old (large!) compressor oil level question

I bought a used compressor and intend to change out the oil and install a HTF auto tank drain on it. The air pump does not have a dipstick or sight glass on it. Since the oil bung was in a tough location, most of the old oil ended up on the floor and could not be replaced with a similar volume. The pump is a Vee twin or four cylinder (thought is was a two, but the heads have a "8"-ish shape to them.. Pics are posted on alt.binaries.pictures.woodworking). Can anyone recommend an oil level relative to the crank or other point? Also, where can I learn more about this machine.. The 60?gal tank has a welded tag identifying it a 1956 John Wood of Conshohocken Penna. The motor is a 3 hp single phase AJAX, and this bad boy weighs in at about 400#.
many thanks!
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Only 400#'s??? No dip stick, or sight glass? Could the sight glass have been painted over? Is the fill hole vertical or does it face up? Typically vertical/fill locations are filled until the oil begins to pour out.
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The fill is top dead center between the cylinders. Mike hit it right on the nose. There is a painted-over sight glass behind the ball valve in the picture (held in by six really good sized bolts!). A little lacquer thinner cleared her right up. There is no gradation on it, so I'd presume that center of the sight glass is ideal? That would put the oil level even with the lower edge of the crankshaft diameter.
I'm guessing at the 400# number... I loaded it into a tilt trailer with a winch and took it out with a backhoe. Good thing about a bad back, you don't try the impossible anymore:)
Thanks gents, any more advice about the care and feeding of these old brutes? Thanks!

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Not a 4 cylinder - 2. Can't tell if it's a single stage or a two stage, but with what looks like both cylinders being the same size and the manner in which it's plumbed (air intakes), I'd guess two cylinder single stage. It looks like there is a site window behind the green valve handle. That round thing. It may be painted over by the looks of it. See if you can clean some of the blue paint off of the center of it. I'll bet you find a site window under there.
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I'm curious Mike, what leads you to say not a 4 cyl compressor?
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Leon wrote:

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Gerald Ross
Cochran, GA
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I doubt that but that seems as likely as a single cylinder and piston under each head. I'm not an expert buy have replaced the reed valves on a compressor and it is quite common for a rectangular exterior cylender to have 2 cylenders. I was wondering if Mile had some kind of knowledge of there being retangular cylendars with 1 piston and cylinder in each. This being an old compressor I guess there could be the possibility that there are a couple of actual valves in the cylinder that would occupy the rest of the space similar to old lawn mower engines, flat head V-8s.

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The intake and exhaust look to be just one each per side.
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I'll accept that. I might have jumped the gun with my original response. I can easily see a reed valve arrangement working.
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To be fair, there was nothing inherently wrong with your assumption. I knew that my compressor was a double cylinder in a single bank but until I pulled the head and reed valve plate off it was difficult to understand how all that worked in there. Not totally unlike some 2 stroke engine where the intake air path is not as direct as one might think.
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