It does not look an ideal gear for anything, but I don’t suppose Fowler reversed the geometry so that the forward gear was the one with the better steam distribution?

Road vehicles, apart from rollers, don’t need work very efficiently in reverse so although still a compromise, making the forward gear the more accurate would at least partially “work round” the problem.

So what happens if you move the eccentric setting 180º – putting it on the opposite “side” of the crankpin?

The curvature of the radius-link may be the main culprit: the smaller its radius the more the distortion and this could be with a very small radius change.

The Firth gear is new to me but it looks like a “straightened out” Hackworth gear.  I have observed when trying to trace a problem with a ‘Wren’ locomotive that with Hackworth gear the more the die-block guide is set from the mid-gear alignment the more the connection-point locus distorts from an ellipse to a stylised whale. I’ve think I’ve also found this on paper with the similar Marshall and Bremme gears. These simple radial valve-gears don’t like being over-driven.

Tightening the radius link curvature might increase the effect, and success of this gear seems to depend on a fine balance between the radius and the angle to which the link is rotated.

I do wonder why Fowler used this, and how they made it work: they didn’t cheat by using unsymmetrical ports or valves, did they?

I’d suggest first seeing what happens with the eccentric put on the other side, then modifying the radius-link radius a very small bit at a time to observe the effects in full gears, mid-gear and a couple of intermediate notches in both directions.

If Fowler used it on Rollers, it would seem thatn they had found means of making the engine as reasinably efficient in nreverse as in forward gear.

For a ploughing engine the heavy work mwould be in hauling the plough,and reverse running might be rarely needed (other than for manouvering), since th rope could be paid out using a brake to control drum speed.

Would any full size Fowler enthusiast, ,or somone from Old Glory be able to throw more lightb onn the matter?

Bear in mind that what worked in full mscale, may not function quites mwell when scaled down.

Might be worth seeing what MERL have on record, looks like they have the Firth Valve gear for the BBS and some of the valve parts for the Z7S. You could overlay Hainings ontop of those and see how things compare.

The BBS drawing is the same as the one in Gilbert’s book but will be a lot bigger at 3″ to the foot.

It does seem that the basic radial gears are best for short valve travels: Walschaerts and Baker overcome the problem by adding the combination-lever.

The American builder, Case, successfully used a form of Hackworth gear, with straight link, on its traction-engines.

The Bremme version, using a swinging link in place of a sliding die-block, was effective on marine engines but those were intended to run at a fixed cut-off Ahead for days on end. Astern is normally only for short manoeuvres so an unsymmetrical action would not matter so much provided Ahead is efficient. The Marshall gear is similar to it.

All these gears work by constraining a pin-joint to an elliptical locus, and they normally give constant lead at all settings.

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As you suggest, Fowler may well have used this gear for its compact nature, and my book from their era hints at similar reasons for marine-engine use, where the crankshaft needed plenty of big main bearings. Such gears also use less metal and less joints.