Removal of the engine

It will be recalled that nothing would persuade the engine to turn - it was seized solid and so removal and examination of the engine was the key first step to determine whether the whole project was feasible. This process was carried out in the garage where the car had been rested-up since going into storage in 1971. A gantry and two chain hoists were employed in order to both lift and tilt the engine.

But first the ancillaries on the engine (carbs., starter motor, etc.) were removed to enable easier access to what is a very large engine, in a not much larger hole! The Gearbox was then parted from the engine, and then we were in business.

Extraction of the engine was pretty straightforward, but as can be seen we took the necessary precaution of tying the legs of the gantry crane together to stop 'spreading' under the massive bulk of the engine!

Incidentally, in the right hand picture above, note one extraordinary feature of the W186: three horns, one of which can be switched in or out of circuit, presumably to give that extra presence when hammering down the Autobahn!

Whichever way you look at it, this is a dirty big engine....

The engine was then laid on its left hand side on a reinforced 'Euro pallet' to which castors had been added for mobility. The whole thing was then transported to my garage for disassembly.

First task: getting the pistons out

....Well it was off with the sump, camshaft cover, timing chain and camshaft. A scene of unremitting grime presented itself: all surfaces, nooks and crannies were plastered with a baked-on mixture of oil, rust and Kenyan red sand! Inside the sump, gelatinous oil/diesel fuel sludge was in profusion, stinking out the garage and getting everywhere. The next important task was to remove the cylinder head. It is testimony to the quality of fasteners used by MB (hardened cap-head bolts) that all the head bolts were removed without damage or stripping, despite needing armfuls of torque to loosen them. The head was then parted from the block revealing extensive coking, some apparent damage to the cylinder bores, but generally no real problems. When the head had last been removed (big ends were done about 1969/70) I suspect that the original head gasket had been put back. Needless to say, the gasket was in a very poor condition with some evidence of leakage past the rings. However, I assume that the low compression ratio and generally low rating of the engine, plus the amount of gasket cement used had meant that the engine had survived.

	'Top end': overhead camshaft arrangement. Note the torsional mass-damper coupled loosely on the end of the timing chain sprocket (to reduce variations in camshaft speed throughout a revolution)
	Artwork from the original brochure for the MB 300 showing the OHC configuration. (The lower picture extols the virtues of the engine mounting system.)
	A very grimy 'bottom end'. Note the torsional mass-damper coupled loosely on the end of the front end of the crankshaft (left). A further measure to achieve smoothness at any price!

With the head off, the crank still would not budge, but where was the seizure? Close examination of the behaviour of the apparently loose big ends when torque was applied to the crank suggested that No 5 cylinder was the culprit, there being absolutely no movement whatsoever in this cylinder. The next task was clear: the crank had to come out. First the big ends were removed, a task made very difficult by the last person to do this replacing the castelated nuts the wrong way round with the narrow flats in a recess next to the big end - how did they ever tighten them? Anyway, Ray ground down a socket to fit and then off came the big ends. Now the fun really started!

With a little brute force and a lot of ignorance, the connecting rods were then forced up the cylinders by levering against the still locked crank. Nos. 1, 2, 3, 4 and 6 came out reasonably easily, but the big surprise was that each ejection was accompanied by a sweet 'tinkle' of broken piston rings: in every case one or both compression rings were cracked. However, No. 5 was not playing the game - it was time to get serious! With Ray on the lump hammer and me holding a wooden drift against the upper bearing shell of the big end, a series of giant clouts were administered and, progressively the piston inched its way out of the cylinder. Not quite a tinkle this time, but some things did fall: a number of rounded sections of the third ring. Rings one and two were complete but coked tight within their grooves, but most of ring three had gone. Worse, the remnants of the rings, being much harder than the aluminium alloy of the piston, had obviously hammered up and down and, in doing so, had worn a hole in the piston about 10 mm across, and bounded top and bottom by the harder rings two and four respectively. It looked as if a mouse had been nibbling away at the piston!

Piston 5 showing the damage caused by piston ring fragments and the top two rings coked-into their grooves.

A call to the US established that replacement pistons would be circa £1500 a set, and therefore I knew I would have to work with what I had got. The pistons were then generally cleaned-up, but No.5 was selected for special treatment. The gudgeon pin was removed and attempts made to free the top two rings by gentle means, but they were stuck. I gave the piston to brusque antipodean 'Grimbo' at work with the request 'please remove those rings but for God's sake be careful and don't go over the top!'. "No worries" was the reply, and half a day later I returned to my desk to see a much cleaner bare-looking piston on my desk. "How did you do it?" I inquired. "Well, I tried being gentle (Grimbo, gentle?) but I gave up and got the Propane torch on it - that burnt off the carbon and soon shifted 'em!" "But hasn't that distorted them?" If it had, then it was too late anyway, but I needn't have worried - the piston was OK.

So, how to fill the mouse hole? I was advised by a neighbour of a good aluminium welder in Redditch, whose name escapes me but this guy is a real hero (and when I remember his name then he will get a big plug). "....well let's give it a try..nothing to lose really..no good as it is, yes?....". He struck an arc and carefully built-up aluminium around the hole. After about ten minutes work, and numerous rods of alloy, it was done. It looked a mess but was clearly sound. "Great job...what do I owe you?"..."Oh,....nothing, that's OK." I reached in my pocket - no wallet, and just a quid in change, but he wanted none of it, and he'd just saved me about £450 for a single piston! It's folks like this that make the world a great place! I think car restoration brings out the best in people sometimes, particularly if they can see that your back is to the wall. Late news: in revising this page I just called the neighbour who recommended this particular aluminium welder to find out his name and address....and it looks like he's gone out of business! Well what can you say?!

Piston 5 after repair.

Finally, Ray (another unsung hero who has helped me so much) turned-off the excess aluminium, recutting the ring grooves bang-on. So there we were, back up to a full set of pistons. Detailed examination showed that most of the ring grooves were around top limit on width, and one piston had a hair-line fracture down by the skirt. Yes, a new set of pistons would have been desirable but, with new rings, and in such a low-rated engine, the originals are probably good for another few years yet, particularly at the current rate of restoration!

Part Two: Strip and Examination