I have just been pointed to this quotation regarding the reliability of Scot middle big ends. It's from a book published in 1970 written by E.S Cox, J. Powell, W.A. Tuplin, P.G. Johnson, and comes from chapter 3, “Mechanical Development”, pages 13/14 “Of all 3 cylinder engines of which there is a record, the Scots were the most trouble free as regards their middle big ends. Gresley’s anxieties on the LNER are well known and Stanier 5X class (Jubilees) came into their share of troubles to the extent that many of these engines were fitted with tell tale stink bombs to give warning to the driver when disaster was impending but the Scots throughout their lives caused little anxiety in this respect” In another thread, someone mentions roller bearings, this is not mentioned in the book.