The Unofficial Mamod and Other Steam Forum

dorsey · Forum Newbie Joined: 06 Oct 2009 Posts: 13

Yes, it is rust on the spring and also on the inside of the cap. I had a similar thought because there is no rust anywhere else.

Just before I wrote this response, I used a strong light and a magnifying headset to re-examine the spring. I saw what looked like 1/4" of pencil eraser stuck in the bottom of the spring, so I twisted the spring out and cleanly broke off what appears to be a rubber plug of some kind, so that might be the fill hole after all. Frankly, it has to be that because there isn't anywhere else to add water. It seems like an odd arrangement - maybe I'll learn more when I pick the remainder of the plug out of the hole, hopefully without any of the pieces falling into the boiler.

Is there another engine that uses this kind of filler/pressure relief valve that anyone is aware of? I can see how high pressure against the plug would overcome the spring's resistance, and the steam has plenty of space to vent once that happens. Or, this could be a naive repair of some kind because the original parts failed or were lost. I mentioned that the piston and connecting rod are missing, so that would be consistent.

I finally figured out how to post photos (duh), but my camera won't get the kind of close-up detail I need to show these parts.

dorsey · Forum Newbie Joined: 06 Oct 2009 Posts: 13

I made a piston and connecting rod assembly, so I only have to fabricate a replacement pressure relief valve and I can fire it up.

I made the piston out of 360 brass and the connecting rod from 5/32" drill rod. It's a beefy assembly, but I like the look. I threaded the piston to the rod using 5BA threads, so that 5/32" diameter left a little shoulder on the rod.

The piston has close to .001 clearance in the cylinder and the connecting rod has .003 TIR runout. I made the journal bearing .005" larger than the 1/8" crankshaft journal diameter to allow for misalignment. Note also the two oil grooves in the piston. They're large enough to fill with graphite string packing if needed, but I think the cylinder oil will be sufficient.

I found that the bearing plate against which the cylinder oscillates (don't know its proper name) was not aligned perpendicular to the crankshaft, so I also tweaked that to within .001". It should seal pretty well now and I probably could have tightened up the rod bearing clearance had I done that first.

There's light, but perceptible, drag when I rotate the assembly by hand, but I think that will disappear after some running.

Bugsy · Posted: Mon Oct 26, 2009 7:04 pm Post subject:

Great job, Dorsey!
I'm looking forward to seeing it running.

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Steve_S · Posted: Mon Oct 26, 2009 10:26 pm Post subject:

Yes, great job! It's a very interesting engine... it's always good when something a little unusual comes along.

Burnmafingers · Posted: Mon Oct 26, 2009 11:32 pm Post subject:

Spanish/Portugese engine?
Welcome to the site Dorsey.

tmuir · Posted: Mon Oct 26, 2009 11:46 pm Post subject:

Cool, I look forward to seeing it running.
I always like seeing old 'dead' engine being brought back to life, especially ones that are a bit different.
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A nice example of an Australian made Scorpion Donkey Engine

dorsey · Forum Newbie Joined: 06 Oct 2009 Posts: 13

I'm going to take a little poetic license on the relief valve, and will post photos when it's complete. Thanks for the encouragement.

I almost passed on this because of the missing parts. About five nanoseconds later I realized how dumb that would be and snapped it up.

dorsey · Forum Newbie Joined: 06 Oct 2009 Posts: 13

I couldn't wait to complete the relief valve, so I ran it on compressed air and it spun like a top! Very encouraging.

Based on old pieces I removed, I think that the original valve used a mushroom-shaped piece of rubber and the existing spring, but that could have been a replacement for original parts that were lost. I have no way of knowing. Here's a shot of the relief valve components I made:

I bought the o-ring at a hardware store for 79 cents, and a 6-32 brass round head machine screw for 25 cents. Using a lathe, I filed down the screw head so that it fits into the boiler, and also filed off the threads below the screw head so the o-ring will fit better and stay in place when the screw is removed.

I machined the "holder" from alloy 360 brass to fit snugly inside the spring and apply pressure to the o-ring against the boiler. The screw aligns the assembly to the boiler and holds the o-ring in place. Here's a shot of the finished assembly:

The next photo includes a ruler to give the relative size:

I'm ready to rock and tool now! Unless I get other suggestions, I'm going to use denatured alcohol as fuel. When I get it to run a steady speed, I'll report the actual RPM.

Lozza1 · Posted: Fri Oct 30, 2009 5:32 am Post subject:

Oh well done Dorsey

That looks like a better engineering solution than the original, and not a distructive change iether

Looking forward to a video

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dorsey · Forum Newbie Joined: 06 Oct 2009 Posts: 13

Well, I finally fired it up and it runs great. I can't get a speed reading because the engine produces so little torque that my Starrett mechanical rev counter causes it to stall. I tried taking a movie of the engine running so I could post it here, but like most such videos, the moving parts are a blur, and show that syncopated, "stop frame"-like motion.

As has been pointed out earlier in this thread, there are a couple of basic design flaws. Because of the top flue sheet, the filler had to be on the side opposite the cylinder. If I'm not careful, when I tip the engine over to fill it, water runs out the high pressure tube. I developed a clever way to grasp it with my left hand while filling it with my right so that doesn't happen, but it's not obvious. Also, there is no speed control other than the relief valve. Finally, I had to add an inelegant cork to plug the fuel filler so the alcohol won't evaporate too quickly.

This was a very satisfying project with a happy ending. I'll keep looking for more such opportunities and I'm happy to have found this forum, from which I've learned much. I'll certainly keep coming back for more.

Maybe someday someone else will stumble here and enlighten me/us about the details and history of Finotti Umberto steam engines.