Notes for an imperial build of the pottymill horizontal engineSince I'm done with my build I thought I should record some notes on what changes I made for this engine for an imperial build. In general I continued to use lengths and widths in mm, adjusting somewhat for common barstock sizes. For example, for 6.3mm square barstock for the cross head slide bars, I used 1/4" since that is nearly identical. Similarly, I chose rod diameters that matched what I could purchase. As I chose parts to build, I tried to always look forward so I could bore a hole first using my imperial tools, and then turn whatever fits the hole as a later part. When operating my dials I had to convert mm to inches. I would write the conversions onto a paper copy of the print in case I needed to align parts later.
Here are some specifics that I jotted on my prints. There is a lot of info here so hopefully I didn't screw up. This isn't any fun at all to read, and you need the prints anyway, so hopefully if you are building this engine in imperial you can skim down and find the part and see if this jibes up with what you were planning to do. I think you'll find most of these are obvious choices. The main thing is that once you've made a choice you need to cross check what other part it impacts. I try to point that out for each part below.
Cylinder Front Cover: I replaced M8 with 5/16-24 threads and used a 3/16" piston rod so drilled accordingly. I used clearance holes for 4-40 screws.
Cylinder Back Cover: I used clearance holes for 4-40 screws.
Piston Rod Guide: M8 replaced with 5/16-24 threads. I used 3/8" hex brass as being approx 10mm.
Cylinder: M3 cover screws replaced with #4-40 screws. 4mm air ports used a #21 drill. 3mm air ports used a #31 drill. M2 valve box screws replaced with #2-56 screws. I turned larger diameter aluminum down to 32mm external diameter, but 1.25" should work fine (I didn't have any handy). I believe I bored the cylinder to 15/16", but I would bet 7/8" would work just fine, too.
Valve Chest: clearance holes for 2-56 screws to attach to the cylinder. M2 replaced by 2-56 for the valve rod guide. 3mm air ports were #31 drilled. 4mm input air port was #21 drilled. I tapped 3-48 screws for the air connection (2-56 would be fine, too). The main 6mm bore was drilled/reamed to 0.249" and I turned the valve piston to fit. The wider 7mm counterbore for valve guide stuffing box was drilled with a J bit.
Valve Chest End Closure: turned to fit the 0.249 valve chest bore. I got the right size by test fitting and made from 5/16 aluminum rod.
Air Connection: I used clearance holes for 3-48 screws. The 2.5mm air passage was made using a #39 drill. I drilled/tapped the input for 10-32 so I could use quick fittings from LMS that are this size.
Valve Guide Stuffing Box: I turned the outside to fit the J-drilled hole in the valve chest which is 0.276, but I did test fits to get it snug (but not press fit -- I could pry it out). I drilled the cup using a B drill (0.238).
Valve Rod Guide: I turned the 6mm end to fit the B-drilled hole in the cup side of the stuffing box. I drilled 2-56 clearance holes for screws.
Piston Valve: I turned it from 0.250" rod test fitting to my under-reamed 0.249 valve chest. I drilled/tapped 4-40 threads and made a 0.125" counterbore for a 0.125" valve rod. These are very tight measurements -- in my first attempt I recall the 4-40 weakened the part too much as I drilled too deep.
Piston Rod: I made this from 3/16" rod which matched the hole in the front cylinder cover. I threaded the piston end with 6-32 and the cross head end with 8-32.
Piston: I tapped 6-32 and counterbored 3/16" diameter for a good fit with the piston rod. I turned the piston to fit the cylinder by test fits. I got the packing groove a little deep on mine, but that may be better than too shallow.
Connecting rod: I turned this from 1/4 x 3/4" bar stock. I drilled the crank end to 5/16" and used a filing button to file the end to 1/2 inch diameter as the button was made from 1/2" rod. I drilled tapped the holes in the other end for 2-56 screws.
Little End Bearing: I made from 5/16 brass rod (fits the conrod) and drilled with a D bit (0.246).
Bearing Brasses and cap: I drilled #43 clearance holes for 2-56 screws to replace the 2.5mm holes. I assembled and drilled/reamed the 6mm through the brasses to an over-reamed 0.251"
Cross Head: I made a note that the overall length of the cross head is 19mm (14mm+R5) giving a shoulder of 3mm. I drilled/tapped M4 as 8-32 to fit the piston rod. I drilled the 6mm with a D drill (I had initially had B but scratched it out. B should work, but then the pin, thrust washer, and conrod bearing must match)
Cross Head Pin: I turned this down from 0.250 stock to 0.246 to fit the D-drilled hole in the cross head. I threaded the ends with 10-32, but in hindsight I'd recommend 8-32.
Slide Bar: Use 1/4" square bar stock. Drill/tap the M4 to match the cross head pin. I used 10-32 but would recommend 8-32 to fit the 1/4" square bar stock.
Thrust Washer: Use 3/8" brass rod and drill with D to fit the cross head pin.
Slide Bars: Use 1/4" square bar stock. I used 2-56 screws so made #43 clearance holes.
Slide Bar Spacers: Use 1/4" rod and drill #43 clearance holes for 2-56 screws.
Valve Rod Guide: Use 1/4" square bar stock. Drill #29 for extra clearance for the valve rod. Drill #43 for clearance for the 2-56 screw.
Slide Bar Pillars: I used 2-56 screws for attaching the valve guides. I used 4-40 screws for attaching to the base plate. I made them from 5/8 brass rod.
Valve Rod: Use 0.125" rod. Thread 4-40 on one end to fit the piston valve.
Eccentric Rod: Use 0.125" rod. Thread 4-40 on both ends.
Valve Rod Coupling: Turn from 1/4" rod. I threaded for a 6-32 set screw (much bigger than M2.5). Drill the 3mm hole as 0.125 to fit the piston rod. I turned and threaded the M3 end for 4-40 threads. I turned this to 0.125 (big for 4-40) and threaded only the end for 4-40. The threads stopped short so the eccentric rod coupling could freely hang on the unthreaded 0.125 section.
Eccentric Rod Coupling: Tapped for 4-40 to accept the eccentric rod threads. Drill 0.125 to fit on the valve rod coupling.
Eccentric Assembly: I used metric measurements except for the offset shaft hole which needed to match the crankshaft, and I tapped for a 6-32 set screw. Make "outer" parts first and turn each inner part to fit an already-made outer part. The offset is important for timing so I kept it at 3.5mm (0.138").
Bearing Stands: Use 5/16" x 1.5" bar stock. Drill the 14mm hole as 9/16.
Main Bearings: Turn 5/8 brass down to fit the bearing stands. I went for a press fit, though loctite is fine. I chose to drill the 8mm as 5/16. If you are making the crankshaft from parts, make this fit the rod you are using for the crankshaft. 5/16 is probably right and overreaming would be a good idea.
Crank Shaft Assembly: I turned this from a single chunk of bar stock and turned it to fit the main bearings. The crank needs to fit the conrod bearing brasses and 0.25" is about right.
Flywheel: I made mine from scratch from 5" diameter steel. The hub needs to fit the crank shaft, of course. I used a 6-32 set screw at an angle.
Base Assembly: I used 1/4 thick aluminum. I converted all the mm measurements to inches and wrote them on the print.
Hope that helps! It's a great engine. Build it and you'll see
Todd