Scott
To answer your last question the distance between the bearing inner faces and the crank webs should be the same plus a small clearance. On my engine, built in the late '70s, I'd estimate that it is a couple of thou. On the plans that came with my casting set there are some build notes and with reference to this it states "The shaft should turn without binding in the bearings but with practically no end play".

In my book, which is "Building a Steam Engine from Castings" by Edgar Westbury, not Andrew Smith, the crank webs are shown as 5/8" wide the same as the drawings. Check the distance between the mounting hole for the bearings in the base casting and the distance between the mounting holes in the bearings and the inner faces of the bearings. They should be the same as the width of the crank web plus a few thou. On my drawings, and in the book, the distance between the holes in the base casting is 1 1/4" and the distance from the mounting hole in the bearing to the inner face is 5/16" so when mounted on the base casting the gap between the inner bearing faces is 5/8", same as the crank width. Obviously there also has to be a small clearance as stated above so the crank can turn freely.
Check these dimensions on your plans, an in the book, and see how they add up.

I'm not sure this has answered your question but I hope it helps.
John

Edited By John Purdy on 21/04/2023 18:44:54

Scott welcome to the world of Model Engineering where every set of drawings has errors, no matter how old they are they will not have been corrected.

No, as John hinted at they face the inside and leave a 5/8" gap for the crankshaft.

If you look at the soleplate drawing it shows the stud holes at 1 1 /4" ctrs

Middle section of bearing is 5/16" so middle to edge is 5/32"

Protruding round section also 5/52"

So when assembled you have 5/32 + 5/32 + 5/8 = 5/32 + 5/32 = 1 1/4

Edited By JasonB on 22/04/2023 07:42:54

The 10V is a nice engine. It’s a well documented build and it will teach you quite a lot about fitment. Do you plan to add the reversing? It can always be added later, but there is less repeated work if it is done from the start.

Best of luck with the build.

My figures of 0.098" give approx 5% compression of the nominal 0.103" section ring I suggested. That is based on the Arnold Throp calculations that can be found in Model Engineer's Handbook and paper versions of Reeves catalogue.

Hi Scott, If you intend to use an O ring then the finish of the cylinder bore is very important if you want to run the engine. Using a small cylinder hone is one way to get a good finish though there are other ways This then means leaving the cylinder bore slightly undersize, say .025mm, then you hone or finish to size. .I only mention this as your comment above indicates you may not be from an engineering background Good luck. Noel.

With out getting out some books, O rings are available in both odd imperial sections and also metric sections so that give or take one can normally find a ring to do the job. Noel.

Ah! One must first determine what "the job" is! The desired result is a smooth moving piston with minimal friction that seals. To do that you must know what ACTUAL dimensions you are dealing with. I.E. ACTUAL bore diameter, ACTUAL O-ring X-section diameter, and desired O-ring compression I have found that 5% compression is the upper limit of compression. If it were me, I would shoot for a for no more than .005" total compression. For example: Assume a nominal bore diameter of .750". Assume actual bore diameter = .753". Assume nominal 3/4" x 1/16" (.0625&quot 0-ring will be used. Actual measured x-section = .071". Desired 0-ring groove diameter would be .753" – 2x.071" + .005" = .616". If you noticed, There is no need to know the piston diameter as it obviously must move freely. The o-ring groove depth has no meaningful relation ship to the piston diameter. Groove width is not so critical. It needs a bit of side clearance. Using published O-ring compression numbers will most likely result in far too much drag. Rick

Probably be lower down the list than mild steel, why not just get some stainless steel, 303 is easy enough to work with.