exhaust channels, generated vibration and encouraged flexing and fracture of the locomotive frames. All of his Pacifics were particularly prone to wheel slip owing to the high power output of the engine in relation to the adhesion factor. The engines were effective, and the design had materially less maintenance demand on the centre valve gear than the conjugated locomotives, but the positives were outweighed by the problems, which were of such significance that Thompson's engines were withdrawn and scrapped before many of their Gresley-designed forerunners. The Thompson Pacifics were ultimately more maintenance-intensive overall than the Gresley engines. Thompson omitted the "banjo dome" that had featured on the Gresley Pacifics since 1928. However, Thompson's successor, Arthur Peppercorn, revived the feature on the remaining batches of LNER Pacifics. From a technical standpoint, a number of the features of the Thompson design were not considered as acceptable or reasoned principles, as they did not follow established locomotive consideration. Divided drive, although it did reduce the total stress/strain on the centre crank axle, did not lend itself to frame integrity, especially coupled with connecting rods of equal length, as the greatest point of rigidity in the frame structure itself, the cylinders, were no longer lined up. Locomotive frames are flexible to handle side-to-side twist, and even axial twist along their length, but fore-and-aft motion between the frames leads to bearing and joint failures. To maintain connecting rods of equal length required the inside cylinder to be placed as far forward as possible, and even so the outside cylinders were behind the rear axle of the front bogie, which was not LNER practice, and subsequently resulted in a very long wheelbase. The front bogie itself was common with that on the B1, but was not sufficiently strong to control yawing of the Pacific classes, nor to straighten the engine out after a curve, and the Thompson A2's all had a poor reputation for being unwieldy, especially for rolling and yawing. However, the steam circuit, which was based on that from the P2's and the A4's, was generally regarded as excellent, as were the 19" cylinders fitted to the later A2/3's: consequently, with their large boilers, the Thompson Pacifics had a reputation for being capable of very high speeds, often being able to hold speeds in excess of 90 mph. The steam circuit aspects of the engines were carried over into the Peppercorn designs. Class L1