The Clayton Type 1 (class 17) locomotive was introduced in 1962 and was billed as the new standard type 1 locomotive which would be purchased in preference to the highly successful English Electric Type 1(later class 20). The principal reasons highlighted at the time was the shortcomings of the single cab configuration of the class 20 which led to significant visibility issues when running “nose” first. It is unclear why a double cabbed version could not be developed although English Electric was heavily committed to other builds including the class 37 and 40 locomotives.

As a second generation modernisation plan design should have been substantially more successful than some of the earlier designs due to the increased knowledge of diesel traction that existed at the time of order. Unfortunately, this was not the case, especially in terms of the engine and major ancillaries.

The class 20 utilised the EE 8-cylinder SVT Mk2 engine running at a moderate maximum speed of 850 rpm. This engine was effectively an 8-cylinder version of the units fitted the pilot locomotive 10000/1 and 10201-203 and the class 40 locomotives. As an engine which had been in production since 1934 it had been progressively developed over the years and was, at least in 1960 terms, virtually indestructible and capable of managing the extremes of a traction environment.  The class 17 locomotive however appeared to have learned none of the lessons of the last decade.  To facilitate the central cab design, it was necessary to incorporate 2 low profile engines.  These engines, manufactured by Paxman were designated as “6ZHXL” and were a 6-cylinder horizontal configuration running at a maximum speed of 1 500 rpm and producing 450 horsepower each, giving a total of 900 HP for the locomotive.  The engines had never been used in a traction configuration before although two had been trialled from 1956 in an experimental diesel multiple unit which operated without significant engine issues however it ran in a naturally aspirated mode without supercharging and produced only 300 HP.  In spite of the lessons learnt the hard way with many of the modernisation designs, no prototypes were constructed and the entire order for 117 units were ordered off the drawing board. Once the first locomotives entered service problems with the engine frames, which were constructed of aluminium, cracking surfaced along with numerous other ancillary issues. The problem was so severe that deliveries of new locomotives were suspended and existing units stored while the problem was resolved.  This resolution including the replacement of the cast aluminium frames with cast iron frames at Paxman’s cost.  There is some correspondence that Paxman requested the change from Aluminium to Cast Iron prior to the service introduction and that British Railways for whatever reason refused.  Once the engines were rebuilt the availability improved slightly, from below 50% to a peak of 60%, however the engines continued to be problematic and required frequent repairs. A discussion that I had with a foreman at Polmadie in 1971 indicated that the rear engine failed more frequently than the leading engine indicating perhaps a cooling or air flow issue and based on photographic evidence there appears to be evidence that the rear engine produced more black smoke than the front suggesting a shortage of combustion air.  After the publication of the National Traction Plan in 1966 it was clear that there was going to be no long-term future for the locomotives and a total of 100 new class 20 locomotives were ordered to replace the entire fleet. The withdrawals started in July 1968 with D8537 which had a life of only 61 months, part of which it was stored. The remainder of the fleet was withdrawn progressively between 1968 and 1971 with the locomotive with the shortest life being D8611 which after being introduced in December 1964 was withdrawn in October 1968, a life of only 46 months! Three of the class (D8512, D8521,D8598) survived in departmental use at the railway technical centre at Derby until 1973 (D8512) and  1979 (D8521/8598) and a single unit (D8658) passed into industrial service before being preserved and fully restored.

The tractive effort curve shows a peak of 40 000 lbf on starting and a continuous rating of 18 000 lbf at 13 mph.  It’s interesting to note that the curve also shows the tractive effort when running on a single engine which given how often journeys were completed on that basis is prophetic.  There was also no unloading of the main generator up to the locomotive maximum speed of 60 mph although there is a slow drop which can be observed above 20 mph.(That air shortage again?)