "Sometimes trouble develops in quite a contrary direction ; that is to say, there is a shortage of.petrol instead of a surplus. This is generally caused by air leakage inone or the other of the joints of the apparas tea. It may be due to a fractured or punctured pipe. Occasionally an airlock, -either in the main or supply tank, in the piping, or in the _Autovae tank, may have the same effect. If the strainer gauze requires clea.ning, that will, have the same result by impeding the flow of the fuel "These difficulties may generally, be avoided if the driver occasionally looks over the apparatus, cleans it, and examines it; particularly should he pay attention to the small cup-shaped gauze which is fitted between the petrol delivery pipe and the Autovac.

" Another cause of Autovac failure is the stoppage of the small air intake pipe. This causes a vacuum to form in the main tank and stops the supply to the engine.

I think I have enumerated all the various Autovac troubles which I have encountered in the course of my experience. It must not be thought, however, that because I have been so ready to enumerate its faults that I am in any way averse to this fitting."


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