Technical

As the piston seal and the stuffing box seal age they become hard and do not seal as well. Tightening the stuffing box screw with the special tool either goes hard and stop the loco working making wheels hard to turn or slackening the screw causes loss of performance especially at slow speed.

The main test of these seals is the performance at low speed. If poor loco will not crawl smoothly but will cog along. this is caused by the piston going forward loosing its steam via the stuffing box seal or by-passing the piston before it can slowly expand pushing the piston forward. At high speed there in momentum and the steam also does its job expanding and pushing the piston before too much steam is lost.

There will also be an issue with the steam by-passing the piston seal with the piston going backwards but the main problem of the stuffing box seal is not there. In practice the stuffing box seal usually fails sooner than the main piston seal.

Indicators of the piston main seal failing are seal from the chimney at heat-up. starting the loco will stop this as then hot steam heats up the cylinders and makes the main piston seal seal up. I have found the stuffing box seal does not seal up so well and it will not once it goes hard with age.

I would also mention that changing my piston and stuffing box seals with Hornby seals on my A4 greatly improved the very slow running but I then changed for the OO Steam Club VITON seals and the improvement was remarkable. Loco is smooth at about a turn of the driving wheels every 2 seconds or more. The steam just very slowly expands as the piston goes forward with no loss perceivable.

For the stuffing box seals on NEW seals I tighten up the stuffing box seal screw with the special tool from the steam Club and then back off 1/4 of a turn. Steam a couple of times and you have perfection. Please note VITON is massively superior to the Hornby spare seals which are now getting old on shelf life. Remember NOT to touch VITON if it gets burnt as it produces Hydroflouric Acid when brought into contact with water (ie skin moisture), possibley the most dangerous acid to humans. You do not feel it burning until too late then amputation. Max temp I think is 260 deg C. Read the safety sheets. OK in operation for out 130 deg C.

Hope this helps

Martin