DEALING WITH ENGINE TROUBLES.
Page 29
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Our Driver and Mechanic Readers Describe Methods of Overcoming Certain Difficulties.
0"OF THE meet frequent of engine troubles is an elusive knock. "M.M.," of Earl's Court, tells us that he has experienced cases in which the mechanic has taken -the greatest care in his work of overhauling and assembling, yet, so soon as the engine is erected and started, a mysterious knock is heard, and a considerable amount of time is lost before its source is discovered and the trouble eliminated. There is one cause which is often overlooked, and which is most prevalent on certain types of engines after they have been hard at work for about, 18 months or so. Each cylinder wears, and a ridge is formed, not, as so many people imagine, at the limit of the movement of the top edge of the piston, but at the corresponding point for the top ring. Now, whenever an engine is overhauled, after the lapse of about the period named, it is usual to fit new gudgeon pins and bushes, and sometimes new big-end bearings. Even if the latter are not actually renewed, they are set up, as are also the main bearings, and the net result is to increase the effective length of the piston and connecting rod, compelling the piston once more to reach up to that point in the cylinder where it used to end its stroke in the early days of the engine. This brings the top ring of the piston up against the ridge, and a mysterious knock results.
" MAL's ' method of overcoming this trouble is a simple one, but none the less
effective for that reason. The accompanying sketch is, as he says. in his letter, almost self-explanatory. Both the top edge of the piston, and that of the sipper ring, are rounded off, to the extent requirede-usually from 3-32 in. to
in. This is better than the more frequently adopted expedient of putting a sheet of Hellite, or other jointing material, between the cylinder and crankcase, en as to lift the cylinder and offending ridge) higher out of the way of the piston, as the latter method is open to the objection that it affects the compression ratio of the engine. We award the writer of this letter the 15s. prize this week.
The same correspondent also writes to say that he has observed, in the course of overhauling a large number of engines, the prevalence of slack gudgeon pins. He has been so impressed by the undue proportion of these that he has been at some trouble to discover the reason. His view is that in most cases the trouble is doe to wrong fitting of the taper pins which are used to secure the gudgeon pins in the pistons. As a rule, according to his experience, these taper pins are made much too short., so that the head of the pin comes up against the boss of the piston before the taper pert of the pin is home in the gudgeon pin. The result is that the last-named is free to move a little in the hole in the piston, and, in course of time, wears, and becomes slack, as " MM." has noticed, Correction of this defect in the fitting of the taper Pin invariably effects an improvement in the longevity of the gudgeon pins.
In the accompanying sketch two designs of piston boss are shown. When an example like that indicated at A is encountered, the only cure is a new set of taper pins, made a little longer in the shank, so that the taper part engages the gudgeon pin properly before the head
of the screw makes contact with the metal of the piston. When pistons nagee as shown at B are round, then it is simpler to reduce the depth of the hoes. For this purpose a special tool like that
shown in the sketch is required. Its construction and working are clear from the illustration.
For hollow gudgeon pins "M.M." prefers to have soft metal plugs, and makes them as shown in the sketch, so that the nipple fits inside the hollow gudgeon pin. A hole is drilled in each for the insertion of a long, thin punch, so as to facilitate removal of the plugs when dismantling the engine.
" 11.11.M.," of Darwen, finds the replacement of Dennis pistons to be rather an awkward job. Most of the trouble, apparently, is encountered in connection with the scraper rings on the piston skirt. There are two per piston, and both fit into the same wide groove. Stops are fitted on opposite sides of the groove to prevent these rings from turning round and lying with the slots in line, In his experience, the replacement of the cylinders over these pistons usually requires at least two pairs of hands, and frequently more than two. The rings apparently have a habit either of slipping out of the groove at a critical moment, or of getting away from the stops and lemming. " H.H.M." sends a sketch of a simple tool for holding those rings in position while the cylinder is being replaced. He is of opinion that this design, with two bolts, is more convenient in use than the usual kind, with only one bolt.
When replacing the cylinders, commence by turning the crankshaft until the piston heeds are level one with another. Place a clip over the two scraper rings on each piston, taking care not to fasten them too tightly, or they will not come off when they should. Lower the cylinders in the usual way over the top rings and gudgeon pins until they rest on the clips. A few light blows on the top of the casting with a piece of wood will then suffice to force the cylinders over the rings and the clips off the skirts of the pistons, when they may be removed by taking out a bolt from'each.
It will be found that there is ample room between the bottom face of the cylinder castings and the holding down studs to allow the clips to be removed. The same device will be found useful on many other types of engine.
" RM.," of Brighton, sends a useful tip which he says he has found to be of particular service in connection with Tylor engines, when addicted to pinking. He turns the cover plates which carry the sparking plugs until the latter are as near the centre of the compression space as possible. This expedient brings the sparking points at least one inch nearer the centre than usual, and, in his case, the improvement in the running of the engine is most remarkable.