A report of the principal problems with NBL built locomotives had been prepared by Swindon in September 1962 and read as follows:
(1) In early NBL built MAN engines, great difficulty is experienced in fitting replacement parts, due to non-standard building. Also many failures have been due to poor workmanship e.g. badly fitted taper seats for gears and flanges, additional drilled holes in castings causing oil leaks, balance weights slack on the crankshaft, split pins and locking devices missing, piston rings butting together, and general pipework of poor quality.
(2) Cracked or porous cylinder heads and/or cores plugs leaking.
(3) Exhaust manifold system complicated and unsatisfactory, expansion joints seized, stud threads stripped on cylinder head and manifold joint. Pieces of baffle plate become detached damaging turbo-blower blades. Experiment being introduced with stainless steel bellows to take up expansion and vibration.
(4) Cam followers wear, seize, and cause damage to the camshaft. Occasionally worn followers free themselves, and rotate, and if the tappet clearance has been taken up by adjustment, the push rods are bent by the action of the cams.
(5) Broken top chrome piston rings, thought to be caused either by lack of lubrication due to the severity of the new ring pack introduced to reduce oil consumption, or by ring butting due to build-up of carbon in the ring gap and behind the ring. The ring gap has been increased, but no evidence is yet available of any improvements noticed.
(6) Faulty CAV injectors, with a fractured collar on the spindle, lead to overheating of the burner insert which melts into the cylinder causing damage to pistons, liners and valves.
(7) Big-end bolt breakages mostly attributed to fatigue failures caused by inadequate tightening of the bolts when assembled.
(8) Big-end bearings damaged by the copper blanking plugs working loose in the crankshaft journal oil-ways; modification introduced with thicker collar and hole to facilitate riveting.
(9) Heavy oil leakage from white-metal oil seal at front of crankcase; repair entails re-metalling and reboring.
(10) Failure of ball races on fuel pump drive shaft and cradle.
(11) Insufficient drainage on the combustion air delivery box in the vee of the engine, combined with poor welding causing fuel leaks, leads to accumulation of fuel oil in the air box.
(12) End cover tapered locating dowels work out of the cylinder block causing oil leaks. A dowel retaining clip is now fitted to overcome this defect.
(13) Pistons suffer from thermal cracking of the crown and have to be scrapped .
(14) Excessive wear of piston top ring groove is thought to be caused by hard carbon building up in the wear-groove in the liner, and plain liners are now used whenever possible.
(15) Overspeed device is unreliable due to design weaknesses which are being discussed with the supplier.
(16) Scoring of the lower liner register during assembly has led to water leaks.
(17) Premature flaking of the lead flashing on the 'Glacier' big-end bearing is at present being investigated.
These same remarks also applied to the 58 engines in the Scottish Region in the Class 21 diesel-electrics as they were also fitted with the MAN L12V 18/21 engine, but in addition, they suffered from unnecessary delays in service due to the overheating trip shutting down the engine, which could not then be restarted until the whole system cooled sufficiently. This was altered to allow the engine to idle so that the fan could still operate. They also found that fractures in the connecting rods originated from the serrations on the mating faces due to insufficient radius at the base of the serrations. New rods were fitted, with serrations having a fuller radius, and proved satisfactory.
One root cause of a lot of the troubles with all of the MAN engines was the coupling in the drive to the fuel pumps. Due to the hardening of the rubber inserts, misalignment altered the pump timing which in turn caused overheating in the cylinder heads leading to piston seizures.
Many big-end bearing failures occurred due to the bearing turning in the housing. It was found that it was possible to have as little as one thousandth of an inch nip between bearing and journal. Larger shells with a higher tightening torque were introduced, but there was really insufficient depth of thread in the caps to take this increased torque.
The piston problems were never properly solved and attempts to get UK piston suppliers to supply pistons with Alfin bonded ring inserts did not materialise. They probably felt that it was a thankless task to undertake for such a small market, so the problem was left to Mahle, who were the original supplier. A suggestion was made in 1967, to test pistons made from silicone-aluminium, but this was never started as these engines were soon to be phased out.
Exhaust manifolds were a constant source of trouble having a partition designed to provide a two-part entry into the turbo-charger. This partition fractured, as did the entry pipe itself, but Glasgow works eventually made their own manifolds in mild steel, which were an improvement on the original and a lot cheaper.
In March 1963, the engine in D6123 in the Scottish Region was replaced by a Paxman 'Ventura' engine rated at 1350 hp. This 'Ventura' was similar to the two fitted in D830 (produced at Swindon in 1961) but at the higher rating, which the electrical equipment on the D6100 Class could accommodate. This was followed by nineteen more 'Ventura' replacements in 1965 but the remaining thirty-eight in that Region were allowed to run on until withdrawal in 1968/9. Those in the Scottish Region had suffered more from crankcase cavitation erosion due to water problems. In this Region the water treatment had at first been sodium dichromate, then sodium benzoate nitrate and finally borax/sodium metasilicate, whereas the Western had used the German recommended soluble oil treatment. Reclamation schemes were tried out using shot blasting and spraying with molybdenum and ceramic, but these were not in action long enough to be conclusive.
Some mention should also be made of the similar engines used in the 'Blue Pullman' railcars although these were German built and only rated at 1000 hp. Their performance was generally admitted to have been superior to the NBL built engines, though railcar duties are reckoned to be easier than those on a locomotive installed engine. These ten engines went into service in July 1960 on the LMR transferring to the Western Region in September of that year. They remained in service until 1973 when all the 'Blue Pullman' trains were withdrawn.
During the last few years of their reduced operation these engines had settled to an acceptable life cycle of 4000 hours between overhauls in the 'Warship' locomotives (D833-865). Problems were still occurring in the high-pressure fuel pipes due to cavitation erosion and fracturing of end nipples. Fuel injectors were changed every 3000 hours and all flexible hoses at 4500-6000 hours, according to overhaul procedures. The one feature of these engines that performed very satisfactory was the Napier turbo-charger, which had not previously been tried on MAN engines, and which amazingly worked better on these engines than on many of their own English Electric engines.