4-cylinder compound de Bousquet-de Glehn Nord 3500

the Acme of steam locomotive design art, epitomised in Photo 1:

1

While making replacement fake valve gear for the 2-cylinder simple, slip-eccentric, version (pictured on page 9 of NL&J 264) it had become more & more apparent that, using high-carbon steel, it was quite robust enough to drive a real valve.

An irresistible slippery slope led to photo 2:

2

At 1/32nd. size the gear gives a valve travel of a full 4mm which allows for quite practical port & valve proportions, as per photo 3, running well on air.

3

Thus encouraged, the low pressure engine was made using the same geometry, the difficulty of erecting the motion between the frames being overcome by pre-assembling the expansion links & radius rods etc. in a "box", photo 4.

4

This is then slid between the frames and connected to the eccentric rods, photo 5.

5

The box is secured by just one screw each side, held in alignment by the slidebar rings locating on to the cylinders' back covers, same as for the HP set-up, photo 6:

6

Crossheads are assembled from two halves, photo 7 . . .

7

. . . then pistons & rods screwed in from the front. All glands & piston seals are Viton O-rings. Photo 8 is a view from below of progress to date:

8

Photo 9 and the final Video show it all turning over on compound air; the stainless steel pipework is a reheater.