Rob Roy. 0-6-0T in 3½" gauge by Martin Evans.

If built to the drawings (and I don't think they have been corrected yet), the valve gear and the valve
spindles don't line up. The difference is 1/8" (?), and the solution is either to make a stepped valve crosshead, or to make the valve face 1/16" further out from the cylinder, and the valve spindle 1/16" higher in the steam chest. (Thanks, to Steve Papworth.)

Change the pitch of the thread on one end of the valve spindle, to allow the setting of the valves to be more easily modified.

Fit thin locknuts to both valve and piston glands.

The bottom section of the crosshead pump is too close to the cylinders.

The blast pipe is in the wrong position.
Increasing the height of the blast pipe by about 3/8" seems to improve draughting.

The dome bushing on the boiler should be 3/4" diameter bore!

CONWAY. 3 1/2"  0-4-0ST by Martin Evans.

The boiler barrel should be 14.1/4" long, not 14" as on early drawings.

BRITANNIA 3 1/2" Pacific locomotive by L.B.S.C.

Drawings L.O. 81 sheet 8.
The side of the cab is stated as being 3 5/16" high............
Oh no it's not.......... it should be 3 15/16"

Also take no notice of actual roof dimensions, I had to add 3/16" at the rear and remove almost 1/2" from the front as the angled specticle plates do not tally with the roof!!