Checking out the cylinder block

Now that the pistons were out, the bores could be inspected, and it wasn't all particularly encouraging! No. 5 had suffered some damage coincident with the 'mouse hole' in the piston, but No. 1 was of greater concern with extensive areas of pitting: one of the valves must have been open for 24 years with the piston at the bottom of its stroke!

The side covers for the water jacket were then removed and, what a surprise, they were absolutely clogged solid with grime, scale and rust. How did this engine stay cool? Again, higher rated engines would have expired long back with this degree of neglect. A hammer and screwdriver were necessary remove most of the rubbish stuck to the block before the whole thing was lifted - just - into my degreasing tank. I seem to have spent weeks stood at the degreasing tank whilst my 'caravan' electric water pump squirts black and sordid paraffin over the aforementioned grime. I got through a number of pairs of gloves, stiffened by the paraffin, and many of my wife's brushes from the kitchen. But, on the whole, I found degreasing most therapeutic, and very rewarding notwithstanding its uncanny similarity to 'washing the pots' which, as all men know, is the worse job in the world!

Removal of a side cover revealing a clogged water jacket. Note the convoluted oil temperature stabiliser coil integral with the side cover: heats the oil when cold, cools it when hot.	Degreasing: the most theraputic aspect of engine restoration. (Any resemblence of this activity to washing the pots is purely coincidental!)

With a clean but worn block, and a set of dubious pistons, I set off to see the legendary Phil Moss of Moss Engineering, Ledbury (01531 632614). Phil is a remarkable man, perhaps not as young as he was, it is fair to say, but a die-hard motorcycle enthusiast who really practices what he preaches. The previous day he had been lapping Castle Combe on his Ducati! Anyway, with gauges and clocks in skilled hands, Phil called out the state of the cylinders: "Six thou wear...two thou ovality.....five thou wear....one thou ovality..." and so on. The verdict was not as bad as I had thought - if the cylinders were honed then the worst of the damage could be accommodated, so this is what we agreed. A very reasonable £15 lighter, I left Ledbury with bores looking like new, and a wealth of good advice. As I crossed the Malvern Hills (they look so different from the Herefordshire side) I felt confident for the first time that we had got the measure of the engine's problems for the first time, and that success was achievable. After about three months effort, it was a good feeling!

For the cylinder block, it was then back in the tank for another swim in paraffin.

Bottom End Troubles?

As mentioned above, I had been advised that the big end bearings had been changed just a few years before the car was stored. On examination, I could confirm that the bearing shells were not original unless Benz originally fitted Glacier bearings (about as likely as them fitting a Lucas distributor!). But they were badly worn down to the base metal, particularly in the upper halves which, presumably, do the most work. 'Miking-up' the big end journals showed them to have been ground down by one size and, fortunately, they looked to be in excellent condition. The main bearing shells and journals both looked A1 which was encouraging. However, the condition of the big ends after 20,000 miles at the most set bells ringing.

Had the engine suffered oil starvation? I suspected so for a number of reasons. Firstly, the workshop manual for the W186 makes reference to three different specifications of oil pump during the first four years the engine was made. With each iteration, the delivery of the pump was increased, with mine being of the second, intermediate type. Perhaps by specification, the oil pump was only just up to the task, and certainly not after running with years of neglect. Secondly, the oil filter, a superb wire-wound type, was (no surprise here!) clogged solid with crud. Paraffin would not touch it: eventually I resorted to hot Genklene in an ultrasonic vat to clear it out. Granted, a by-pass valve is fitted across the filter, but this only opens at higher than normal pressure drops, and therefore delivery flows of oil to the engine must have been affected. Thirdly, the oil strainer in the sump was heavily clogged and, fourthly, the oil pump clearances were indeed on their upper limit against the probably conservative specification in the workshop manual. It was clear that a major overhaul of all elements of the lubrication system was an essential requirement.

"The difference between us as Engineers and everyone else..."

So Rex at work used to say "....is simply this: attention to detail." Dear old Rex used to 'go on a bit' to put it mildly, but he was often right. And if attention to detail defines engineers, then the engine builders at Stuttgart were Master Engineers. Contemporary reviews of the W186 define the engine proving schedule as follows:

• Run on testbed for 20 hours.
• Strip Engine and examine parts.
• Test for further 90 minutes.

In addition to which each car was then road tested. Assuming the odometer on my car was originally set at 000000, it was tested for 46 miles before going in the crate!

With the strip and rebuild activity, identification of parts was an obvious issue and, during the degreasing process, this became apparent. For example, not only do the conrod and big end covers carry unique numbers stamped on, but after 45 years, the engine number (2042) is still visible on the side of each conrod.

Further, almost every casting or machined part carries an inspector's stamp, presumably a carry-over from wartime when the same plants were making aero-engines for the Luftwaffe. Either way, when dealing with this engine there are areas of breathtaking attention to detail, and gross over-engineering which leave one gasping with amazement. The following is an example of what I mean. The air cleaner assembly includes a 'horn' style cranked intake (lower right in the illustration below).

On the face of it, you would expect this to be a pretty straightforward part, but not on a 1953 Mercedes! Believe it or not this is an assembly of 9 separate sheet steel sub-components, generally hand beaten to form, which are then fastened together using two lengths of seam welding.......and 139 spot welds! Compare this to the materials and methods used for similar parts in modern motor cars: the men who built the '300' knew they were making the best engineered car in the world at that time, and therefore nothing was to be left to chance, even if it required 139 welds to hold it together!

Part Three: Rebuilding