Setting the valve timing.

Modelling the Kerr-Stuart Brazil Class 0-4-2ST locomotive in 16mm scale. - Keith Bucklitch

Setting the valve timing.


Assemble the valve gear. Bolt the trunnions to the frames.


The return crank should be at exactly 180 to the main crank. Screw the top end of the vibrating rod on the pin of the slider sufficiently to allow free movement of the die block in the slide. Insert the die block into the slider and mount the lower end of the vibrating rod on the return crank. A 10 BA nut will secure this. Lay the assembly to one side for at least 12 hours to allow the adhesive to cure before proceeding with the valve setting.


Connect the reach rod to the reversing arm and the reversing lever. Adjust the position of the reversing lever assembly so that the slides are perpendicular to the centre of the driving axle. Temporarily clamp in place to the frames. Remove the cover from the steam chest. Connect the valve lever to the vibrating rod (make sure the pivot screw does not foul the slide) and temporarily to the valve rod. Set the reversing lever in mid (vertical) position. Rotate the wheels in a forward direction and observe the movement of the valve. You need to set the valve travel to be equal in each direction related to the steam ports. Screw the valve rod in or out to fine tune the travel. Do this on each side. Move the reversing lever backwards to the forward travel position. Rotate the wheels forwards and observe the valve movements. Watch carefully and stop the rotation just as the edge of the front steamport can be seen. Note the position of the cranks. Hopefully the piston will be on or just past front dead centre. Continue the rotation of the wheel and note when the rear port just begins to show. Again note the crank position. If this is on rear dead centre, the valve should be set correctly. Repeat for the cylinder on the other side.


Set the reversing lever to the reverse position (slide the lever forwards). Rotate the wheels in reverse and note when the ports are just opening. Note the crank position. If all is well, both ports will open just on or past dead centre. What can you do if they don't? Try to increase/decrease the angle of tilt of the slide by altering the pivot point of the reach rod. If valve travel is unequal then the position of the reversing lever may need to be adjusted. If all else fails, check the dimensions of a) the return crank, b) the vibrating rod, c) the valve. You may need to remake any of these components.


When you have successfully set the timing it is time to try the chassis on air.


Attach the reach rod to the reversing lever and the reversing arm on the weighshaft. Set the reversing lever to mid position an adjust the position of the stand so that the slides are perpendicular to the centre of the rear axle. Clamp the reversing stand to the frames in this position. Connect up the steam pipes. Squirt some light steam oil into the steam pipe. Attach a source of compressed air. If you do not have a compressor, then a foot pump may work, preferably operated by someone else. Alternatively, take your spare tyre down to the garage and pump it up to 40 psi. Make up a connector to fit the tyre valve and link it to the steam pipe with some rubber/silicon tube and a suitable connector. You may also need some means of controlling the air flow in order to adjust the engine speed. Carefully move the reversing lever backwards. If you are very lucky, the wheels will start to rotate in a forwards direction. More likely what will happen is that you may need to turn the wheels and will detect a kick from one side or the other as you do so. Try the lever in the reverse position to see if the wheels run backwards. If nothing happens in either direction, then disconnect the steam tee, and connect the air supply to each engine (i.e. one cylinder) separately in turn. Again move the reversing lever and note if the engine runs in any direction. Note whether it runs forwards or backwards and adjust the valve rod backwards or forwards a half turn at a time. (Remember the position of the valve lever can also be critical, so make sure that this is not moving out of position.) Eventually you should begin to get signs of life from one engine and (assuming that it is not too tight) it should begin to run in either direction. Repeat with the other side of the loco, and when both sides are running, albeit jerkily then reconnect the steam tee and see if both side will now work. Note the forwards and reversing positions of the reversing lever, mark and file the notches for the latch.


If you can maintain a supply of air, then allow the loco to run in both directions for a couple of hours or more, applying oil frequently to all moving surfaces, and also into the steam pipes from time to time. As the chassis frees up so the starting of the loco should improve so that eventually it will start when you set the reversing lever and turn on the air supply.


Finally, don't forget to pump up your spare tyre once more before replacing it in the car!


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If you have any questions, or comments or find any errors in these notes please contact me by email. Email Keith Bucklitch.

Copyright © 1998, Keith Bucklitch
Last Edited - November 1998