Know-how 14: How did this happen?
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IN a previous article, I wrote of the young fitter and his first solo towing Job that did not quite go according to plan; here is another solo effort, but of a somewhat different kind.
A vehicle left the workshop after engine dock and somehow got back into service with the pistons fitted the wrong way round. This is normally considered an impossibility. The valves need to have machined recesses in the piston head and these recesses are off centre with the piston. Therefore with the piston fitted the wrong way round, the engine should not have turned far enough even to set the tappets.
However, that the engine did start and in fact ran for two weeks is borne out by the appearance of the piston in the picture. It will be seen that the valves had made a new bed for themselves in the piston crown (and survived for 1,300 miles). How could this come about, and how did it pass the tester, an experienced chargehand?
The vehicle in question was a Comet 400, and it required piston and liner change. The job was given to an apprentice in the final six months of his apprenticeship. He was already at a point in his training where he could tackle most jobs on his own, with just the right amount of supervision to keep him on his toes without destroying his confidence.
He stripped the job down and since the pistons were to be renewed he parted them from the con-rods and put them on one side without paying much attention to them as he had been told that they were beyond further service.
With the new liners fitted, he arrived at the point of fitting rods to new pistons. With the rods already marked to the camshaft side, he applied a bit of his earlier petrol-engine logic.
Without reference to the foreman's Comet manual, he came to the simple conclusion that as usual on push-rod engines, the valves must be on the opposite side to the camshaft, and fitted the pistons accordingly. Incidentally, these pistons were not marked for fitting, although it is usual to find the word "Front': clearly stamped on the head of the piston.
Even when the head was fitted, he failed to note that the rockers stood well to the camshaft side of the engine. The mistake. shauld have come to light when the tappets were set,
but in this instance he again pressed on with his idea of doing the job entirely singlehanded, and used the starter button to move the engine.
Finally the engine was started, and as one would expect, it sounded somewhat "hard". When taken out on test by the ehargehand, accompanied by the apprentice, the engine ran noisily but well, the one remark of the tester being, "Set that pump back, it is miles too early".
This the apprentice did, but could not remove the "crackle" within the range of adjustment. So he concluded that the injectors were set higher than standard, and returned the pump timing to its earlier spillpoint setting, fairly satisfied that his first solo engine job was a success, and in "cracking" good fettle.
The cracking part came home to roost a fortnight later, when bent and blowing valves caused the head to be lifted and the mistake was discovered.
There is little doubt that a reluctance to ask the foreman for the manual by an otherwise confident apprentice was the main reason for this error. And it shows that more than ever in this age of increasing complexities in design there is a need for the diagrammatic wall chart. which will go a long way to removing both embarrassment and error from the path of the trainee.