Professional Engine Rebuilds

They Can Always be Relied Upon – Right?

I recently completed the recommissioning of a very nice S7 that had been professionally rebuilt and nicely restored some years ago but then ridden for only a few hundred miles before being stored. The paint and the general standard of finish was excellent, one of the nicest looking examples I have seen. Although the bike had been stored for many years, it was obviously kept in a warm and dry environment, judging by its general condition. But its owner wanted to be sure that the passage of time had not bestowed internal corrosion, hardened seals and sludge where the oil should have been.

The only external signs that all was not quite perfect was a leaking cylinder head gasket, around the front of the number-one combustion chamber, and a curious stiffness of the engine. The leaking is usually a sign either that the head was not tightened down properly following a rebuild or that the thread had failed on the front centre stud. In any case, replacing all of the seals, changing the oils, cleaning out the sump and doing some basic checks are wise precautions on an engine that has been sitting around unused for a while, particularly if its history is only partially known.

Removing the cylinder head revealed that it had probably done less than 500 miles since its rebuild. There was very little carbon in the combustion chambers and the bores still showed their honing marks. The camshaft, cam followers and valve gear were also in excellent condition. However, the engine was mysteriously tight to turn over, more so than is normal, even in a recently rebuilt engine. This made the engine very difficult to start and clearly needed to be investigated.

Dismantling the engine revealed a number of unusual things. The bronze bush that serves as the bearing for the half-time gear had seized onto the bearing pin and was turning inside the gear, which had caused the gear itself to wear. This can happen if the bush is not reamed to the correct clearance when fitted. That isn't good because a new replacement half-time gear is no longer available and, even if a spare can be obtained, the crank-shaft gear and the half-time gear are a matched pair. Mixing them can result in a noisy engine.

Fortunately, because of the very low mileage of this engine, the damage was slight and the half-time gear could be re-used once a new bush had been fitted and correctly reamed.

The next discovery was that one of the big-end nuts required a lot more force to release it than the other three. Seemingly, it had been tightened to a higher torque than the others. It isn't all that uncommon for that to happen. The reason is usually that an inexperienced engineer has tightened the nut to the recommended torque and then realised that the crenelations on the nut don't line up with the hole in the bolt for the split-pin. It is tempting to continue to tighten the nut until it does line up but, instead of stretching the high-tensile steel bolt, it deforms the much weaker aluminium of the connecting rod. In extreme cases, like this one, the connecting rod becomes so distorted that it can't be passed up through the cylinder bore to remove it so some metal needs to be ground away from the side of the rod to facilitate removal.

Again, it is fortunate that the engine had not done a lot of miles because this kind of damage usually causes a failure of big-end bolt sooner or later. The reason is that the aluminium of the connecting rod does not crush evenly. It crushes more at the outer edge than it does towards the centre. That means that the head of the bolt is tilted, stretching the bolt more on one side than the other, causing a very high load just where the bolt head meets the shaft, its most vulnerable point. A guarantee of a failure that can often cause extensive collateral damage. The misshapen rod and the abused bolt were only fit for the bin.

The picture on the left shows the wedge-shaped gap under the big-end bolt head. When the bolt is tightened, it will tilt the head. You can also just about see the slight bulge in the aluminium, where it has been distorted by excessive load.

On the right, we can see the big-end nut being “adjusted” on the lathe to get the correct alignment of the crenelated nut with the hole drilled through the bolt.

The third discovery was that the end-float on the crank-shaft was rather less than zero. When the rear main bearing carrier was fitted loosely, the crank-shaft would spin freely but, as the carrier was tightened, the crank-shaft was clamped as if in a vice, putting a load on the thrust face. This was the cause of the tightness. End-float should be between 0.004” and 0.007”. There was no thrust shim fitted in this engine so the extra clearance had to be gained in another way. Metal had to be removed from the thrust face of the rear main bearing bush. In this case, quite a lot of metal, so it is fortunate that the white metal coating on the bush is very thick on these engines. The damage to the thrust face of the bearing can be seen in the picture on the left.

Buying an old motorcycle that has had a recent rebuild but has subsequently done very few miles on the road can be a sobering experience. There are a lot of rebuilds out there, some even by reputable engineers, that conceal a number of issues within. The only way to find them and fix them is to pile on the miles. If somebody spends a fortune restoring an old bike but sells it before it is fully run-in, there is usually a reason. Often, it is a perfectly innocent reason but sometimes not. And when the new owner discovers what it is, he or she may not be as pleased with their purchase as they had imagined. If you buy such a bike, it is wise to have it inspected. It can be expensive and frustrating to find faults like these the hard way.