The Craftmans Shop > Model Engineering

Rockerblock engine--something a little different--

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Brian Rupnow:
Okay--Back to business. Immediately after I had machined the gears my phone rang about 5 times in a row, each time with a customer I couldn't turn down, and each customer wanted to be looked after "right now". Being a slave to my bank balance, I said yes to all of them, and haven't had time to play "small engine" since. I have discovered one thing, and I'm not sure yet whether it is really a problem or not. When I make built up crankshafts from 3/8" diameter stock, I ream the holes which I am pressing shafts into with a 0.3735" reamer. I use 3/8" drill rod for the shafts, which always seems to come in at .0005" oversize. This gives a VERY hard press fit, with .002" interference, and I have never had one "slip" after the fact. I don't use 3/8" cold rolled steel because it comes in at about .0005 to .0001" undersize, and though it does give a press fit, it is quite a light press fit, and it WILL slip after the fact. Last year I purchased a 0.4985" diameter reamer in case I wanted to make a crankshaft using 1/2" nominal shafting. The problem is, that I just stopped at my metal supplier today  and micrometer measured all of their "01" drill rod, and it all comes in exactly at 0.500" diameter. So--I will only achieve a .0015" interference fit. I have to think on this a bit before I proceed. ---Brian

Brian Rupnow:
What to do--What to do??--Well, we'll make a little test. I drilled and reamed a 0.4985 hole in a piece of 3/8" mild steel, and I turned a very slight "lead" on the end of a piece of 1/2" drill rod, and I pressed it to see what happens. This type of pressing can not be done on a manual arbor press. It gets done in my vice. This was tight, but not "Oh my God, I've just given myself a hernia" tight. (That's the way it is with a .002" interference.) So, to farther quantify the results (We're getting really scientific here), I put the piece of plate in the vice, clamped my vice grips on the 1/2" round stock, and gave it the old "Reef your guts out" test.--And it slipped---Just like I thought might happen. The fit is tight enough to withstand moderate abuse, but not exceptional abuse such as backfire, pre-ignition, or nuclear Armageddon. I do however, think it would work fine if the components were cross drilled and pinned. Probably with .094" (3/32") dowel pins.

Brian Rupnow:
I've been thinkin', Lincoln---if I do get the flywheels waterjet cut, why not do something fancy? so, I got my old Philip Duclos  book out and using the information in it, created curved spoke flywheels. I think it adds a lot of "pizazz" to the overall engine.

Brian Rupnow:
I just ordered Jerry Howells gas valve plans and components to run this engine on propane, and ditched the gas tank. There is just too much beautiful "monkey motion" in those long rocker arms to cover them up with a gas tank, in my opinion. Also, if I do get the water-jet cut flywheels, there will be a relief machined in both sides to accentuate the spoked area.

Brian Rupnow:
Running the engine on propane is not going to let me mix a little two cycle oil with the gas to keep the viton o-ring lubricated. The simple fix for this is to add a small oil cup with a very small "dribble hole" of about 0.030" diameter to the rear of the cylinder, tucked up tight against the back of the cooling water reservoir.  This will let enough oil onto the piston skirt with each revolution of the crankshaft to carry oil thru the rest of the piston stroke and spread it throughout the cylinder while the engine is running. This was very common on the old hit and miss engines.

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