If the loco is doing the same revolution each way then 9 times out of 10 it will be a valve gear issue. More often than not one piece is out of timing or slipped and unable to complete a full cycle. If you can post pictures of each side we might be able to spot something. Jon . Yes it's the Dapol N gauge valve gear spanner that fits. This would need to be obtained from DCC supplies or Dapol direct. Maybe peters spares .