"Catherine", a USA "Sweet Pea" engine build

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Benjamin Maggi
Posts: 1220
Joined: Wed Apr 25, 2007 10:38 pm
Location: Albany, NY

Re: "Catherine", a USA "Sweet Pea" engine build

Post by Benjamin Maggi » Fri Nov 08, 2019 9:02 am

As evidenced by the pictures, the crankpins hadn’t been secured to the wheels yet. However, it was time to start fitting the pieces of the valve gear together to see how it all worked. At this time it is all just dry fitting and there are still lots of variables that need to be accounted for but it is a start. So, the pieces were temporarily put together and clearances were checked. But, a critical piece still had to be made: the valve gear extension rods. These connect the slide valve stems to the vibrating levers which are on the crank pins. Everything is connected with forked ends and spindles and fitted pins, so it was a lot of easy work on the lathe and mill to make them all.

The forked ends were made from some ½” square CR1018 steel that I had on hand. Sad to say, but these are actually the third set that I had to make for the project. Where the fork attaches at the rear to the vibrating lever there is absolutely no clearance between the back of the fork and the front of the slides and the design of the pin which holds the fork together is that it slides in from the back and it retained with a nut or an e-clip on the front. The clip/nut prevents the pin from sliding backwards and fouling the slide, and the pin has a larger diameter (1/4”) through the back of the fork than the front (3/16”) so it can’t shift forward. A simple plan, but I messed up the first set by switching the diameters of the holes so that set was ruined. The second set was made because I purchased replacement steel that was not actually ½” square, and that threw dimensions off. (Lesson: measure the stock you are buying at the store!) I made sure to drill through the 3/16” hole before cutting the slot so that the hole would be nice and straight.
Roughing the fork ends.jpg
Roughing the fork ends.jpg (21.15 KiB) Viewed 140 times
Then, the slots were cut on the mill which was a little bit scary for me as I used a small mill cutter so I took it nice and slow. Those who work in ¾” scale are probably used to this type of thing. Once that was done, the ends were rounded on the belt sander and the one half of the fork’s hole was opened up to ¼” and then reamed nice and true. Finally, a curved profile was added to the front of the fork which made it look a little bit more elegant. Even on a basic quarry engine, the builders took some pride in their work.

The other end of the rod went into a fork that was a bit more basic in design. It was milled the same way, though the hole through it was a constant ¼” diameter and there is plenty of clearance on both sides for a hub on the rear and an e-clip on the front to retain the pin. The pins I used throughout were steel clevis pins that I cut to length, turned down in diameter where necessary, and slotted with a tool made from a portion of an old hacksaw blade.
Forks with pins assembled.jpg
Forks with pins assembled.jpg (20.94 KiB) Viewed 140 times
All the holes for the pins were reamed and the pins fit in nice and smooth with no slop. I was quite happy with the results.
"One cannot learn to swim without getting his feet wet." - Benjamin Maggi
- Building: 7.25" gauge "Sweet Pea" named "Catherine"

User avatar
Benjamin Maggi
Posts: 1220
Joined: Wed Apr 25, 2007 10:38 pm
Location: Albany, NY

Re: "Catherine", a USA "Sweet Pea" engine build

Post by Benjamin Maggi » Fri Nov 08, 2019 9:17 am

The design calls for the valve gear extension rods to have an outward bend in the middle, allows the rod to clear the motion plate bracket. However, because we had shimmed out our cylinders it wasn’t clear exactly how much clearance we would need. But, after centering the wheel/axles in the frame I measured out the distance of the vibrating lever and valve spindle and it was just about ¼” of variation. So, the rods which are just ¼” diameter steel were roughly cut to length and then marked out for the location of the bend, which is over a distance of about 3”. Then, the rod was put in a press with some steel plate and shims and the press was allowed to do its magic. Actually, we did a test rod first to make sure everything would work and it did. The rods needs just a slight adjustment at one part and that was that.
Bending the rod in the press.jpg
Bending the rod in the press.jpg (14.65 KiB) Viewed 137 times
One end of the rod, which connects to the valve spindle, then had the end piece welded on. It was important that this was square and also in line with the bends of the rod. My friend did the welding, and again it came out fine. The picture shows them before cleanup, but a wire brush and the belt sander made them shine like new.
Welded rod ends - before cleanup.jpg
Welded rod ends - before cleanup.jpg (18.9 KiB) Viewed 137 times
We left the rods over-length right now and didn't weld up the forked assembly on the other end that connects to the vibrating lever, as there are still too many variables left in the valve gear to figure out. Until the suspension springs are in place, the proper orientation of the slide blocks is set, and the valve gear is timed, etc. We will need to make a dummy set of adjustable valve gear extension rods before it is all over, but that should be easy. Until then, I put everything I had together and wrapped tape around the pin joints so that I wouldn't have to constantly add/remove the e-clips that hold it all together.
Valve gear temporarily pinned together.jpg
Valve gear temporarily pinned together.jpg (33.1 KiB) Viewed 137 times
"One cannot learn to swim without getting his feet wet." - Benjamin Maggi
- Building: 7.25" gauge "Sweet Pea" named "Catherine"

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