Re: Stanley Throttle Rebuilding
Posted by: Mark Drake
Date: March 13, 2013 09:57AM
It’s a bit of a lengthy one this, but throttles can be a right pain if not done correctly, and I’ve had to learn the hard way!
The throttle on my 1922 740 was in a truly shocking state when I got the car; poppet, barrel, spindle and body were all in a bad way. I ended up having to make a new barrel and spindle as they had both come to the end of their useful life, the actual poppet could be re-faced. I decided on stainless 304 for both, as it machines nicely and conveniently has a coefficient of thermal expansion very slightly less than the bronze body. This is important otherwise the barrel could jam in the body when hot.
Unfortunately the valve bore in the body was in terrible condition as was the valve face so I also decided to rebore and re-face and alter the poppet length accordingly to make up the lost length. Here’s the sequence:
1. Mount body on aluminium mandrel in lathe which has been turned to just fit in the valve bore.
2. Lightly skim the outside section of the body (the narrower part). This now provides a nice long truly round outside diameter which is reasonably true to the existing bore.
3. Mount body in the 4 jaw chuck, gripping on the fresh OD and clock up on the same diameter. Hopefully the bore and face can be seen with minimal runout.
4. With a fine feed, small boring bar and carbide bit, I gently bored out the valve bore until it just cleaned up. I think it took about 0.050” to give a satisfactory surface.
5. With the same boring set up, I cleaned up the poppet face in the same way, and then machined a 45 degree (I think) chamfer between the bore and face. This chamfer face is small – only about 0.030” – 0.035” long.
6. The barrel and spindle are very much copies of the originals, except that I changed the length of spindle between barrel and poppet to match the new position of the poppet face in the body. Another change was that I increased the size of the lead-in cut out (in the barrel port hole), I used a 1/16” slot drill and made the cut out about 1/16” long. Make sure barrel OD and poppet seal face are finish machined in the same chucking and with the spindle running clock-true, and the same angle on the poppet as the seat.
7. Attach the barrel to the spindle by cross drilling 1/16”, very lightly countersink both ends of the hole and then press in a stainless pin and upset the ends into the countersinks. Lock the poppet on in a similar way.
The fit of the barrel in the body – When I made it I reasoned that the fit should be in the range 0.0005” to 0.001” clearance. Now that it has been in service for a couple of years I would say that this is correct, I don’t get any tightening of the throttle in service, nor has it ‘picked up’ with the bore.
If you increase the clearance too much for an ‘easier throttle’, you will find that the throttle is a devil to open from fully closed. The so-called balanced throttle relies on the poppet leaking a tiny amount allowing some pressure to build up in the steam trapped in the temporary chamber between the poppet and barrel valve. This reduces the force required to start opening the throttle, making the throttle more user friendly.
Refacing the seat (assuming that you’ve re-machined as descried above) – with great care you can use fine grinding paste, but on the poppet face only!!! Take great care and this is possible – don’t push too hard or the carborundum will embed in the bronze. Actually – for well machined, fresh parts I don’t think that any refacing at all is needed, or maybe a bit of working together dry. It doesn’t take much road running to get it to settle down.
Finally (!) – Notice that I suggest a 0.030 – 0.035” long poppet face (in the body only). This will reduce the number of occasions that a particle of scale trapped in the poppet when closing will create a permanent leak across the face. If the valve bore and face came to a 90 degree corner without a chamfered section, then any scale or debris will damage the poppet and it will always leak until attended to. When I first rebuilt the throttle this is exactly what happened.
Finally finally (!!) - when re-installing the throttle, ensure that the port in the barrel valve aligns nicely with the steam way in the body and that the poppet closes tight just before the throttle reaches the stop. Best way is to use a small inspection mirror to see what's happening in the steam way before re-attaching the steam pipe. Stanleys used to mark the spindle with three countersinks at the opposite end, aligned with the port in the barrel to help with the line up.
Hope this all helps,