# Double Scotch



## SignalFailure (May 13, 2008)

Here's my take on 'Double Scotch' (see this page http://homemetalshopclub.org/projects/scotchx2/scotchx2.html for the original), a beefed-up version of Elmer's 'Scotty' (my version of that engine is visible in the video for comparison)

These are the things I changed:

I converted it all to metric to suit my limited gear
The base and frame are mild steel and the cylinders are cast iron
The main yoke is simplified to avoid lots of filing  
I added a large thin washer to fit between the eccentric part of the flywheel and the main bearing
I fitted a brass tube inside the valve's passage
The main yoke is silver soldered to the piston bar rather than using screws
I used brass for the valve yoke rather than aluminium
The valve discs were loctite'd on rather than soldered

There are probably a few others but they don't spring to mind!  The finished engine is just shy of 2lb in weight!!

For anyone planning to build one of these, these notes might help...

I changed the materials because I don't have any aluminium stock and because I don't own a milling machine (aluminium is less tolerant of the sort of errors this tyro makes with saws and files). 

The passage for the valve was a nightmare! The designer suggests drilling carefully from opposite ends but that doesn't sound like it will give an accurate result - I used a long series 6mm drill and lined the hole with a brass tube (inside dia 5mm).

The plans call for cylinders with blind bores but I found it easier to bore straight through then fit some simple brass covers (1.5mm 'step' to locate them then loctite holds and seals them fine).

I made a freehand flywheel (no measurements) of about the right diameter and secured it to the crankshaft with a socket headed screw that bears on a piece of steel rod (as per Elmers 'Scotty') - it's quick and easy to do and it works.

I decided to solder the main yoke to the piston rod as I don't own any tapping gear sub 2.5mm. The rod and yoke could be thinned substantially which might improve performance?

Due to an errant hacksaw I ended up with less substance above the valve passage than was required so the air/steam inlet is a loctite'd push fit rather than threaded.

The valve yoke has a huge kick (i.e. it tilts as far as it can on each stroke) so I made a (roughly 1/2" dia, 1/16" thick) bronze washer to fit between the meain bearing and the eccentric (which is just part of the flywheel) to reduce the kick.

So far the valve yoke tends to 'ping' off after a couple of minutes running - it's only held on by tension; too much and the engine won't turn over, too little and it just falls off. I suppose it will improve.

IMHO (and it is humble compared to some of the genius on here and elsewhere!) the main issue with this engine is the valve arrangement - if you plan to build one I would try to come up with a better system.

I didn't like using as much loctite as I did for this engine but on the valve rod I had no choice - one the discs were to size for the ends of the tube I couldn't push them right through to the opposite side (commercial tube must have had some minor variation in inside dia), so I had to make one pair, push the rod through then glue on the other pair.

It's all a learning exercise I guess 


[youtube=425,350]olJM1z1mEqs[/youtube]


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## DickDastardly40 (May 14, 2008)

Success!

Well done mate, seems to run really well. What pressure is that at, I assume still the aquarium pump?

Congratulations.

Al


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## Brass_Machine (May 14, 2008)

That has a mesmerizing movement. Cool!

Eric


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## SignalFailure (May 14, 2008)

DickDastardly40  said:
			
		

> Well done mate, seems to run really well. What pressure is that at, I assume still the aquarium pump?



Cheers mate. Yep, it's on the aquarium pump which is rated about 5 psi. After an hour or two of running it now starts with the tiniest tweak and runs pretty much non-stop. I fixed the falling-off-yoke by bending the ends outwards a tiny fraction to increase the tension (they should be at 90 deg but, because the length after bending was about 0.5mm too long, they were slightly less than that so now they're closer to where they should be  )

Thanks Eric, it is quite interesting to watch.


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## BobWarfield (May 14, 2008)

That's cool!

I hadn't seen the Scotch style before.

Best,

BW


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## SignalFailure (May 14, 2008)

Thanks Bob. I've changed the video - it's a bit longer now and includes a commentary (sorry about my soporific style!).


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