The Offy's lubrication system isn't trivial. Although I browsed its design while working on other parts of the engine, the last week was spent totally focused on it. Its complexity is difficult to convey in a few 2D parts drawings, and so I created a 3D model based upon information in the manual that I modified for the split crankcase.
The two pumps (pressure and scavenger) are machined into a stacked pair of 3/16" aluminum plates and topped off with a 1/16" thick cover. The pumps are mechanically coupled and sit between the crankshaft which drives them, and the water pump which is driven from their common shaft.
The pump assembly is located inside a recess in the front of the crankcase, immediately behind the front cover. Drilled passages in the floor of the recess behind the pumps carry oil from the bottom (pressure) pump to the main bearings. Waste oil that collects in the crankcase is drawn out by the upper (scavenger) pump and returned to an exterior oil tank. Compared with the original one-piece crankcase, the redesigned main bearings in the split crankcase greatly simplify oil distribution in the bottom end.
An interesting feature of Ron's crankcase oil returns is their different diameters. They're designed to equalize oil accumulation between the bearings and reduce chances of the scavenger drawing air. I carried this detail over to the split crankcase as well.
Getting oil to the engine's top end is considerably more complicated, and care will be needed to prevent leaks and pressure loss along its tortuous path. The top end will actually receive oil from the scavenger pump. A portion of the oil that would otherwise be returned to the tank is diverted to the top end through a needle valve located on the side of the engine. This oil, though, must be pumped across any clearance that exists between the pump assembly and the side wall of the crankcase recess that it's mounted in. So, this fit must be snug.
After crossing this boundary, the oil flows through a vertically drilled passage in the side of the crankcase and then into a horizontally milled trough underneath the gear tower. It then continues up between the gear tower and engine block inside a milled channel in the rear of the tower. Before crossing another boundary to reach the head, a portion is turned back into the tower in order to drip lubricate its gears and bearings. The remainder enters the head on its way through the cam blocks for distribution in the top end via the hollow camshafts. Top end waste oil will be returned to the crankcase through the four vertical tubes already installed in the block.
The 'snug-fit' requirement (among others) concerns me, and so I took a break from modeling to find out how much of an issue it might become since an o-ring seal in this location doesn't seem practical. I wanted to experimentally determine how close of a fit I might expect for the pump assembly inside my already machined crankcase. A thousandth clearance should allow the assembly to come in and out of the recess without damage to either and maybe keep leaking oil and pressure loss at manageable levels. Another issue, however, is that the rear bearing for the pump assembly will sit in a pocket in the bottom of this recess, and it too has already (and maybe prematurely) been machined.
The recess was part of the lower half crankcase machining done much earlier on my Tormach using CAM tool paths that I can reuse. However, effective cutter diameter and machine backlash can unpredictably affect the fit I'm hoping to achieve. In order to get the best possible result, I machined three trial pump blanks using the same tool paths and end mill that was used to machine the recess. I modified the CAM of two of the blanks by adding a thousandth to the outer perimeter of one and subtracting it from the other.
To verify the alignment, a bearing pocket was added to the rear of each trial blank using the same CAM and cutter that was used to machine the pocket in the crankcase. A dummy bearing was used between them during the fit checks. The CAM parameters for the trial blank that gave the best fit were saved for use later when the pumps are actually machined. Even though there was a perceptible gap between the two, it was small enough to prevent a .001" feeler from passing between them.
I've provided some renderings from the modeling. I made use of a SolidWorks feature that I recently discovered that permits an x-ray view behind an individual surface. The head and cam box models aren't yet finished, and so they aren't included. I also have some o-ring and gasket details to work out.
After a full week in front of the computer, I desperately needed to return to the shop and make an actual part. I was fairly certain that its design won't change, and so I machined the oil manifold that will eventually connect the engine to its oil tank. A hose on its top barb will supply oil to the pressure pump, and the bottom hose will return scavenged oil to the tank. - Terry
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Terry: My engine has been running for twenty years now and I have never had a problem with the lubricating system I designed. I pondered a long time on just how to do it. I admit it is a bit difficult to employ, but it has worked flawlessly. I am happy to say that you were able to understand exactly how it works. Using the scavenged oil to lube the upper end and making the flow adjustable with the needle valve on the side is probably a little unorthodox, but it all works perfectly. I came up with this design at the very end of construction. I left it till the very end because I knew it would be difficult to get it all to fit and actually work. I don’t know how it is done on the full sized engine. I had very little info from which to work with. It is what it is!
