HINTS ON MAINTENANCE.
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How to Get the Best Out of a Vehicle, to Secure Reliability and to Avoid Trouble.
CONTRIBUTIONS are invited for t:::.: page from mechanics, works staff and draughtsmen, and fleet managers, drivers, garage foremen, and will be paid for on a generous scale. Every system, make, and type of commercial motor vehicle will be dealt with, and the matter should be written with a view to the disclosure of workshop and garage practice in the maintenance of a vehicle—practices which, whilst they may be quite normal, are peculiar to the particular vehicle and may not be generally known to those responsible •for its running. Expedients and suggestions for overcoming roadside and other troubles are covered in the following page, headed "Roadside and Garage," Communications should he addressed to "The Editor, The Com, mercial Motor, 7-15, Rosebery Avenue, London, E.C. 1."
• 168.—Fitting New Piston Rings to Dennis Engine.
When fitting new piston rings to the pistons in a Dennis engine, the loose rings should be entered into the bores of the cylinders clear of the radius, and squared up with the .end of the piston. The gaps for the various types of engines should then be measured and should be as follow : During the course of an overhaul or repair, difficulty is often experienced in removing the gudgeon pins from the pistons. The latter should not be sub4ected to rough usage, and if a hammer be brought ante play to drive out a• tight pin, it is difficult to find a suitable rest for the piston, with the result that the latter is often damaged. A safe and simple method of supporting a piston whilst the gudgeon pin is being drawn out is to make use of a small, stout box, across which can be stretched two strips of fairly thick leather held in position at each end by two wood screws and ,washers. The piston can then be laid on the straps, with the connecting rod supported by one end of the box.
Great care should be taken before attempting to drive out the pin to ascertain whether one end of the latter is smaller in diameter than. the other—in other words, as to whether the pin 1,, taper or parallel ; also, care should be taken to remove any locking setscrews which may be employed for holding the pin in position. The blows must, of course, be delivered at the small end of the pin.
When replacing the gudgeon pin, this can be done in a similar manner, but great care should be taken to avoid springing the piston, the skirt of which may easily be forced into an oval shape instead of being perfectly circular.
170.—Tuning Up Engine on J-type Thornycroft.
After a Thornycroft vehicle has covered a considera.ble mileage and the engine is beginning to lose power, take off the inspection plates and look for any slackness between the big-ends and the crankshaft. If there is none, or if the slackness is ex tremelyslight, replace the inspection plate, noting that the joints are perfectly oil-tight. .Next remove the valve caps and the valves, examining the tappets for any signs of wear.
If the valve Beatings are in order and the valves are not burnt, grind the latter in, to give a good • seating, using the finest emery or •carborundurn powder, mixed to asmooth paste with oil.
It is better, before putting this paste into position, to scrape off any carbon that may have lodged on the valve itself and around the seating. Afterwards turn the starting handle rapidly so as to blow out as much of the deposit as is possible.
The grinding paste should be smeared round the face of the valve, using only a very small quantity at a time, and during grinding the valve should be rotated and lifted off its seat after every few turns. Now turn attention to the valve caps. It is almost as essential to make these gas-tight as it is in the case of the valves. The copper-asbestos gaskets may require renewing, and, in any ease, test the gastightness of the caps by ,placing a little paraffin round them whilst the engine is running.
The starting plugs should,. of course, be cleaned and the gaps adjusted. The latter should be about .5 mm. or between 1-50th in. and 1-32nd in.
When adjusting the valves, do this while the engine is at its normal running temperature. When this is the case, in this type of engine, the inlet valves can be adjusted almost to touch the tappet, and the exhaust valve tappets should be given only a very slight clearance.
Now examine the leads. See that these are properly connected up to the magneto and plugs and that no loose bits of wire are shorting on the metal parts of the engine, or that shorting is occurring through defective insulation. Only experts should be allowed to deal with the magneto, particularly as regards the contact breaker, which may require adjustinent, and possibly the carbon brushes may need cleaning.
171.—A Method of Refianging Copper Pipes.
This hint applies particularly to the water pipes used in steam wagons. First anneal the pipe at the broken end by heating it to a dull rod and quenching in cold water. Before fitting the collar make certain that the two union nuts are on the pipe, each with its open end towards its respective end of the pipe. Now fit on the collar and grip this and the pipe in the vice, with the flange of the collar against the vice end, allowing the. pipe to prat' tide suffi cientl3r to form the new flange. Insert a tarter punch, and tap gently, turning the punch gradually. When the pipe is expanded sufficiently to fit the bore of the collar, gradually tilt the punch until the pipe flange is formed. Finish off with the flat of a hammer to ensure a good joint.