I have been alerted to this forum and specific exchange via the ‘Mirrlees Memories’ Facebook group (Closed Group open to anyone with Mirrlees background or interests) and decided to make a contribution to this dialogue. I was trained at Mirrlees in the period immediately after Mirrlees and BR parted company.
The JVS12T engines did crack and a repair programme was put in hand to produce modified frames to a Mark 2 version with reinforcing in appropriate places as a short term interim measure whilst a long term Mark 3 frame was developed. The Mark 3 design was a clean sheet of paper exercise and a radical new design was produced. This new design used the same design logic adopted for another order for a larger engine type for the MOD logistics vessels (Sir Galahad, etc) with severe shock loading requirements (with no cracking in service on any of the engines – the last of which were only scrapped within the past decade). In the view of the former Chief Designer this was an overkill exercise for the BR application concerned. I understand that BR had no interest in the Mark 3 frame proposals (having already ordered substantial numbers of replacement engines as we now know!). See Pages 116 to 121 of ‘The Diesel Impact on British Rail’ by R M Tufnell published in 1979 - ISBN 0 85298 438 3
The redundant JVS12T engines were disposed to various organisations including North Lincs Engineering ( a picture exists of numerous engines in a field as purchased by NL) and many went into service in various roles. NL reengineered the sets for new applications but without changing any major components. The NL supplied engines were used for subsequent decades with no issues such as cracks developing into failures. The last NL engine in service was one sold to a scrap merchant on the Channel Islands to drive a metal fragmentation unit which is a very tough duty. The engine lasted for over 40 years in this application and the MD of NL commented ‘Not bad for an engine condemned as scrap by BR!’ It was only at the stage did NL scrap the final remaining engine which had been kept as an insurance policy in case the cracking problem had developed – thus eliminating a perfect piece of kit to permit a conversion of a Class 31 back to a 30.
As a trainee at Mirrlees a visit was organised each month to further our education resulting in visits during my time to the likes of Rolls Royce, English Electric Diesels, Ford Halewood, Fylingdales and British Rail. One of those visits was to BR Crewe where we were shown around by BR apprentices who were quite happy to show us to tell us the ‘full story’ without realising that we were all student and graduate engineers, some with considerable experience and knowledge. We were shown into a former steam workshop that had been converted into a machine shop for repairing diesel engine major components on an industrial scale. We were able to examine in great detail bedplates and frames of all makes and type (except Mirrlees!) which were undergoing a substantial repair exercise with full re-machining. We were told that this was a major issue with the BR diesel fleet at that time. The apprentices were surprised that we knew so much about the things we were viewing! The conclusion of the visit was that many engines in BR use had cracked and not just the Mirrlees J.
In reviewing the BR Diesel Programme there was a rumour that EE were promised a big engine order as compensation for a major order being placed with Sulzer. The Mirrlees J cracking issue would have provided an ideal opportunity for such an order whilst at the same time rationalising on the number of engine makes and types in service. The timing for the replacement engine orders with EE does suggest that this could be the case. The Anson Engine Museum now holds the Mirrlees company records and a few years back an author visited to read the company files in relation to BR and the Mirrlees J, having also researched the BR records in the National Archive on the same subject to finally prove this one way or the other. I haven’t seen anything published so far.
JV12 engines do occasionally turn up and one was offered for preservation in April 2020. If someone is serious in pursuing a conversion back to Class 30 then the best way forward would be via the Mirrlees Memories FB Group and the Anson Engine Museum http://www.enginemuseum.org/
