I sealed and bolted down the steam chest and steel insert, in place, leaving off the cover to get the timing set later on. Marking through the valve rod hole gave me the centre on the brass block for the tapped hole. On the other end I fitted the little crosshead.
The piston crossheads were next and I haven't made any
attempt to reproduce the shape of the castings. Instead, I milled out two blocks from a cast bar and
progressed from there.
The appearance is neat but not very decorative. However, the gudgeon pin has to be
countersunk for clearance on the inside, so on the outside I'll finish with a large chamfered washer and
nut. The photo doesn't show the threaded hole for the piston rod as I hadn't got the position then. I knew
theoretically that the centre is 1" below the underside of the slide bar, but its actual distance might be
a few thou out. This could make all the difference between an easy and a tight sliding fit. To get the
exact spot, I turned a pip on the end of the piston rod and used this as a centre marker. I would have
liked a cast iron or bronze cover, but settled for brass, which I have used before without any problems.
Anyway, it's easy to replace if wear occurs. I've yet to make the oiler pot to go in the centre.
After I'd threaded the crosshead, there was quite a fiddle to get all the pieces together. When I slid the crosshead along the bar it became very tight at one end. On checking with a vernier caliper, I found there was a 10 thou error in the slope. So, when I do the other side, I'll make sure that I fit the bracket after assembly and then hopefully, I shouldn't need to do any packing or skimming.
Having got the set up reasonably tight-spot free, it was time to do something about the
piston rings. Normally, I would get the rings first and then bore to size, but because of the pit problem,
I bored first, which was a good thing, since I had to go oversize. I had no luck in my search for motor
cycle rings as the size was awkward, so, nothing for it but to make my own.
As can be seen in the first photo, I've turned and
bored to size using cast iron bar. I've made rings like this before after reading an article in Engineering
in Miniature Volume 1, which makes it over twenty years old. The width and thickness of the rings is the
same and is 1/25th of the bore diameter which was about .95" Easy enough to make until the parting off stage, which needs a
bit of care.
The second photo shows how I make absolutely sure my tool is at centre height. My rule is
clamped to the workpiece with the new parting off tool and the square lined up to make sure it's upright.
I ground a very sharp tool with a narrow width (.45") for this job. Any jamming could ruin things.
Some rubbing down on fine emery to ensure sides were flat and then saw through with a fine saw. The hard
part is the tempering process. According to the article, a wedge needs to be forced in the cut to hold the
rings apart. The size given is the width x4, but this proved to be too big at this scale and would have
snapped the rings, so I settled for 1/4" instead of 3/8". The next instruction is to clamp them together
between two plates, bind them with iron wire, heat them to red heat and hold them there for 15 minutes and
then allow hours to cool slowly. This would have used a lot of gas and a big burner to do this.
I dispensed with the iron wire as I have a small kiln (used for lost wax casting), which I
used to bring up to temperature.(A piece of aluminium strip melts at about the right temperature.) It
took quite a while to reach the temperature and I let it cool down over night.
The following morning I took
it out of the oven and everything was flaking with oxide. Perhaps I should have used the iron wire and
covered it with soap as suggested. However, they came out in a bunch, stuck together. They easily parted,
but the oxide layer was hard and emery was slow work.
So I put it in my pickle (spirits of salt solution
- lavatory cleaner with hydrochloric acid base) and in ten minutes it had all fallen off. They buffed up
with an abrasive pad as in the photo. I did plenty of spares as recommended. One I was going to test
to destruction to see how brittle they were. I pulled one apart until the gap was 3/4" wide and it
didn't break and there was only a little distortion when released. So I fitted the first piston and tried
it in the cylinder and it seems OK. So theoretically, they should close up as far as the thickness of the
saw cut. I suppose if you really wanted to be particular, you could allow for this when turning the rings
and the gap would be virtually nil. Time will tell if the rings are good. I have got plenty of spares and
it was much cheaper than buying. It was less than a day's job to make them.
Now I have to finish the other side and what is taking a time is curing all the tight spots. You only have
to be a tiny bit out of line say, in the screw in bush and the whole thing seizes up. I decided to have a
break and make a start on the connecting rods.
This is made out of 1/2" material and takes a lot of hard
work. So to reduce that, I cut out the larger end from a piece of bar, bored it and then welded it to the
1/2" bar. Even with careful clamping, it was not perfectly in line and would have needed re-boring. And,
as there are six rods to make, I had them laser cut. Now all I have to do is mill the two remaining
sides to shape. They always look a bit rough at this stage and when this thick, the edges need quite a
lot of work on them. 12mm is probably getting near the limit of the cutter.