MORE BIG-END TIPS.
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Useful Contributions From Our Driver and Mechanic Readers.
The Fitting of Big-end Shims.
THE subject of big-ends still appearS to be of interest to massy of our practical readers, as we have received so many letters dealing with it. We feel that a thorough ventilation of the various methods of construction and maintenance of big-end bearings cannot fail to produce good results.
A correspondent, "R.T.H.," of Bit.mingh4m, raises some points which have not been dealt with by previous correspondents. He also makesa suggestion which might be worth the consideration of both manufacturers and maintenance engineers. He says: " Amongst the very interesting and enlightening letters which" have appeared on this very important subject I have seen only one mention of the plan adopted in the Saline lorry, the method being as follows :—The brasses are made to meet without a shim' and are then carefully fitted to both the rod and the crank journal. The cap is then slightly tightened dawn and when the fitter is satisfied that the bearing is properly bedded, the gaps provided for the shims Are measured with feelers and shims are filed to fit the gaps. The bolts are then removed, the shims inserted and the final tightening down of the bolts carried out.
"The shims in this case do not extend between the brasses, which rely on some kind of dowel to prevent them from moving in the rod. There is little doubt that this method is a good one, but I think that the. shins will form a better anchorage for the brass than nay form of dowel.
"Whilst fully recognizing the advantage of being able to grip the brasses while in their place, and independently of the shim, I beg to make the following suggestion of a compromise between the two systems. I have not tried this plan yet, but unless someone suggests new ideas even if untried, we should never make any improvement, My suggestion is that the cap should be machined as shown in the sketch, and that an allowance should be made in the brasses for a shim. The shims which go between the brasses could then be put in their place' and the Saurer plan followed, and when the bearing is considered to be properly fitted the gap could be measured and the second shim fitted."
More on 011-ways. A READER, "F.S.," of Swanley, writes us as follows :— "Raving been engaged in motorrepair work for the past 25 years, the writer has necessarily seen a good deal of big-end trouble. Perhaps the following remarks Se same may be of interest to other readers. My firm often has to remetal big-ends which have had the caps filed away until the bearing is badly oval. If remetalkd in this cendition the white metal will not stand up owing to being too thin at the top nod bottom where most of the wear occurs. If the old brasses be used, shims must be put between the brass and the cap. "When new brasses are fitted tls
writer prefers to fit shims between the cap and rod only, pulling the brasses tightly together. Bolts should always be easefully examined for stretch, and if doubtful new ones should be fitted, A magnifying glass is useful for this work, as bolts which would pass muster otherwise will quickly be thrown out after this test. Four-bolt rods are better in this respect, and do not need to be drawn up so tight.
" The writer has never seen a fourbolt rod come adrift and smash things. Two-bolt rods often used to. but this iS less frequent now owing to better steel being employed. The four-bolt rod is very tedious and slow to take upDie-cast bearings without shells will not stand up to lorry work; the only way when dealing with these is to tin the rod well and east the bearing in, making sure it really does amalgamate with the tinning. "The cutting of oil-ways is often ores'done, and useful bearing surface wasted, a plain hole widely countersunk often being sufficient. The Ford has none at all and stands up very well, considering the conditions -under which it is often run. A simple V-groove (cut in the direction of running) is very satisfactory under all conditions. "When big-ends run out, one is never safe again until the oil-ways in the crankshaft have been unplugged and thoroughly cleaned out; syringing is not good enough, as a quantity of white metal always remains in the shaft. This should also be done when overhaul.ing an engine which has done a heavy mileage. The writer has seen a shaft the crankpin of which had ,a s-in. hole through it, and this was half full of dirt, which had been thrown by centrifugal force to the outer side and packed hard; this dirt could not possibly have been moved by syringing."
Proud Brasses.
ONE of our correspondents, "D.W.S.," of Aberdeen, adds the following to the discussion :— "Being interested in the letters now appearing in The Commercial .Motor regarding the fitting of big-end bearings. I should like to add a few remarks based on my experience during the past 14 years with various types of light car and lorry engine.
"I agree with the method of • FL' of leaving the brasses proud of the keep and connecting rod, but not to the extent of .002 in. tel he suggests, which gives a clearance equal to .004 in. when the connecting rod and keep are in position. The strain caused on the brasses when bolted tightly together is likely to warp them and to slacken the white metal when under lend; it would also be impossible to have the surface of the bearing fitted to the crankshaft journal with any considerable degree of accuracy if a clearance of .004 in, was left.
"I have found the white metal broken on several occasions, owing to excessive clearance being allowed when fitting the bearings; therefore particular care should be taken by mechanics adopting this procedure.
"Personally, I allow .001-in, clearance on both halves of the hrassee, which gives .002 in. between the keep and the connecting rod when in position. and, although I have always used brass shims on both lorry and light-car engine bearings, I have never seen any signs of the shims being hammered on dismantling the engines after two years' running."
Now that correspondents have covered an the points relating to this subject, we propose to deal with the general consensus of opinion on all the leading points.—ED.
Will the following contributors kindly send their present addresses?-G. Xennings, of illeucester, and P. Scott, of Kirkct-ddy.—En,