15 months with a Kylchap - July 1935 - October 1936 by this text - 'LNER Ecyclopaedia'.
Looks better IMhO, 'though, after rebuild one, or two ... the single blastpipe diameter was reduced and outlet lowered to flush - not sticking out .... looked interesting, and somehow 'streamlined' with the modified single.
Another of Chapelon's recommendations was the addition of a Kylchap (Kylala-Chapelon) exhaust arrangement. In 1934, the P2 Cock o'the North appeared. This was the first LNER locomotive with the Kylchap arrangement, and in 1935 Gresley ordered No. 10000 to have the same arrangement fitted. Due to the unconventional nature of No. 10000, a number of trials were performed with different variations in the Kylchap arrangement. No. 10000 ran through May and early-June 1935, testing final adjustments to these different variations. After these adjustments were completed, it re-entered Darlington on 14th June for a hood smoke-lifter to be fitted. This was a cowl which bridged the gap between the two wings of the casing around the chimney, and was intended to increase the smoke clearance. No. 10000 re-entered service on 10th July 1935.
The End of the Water-Tube Boiler
W1 No. 10000 ('Hush-Hush') entered Darlington Works again on 21st August 1935 for further repairs, having run about 90,000 miles since new. A number of further modifications were being considered, when Gresley ordered all further work to stop whilst he considered a scheme to rebuild it with a conventional fire-tube boiler.
On 13th October 1936, the water-tube boiler W1 made its last journey: from Darlington Works to Doncaster Works to be rebuilt with a conventional fire-tube boiler. Out of the 1,888 days since it was built, No. 10000 spent 1,105 days in Darlington Works.
Al.