[modeleng] Errors in Stan Bray's "Slim Sam" plans
- From: Jeff Dayman <jeffdayman@xxxxxxxxxxxx>
- To: <modeleng@xxxxxxxxxxxxx>
- Date: Thu, 6 Sep 2012 08:33:29 -0400
To Alan Stepney-
Alan, the notes below pertain to errors found in Stan Bray's Slim Sam engine
plans by Chris Stephens as discussed on the ME magazine web forum. He found
a few plan errors which I was surprised to hear about, since I had built one
of these engines for a nephew, loosely based on the Bray plans, and my
engine went together and worked fine. The notes below outline the
differences in what I built vs the plans, and my engine did work out OK, so
my notes could be interpreted as possible "fixes" for the plan's problems in
the errors section of your ME website.
Chris quote-"Jeff, first off the crank disc radius was greater than the
space between the bearing and the base, easily over come by reducing to an
appropriate dimension. Second, the ports in the upright are uncovered
because the cylinder base is too narrow, corrected by making it wider.
Another fault that i corrected was to make the piston longer, this reduced
the tendency for it to rock in the bore and try to lift the cylinder off the
upright. I also added a bush in the upright for the crankshaft which was
clearly shown in the photos but not in the plans. "
I visited my young relative yesterday and had a look at the engine I made
for him. Looks like I dodged a few bullets when I made it via freelancing.
My flywheel was 7/8" dia, rather than 1" as in the plans, so I didn't see
the interference to base that you had. The upright was made from a 3/4 x
3/4" bar scrap, as was the cylinder base so the ports are not uncovered as
they would be if cylinder base were per the plan at 1/2" wide, and the wide
stock did not require a bushing for the shaft as the thinner upright on the
plan would. Not sure how long I made the piston but I remember sizing it by
checking it in place in the bore at TDC and BDC and marking the crankpin
hole and skirt length directly off the engine, so it would have the longest
possible piston and still clear at TDC.
Just out of curiosity I went back to the plans and checked the ports layout.
The horizontal spacing of 1/4" is OK but there are two vertical dimensions
noted for the ports, 11/16" from the cylinder pivot, and in a side note,
17.5 mm radius! Both are wrong though if the 11/16" port to pivot dim on the
cylinder is used. The correct radius pivot to port on the cylinder for
perfect port alignment from my CAD layout is .701" or 17.8 mm, if the
upright ports are made to plans at 11/16" vertically from pivot and 1/4"
between ports horizontally. I'll forward these notes to Alan Stepney for
inclusion in the errors section of his website.
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