The Unofficial Mamod and Other Steam Forum

talluncle31 · Posted: Sat Jul 04, 2009 2:38 am Post subject: Weeden 643 mechanical restoration

Recently I knowingly bought a worn out Weeden Model 643 traction engine. This is the one with a piston valved fixed cylinder. Even with the drive chain removed the engine would not tick over. Usually I do my best to bring an engine back as near as I can to its original mechanical condition, but the cylinder was so poorly designed that I decided to put it back as Weeden should have made it in the first place.

Here’s what I found.

Due to the combination of 0.625” stroke and cylinder port spacing of 0.730” the piston had been made very short, only 0.090” long. It still half covered a port at front and rear dead centre, but this in itself was not a problem.

Because an excessively long cylinder had been fitted there was a total of 0.610” dead space length which had to be filled with steam at every revolution before any work was done. Even when new the steam consumption had been almost twice what it need be.

The cylinder back cover ‘gland’ had worn badly and at back dead centre this allowed the short piston to tip, so steam then blew past and went out of the exhaust without having done any work.

Here’s what I did.

I bushed the cylinder back cover and extended the bush out as close as possible to the crosshead’s most forward position. This gives better support and sealing.

I made a new piston over five times as long, 0.485”, to minimize the dead space in the cylinder. As the piston overlaps the ports I filed two flats on it, one at each end and facing the port, leaving 0.090” of full circumferential contact between the flats. The photo shows a similar piston to the new one used in this engine, the other is the original piston.

The original piston rod, which was 3mm diameter, could not be reused. I had no 3mm rod so the new one is 1/8” diameter.

I bushed the big end, crosshead and valve drive to take out the play.

If the engine had not run well after this work I was prepared to shorten the stroke so the full contact part of the piston could be more than 0.09” long. Fortunately the performance is now excellent and no further mechanical work is required.

Murray Wilson

rangerssteamtoys · Posted: Sat Jul 04, 2009 2:46 am Post subject:

I like what you did. It sounds like you made something good, even better, only this time, good was a pile O' crap

I can only think that something must have been replaced or modified over the years. That piston just doesnt look original, even if it was, the crank might have been modified also.

I would like to see a picture of the whole thing, if possible, a video

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IndianaRog · Posted: Sat Jul 04, 2009 11:49 am Post subject:

Murray, some superb detective and mechanical work there.

I wonder if the original piston might have been lead and broke off some time ago at the junction with the con rod. I have owned 3 Weedens that all used a "poured in place" lead piston where the con rod was simply set into the lead. With time the lead and con rod juncture corroded and snapped off. Ironically I made something like what you show as the "original" setup to get them going again. I used a section of brass rod filed to fit. They worked when I got done with them, but nothing to write home about!!!

You have probably made a retrofit better than anything that left the factory...congrats on saving a classic old TE!!!

Roger
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talluncle31 · Posted: Sat Jul 04, 2009 7:47 pm Post subject:

I don't think there can be much doubt about the old piston being the original, it is very similar to the pistons fitted to the No.1 locomotive.

Murray Wilson

Steve_S · Posted: Sat Jul 04, 2009 9:39 pm Post subject:

That sounds like a great job Murray.