Eccentric rods
IMPORTANT:- For the next loco, when drilling the centre hole in the main axle, thread the pip 3mm

Now then, for the eccentric rod length, here's how to do it:-

This only holds true for an "all square" setup and NOT for inclined cylinders

The crank pin offset from centre and the length of the centres on the return crank MUST make a right angle triangle so that when the wheel is at top and bottom dead centre, the distance from the return crank pin to the expansion link is the same otherwise the valve events will be asymmetrical.

So the offset of the pin in the return crank from the axle centre is an absolute and is calculated from the return crank length and the crank pin offset from axle centre making a Pythagorean right angle triangle.

Make a jig with a bit of half inch drilled with a 3mm tapped hole and a bit of 3mm threaded rod sticking out.
Screw that into the threaded hole in the main axle and with a good fitting bit of bar bored half inch for the jig bit and another hole the size of the return crank pin at the distance calculated by another Pythagorean right angle triangle - set the return crank.

Now that same bit of half inch is used to determine the distance between the centre of the hole in the expansion link when in neutral position (determined by sliding the radius rod up and down and imparting no movement to the valves or the expansion link) and the centre of the main axle with the axle at mid travel (ie running position).

So it's a piece of Pythagorean cake to calculate the length of the eccentric rod. The square root of the distance between the expansion link hole and axle centre squared plus the return crank offset squared.

So in actuality, the return crank and crank pin can be drilled BEFORE the wheels are put on the chassis, as long as the return cranks are clearly marked left and right and outside and inside. That will save a lot of hassle and broken drill bits!

Incidentally, the inspiration for this amazing discovery came to me after and evening drinking a particularly pleasant and peaty Malt - Glenfiddich Caoran Reserve, given to be by my mate Fred Kent from Oz. Normally, I'm a "Famous Grouse" man (if anyone is thinking of contributing, I only really go for the Spey-side whiskies), but that particular version of Glenfiddich is completely different and much better than the usual cooking variety.