SECURING ENGINE EFFICIENCY.
Page 29
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Our Driver and Mechanic Readers Offer Some Useful Advice.
MHE TIME inevitably comes when the bearings of an engine must be overhauled or renewed, and, in consequence, the letter which we have received from " W.F.H.," of London, W. (he wins the 15s. prize), is one which cannot fail to be of interest, since it describes what is, in the writer's opinion, the best way of carrying out these operations.
The first thing to do upon dismantling the engine is to examine the crankshaft to see whether it has been distorted or whether any of the journals have become oval. If there be any distortion whatever, or if any of the bearings have worn oval in shape to the extent of more than about three-thousandths of an inch, then the shaft must be sent to a specialist to be reground.
If, however, the wear be only slight, it is quite possible to correct it by judicious use of a smooth file and emery paper, checking the work by frequent applications of a micremeter gauge.
If the occasion be one on which new hearings are to be provided, they must first be fitted into the crankcase. Great care must be exercised in performing this operation, and if it he found that the outsids. diameter of the shell is too large it should be carefully filed down, commencing at the sides, and not touching the bottom until it can be bedded solidly into its right position in the casing. If the bearing shell be not properly fitted, and if it only "fits where it touches," then the vibration of the shaft while the engine is running will loosen it and a sloppy bearing will result. If the old bearings are being replaced the shell should be carefully examined for cracks and the surface brightened up with a scraper.
When replacing crankshaft journals they should be smeared with prussian blue : the shaft may then be lifted into position and rotated several times. The rotation causes the prussian blue to be removed from the parts of the shaft i where the bearing s in close contact. These parts should be scraped down with a half-round curved scraper. When using a scraper of this type it should be held flat on the bearing, and will then give a . smooth, polished finish to the work if it be tilted, so that one edge digs into the white metal, chatter marks will be formed and the bearing will have a corrugated surface. Scraping should always be done diagonally, across the bearing from corner to corner, and then finished off with light polishing strokes crossing the original ones. The crankshaft must now be smeared anew with the prussian blue, set in the bearing, rotated, and the foregoing process repeated again and again until each bearing is evenly marked over at least twothirds of its surface. Should any bearing be low in relation to the others, it should be packed up with brown paper, and riot With metal fail, which does not always bed correctly and is liable to be springy. After the bottom brasses have been finished, the top ones should be fitted into the bearing caps in the manner above described. If the old bearings are to be used again they will have to be rubbed down on their faces in order to ensure that they properly embrace the ournals. This can be done by placing a sheet of emery paper on a surface plate or some similar object and rubbing the bearing upon it.
If the bearing caps be too deep to allow
the halves o,f the bearings being brought together, brown paper packing should be put in the bearing shell and the caps.
It is not advisable to file the faces of the cap or to reduce it in any way. When the bearings are bolted up, and before rotating the shaft, they should be given smart taps with a hammer on each side and on the top. This will cause them to set properly in their places, and it will invariably be found that it eases them considerably. The shaft itself should, when the work is finished, be rather tight in its bearings, but never so tight as to make it impossible to turn it round when using both hands.
The same method should be applied to the overhauling of big-end bearings, and they should be just. so tight on the journals that the weight of the piston will cause them to fall from a horizontal position.
After fitting, and before reassembling the engine, oil grooves shoad be cut in each bearing, using a sound-nosed chisel for the work. If the crankshaft be drilled and the oil pressure-fed, the oil holes in the shaft should be carefully tested one by one by blowing smoke through them. Finally) smear each bearing with a thick coating of heavy oil.
AN INTERESTING description of a
radiator shield, designed to enable part of the cooling surface of the radiator to be bla,nkettecl of in cold weather, is sent by "T.W.H.," of Leyton. The dimensions given on the accompanying sketch are those which would be required in the case of an A.E.G. lorry : for other types some modifications will be necessary. Two of the spring clips shown are required : they are to he bolted to the radiator, using for the purpose the extreme bolts on the radiator bottom casting. Their purpose is to hold the blanket plate firmly against the radiator itself. The pieces of felt which are shown on the sketch are to be attached to that side of the plate which crimes in contact with the radiator tubes. On the other side is a pair of drawer handles.
A SEMPLE suggestion, to facilitate easy starting of engines during cold weather, is put forward by "H.A.B.," of Rotherham. It is illustrated by one of the accompanying sketches showing a, piece of rubber hose which, at one end, is coupled to the carburetter intake, and at the other is open to the atmosphere. The bulk of the hose lies in a bowl of boiling water.
TEE OTHER sketch on this page shows a• couple of fittings wl:noh
of Shepherd's Bush earriea about with him as a provision against trouble arising from breakage of a petrol pipe. One is a short length of thick rubber tubing complete with a pair of clips and designed to slip over the ends of. the broken pipe. The other also embodies the same length of rubber tube and clips, and also a bend pipe With the union at the top.