Long Beach, California, USA
The engine lubrication "circuit" in BMC A-series engines is not described in any service manuals I have ever seen. For that reason, I thought it might be useful to document it. The following material is based on my recent rebuild of a 1275 cc MG Midget engine, but it's probably valid for other Sprite and Midget engines. Other BMC engines, while not identical, are probably similar to these.
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The circuit is shown in the pictures below. The pictures are of a bare block, and many of the parts (crankcase dip tube, external tube, pressure relief, oil filter, and so on) have been removed. Use your imagination.
1. Oil is collected from the sump by a dip tube, the end of which is surrounded by a screen. The screen is as coarse as ordinary window screening, so it is effective only to prevent the pickup of large objects (buffalo dung, dead squirrels, lost car keys) which might clog the oil pump. The oil intake is shown in the bottom left part of the picture, below.
2. Oil passes through channels in the block to the oil pump (bottom right).
3. Oil is pumped through a channel in the block to a fitting, sometimes called a banjo bolt, on the outside of the block. A second channel carries oil to a pressure relief valve, which consists of a bullet-shaped plug loaded by a spring. If the relief valve opens, some of the oil is returned to the sump through a channel in the block.
4. Oil continues through a tube on the outside of the block to the oil filter. Oil enters the outside of the filter element and flows to the inside.
5. A channel from the filter carries oil to a gallery on the right side of the engine. The filter mounting bracket has a bypass valve to allow oil to bypass a clogged filter. From here, it is distributed to the crankshaft main bearings. Note that the oil pressure gauge is connected to this gallery; so, precisely, the oil pressure indicator shows the gallery pressure.
6. From the gallery, oil passes to the main bearings through passages in the block. The main bearings have depressions in their centers, allowing oil to flow completely around the bearing and ooze uniformly to the outside. This keeps the main bearing uniformly lubricated. At the same time, it allows the oil in the first and second main bearings to flow past the bearing into passages that lead to the front two camshaft bearings. The rearmost camshaft bearing is in direct contact with oil passing through the pump, so it needs no other provisions for lubrication.
7. Oil passes through channels in the crankshaft to the connecting rod "big-end" bearings. The rod bearings do not have or need a central depression, as the hole in the crankshaft journal, which supplies oil to the bearing, is continually moving around the bearing surface. In this way fresh oil is applied to the entire bearing circumference every revolution of the crankshaft.
8. The piston wrist pins and cylinder walls are lubricated by "splash" lubrication. Oil emerging from the rod bearings is thrown all over the inside of the engine block, including the cylinder walls. This oil is also thrown up into the piston, where it lubricates the pins.
9. The timing gears and chain are similarly lubricated by splash lubrication. Oil passes through a hole in the front of the block, between the camshaft and main bearings.
10. A channel from the front camshaft bearing carries oil to the cylinder head. The camshaft journal has a groove that allows oil to flow past it, directly into this channel. The oil continues through a rather narrow passage to the frontmost rocker support, and through the support into the inside of the rocker shaft. Rockers are lubricated through oil holes in the shaft.
11. Some B-series engines have forged rocker arms with oil channels that carry oil to their pushrod and valve ends. Most A-series engines use pressed-steel rockers. A few use forged rockers, which may have similar passages; I don't know. In pressed-steel rockers, such passages are not possible, but anyone who removes the valve cover of a running A-series engine knows that quite a bit of oil is thrown around its interior anyway.
12. Oil that oozes from the narrow gap between the rockers and rocker shaft is returned to the sump via the pushrod openings, lubricating the lifters and camshaft lobes in the process. You can see channels from the valves to the pushrod openings in the rightmost picture above.
13. The block has a second oil gallery on the left side of the engine. Oil returning from the head, via the pushrod openings, enters this gallery, where it is distributed more uniformly to the lifters. Openings to the block allow excess oil to return easily to the sump when necessary; for example, when oil is added to the engine via the port in the valve cover. Some oil may also be splashed into these holes when the engine is running, providing further lubrication to the lifters.
The 948 engine (and perhaps the 1098) has an oil-pump priming plug on the left side of the engine near the rear. This is open to the oil intake passage. To prime the pump of a new engine, or if oil pressure does not come up immediately after an oil change, remove this plug and add oil. The 1275 engine does not have this opening, but the pump can be primed effectively by removing the banjo bolt on the right side and adding oil through that hole.
When a new or rebuilt engine is started, it is important to get oil into the system before any significant loads are placed on the bearings. To do this, first remove the spark plugs and prime the oil pump. Squirt a little oil into each cylinder, as well. Then, crank the starter until oil pressure is observed; this should take perhaps 10-20 seconds. If the bearings and camshaft lobes were coated with assembly lube as the engine was rebuilt, no bearing wear will occur. If oil pressure is not obtained, remove the oil filter and put a container under the filter mounting bracket. Crank the engine again and see if oil comes out of the bracket. If so, the pump is primed and working, and the problem is elsewhere in the system. Check the oil pressure gauge, the filter, and the bypass valve in the filter mounting bracket.
I have heard of individuals checking and priming the oil system, with the engine outside of the car, by inserting a screwdriver bit of an electric drill into the distributor opening and using it to spin up the engine. I have not tried this, but it might work if one is careful. This should be easiest before the timing chain is installed, so the crankshaft and flywheel do not have to rotate.
Correct bearing clearances are essential to maintaining the proper oil pressure, which is necessary, in turn, to support the crankshaft, connecting rod, camshaft, or rocker on a film of oil. The bearing clearances are selected to provide just the right flow resistance so pressure is maintained at a high enough value. This is critical to supporting the bearing loads. The operating pressure is a balancing act between the bearing sizes and clearances, and the oil pump's pressure vs. flow characteristic. Even one bearing with significantly overlarge clearance can drop the system oil pressure significantly.
It should be clear that flow of oil through the bearing is necessary to maintain a film of oil between the bearing and its mating journal. Without this flow, oil couldn't get into the bearing in the first place, as it could not flow through the oil circuit. Also, the maintenance of an oil film inside the bearing presumes some clearance between the bearing and its journal, which inevitably allows oil flow through the bearing.
I don't like saying this, but I suppose it's necessary, now that the lawyers have taken over American society. If you choose to do anything based on this information, but don't know enough about automobiles to be comfortable with it, get some help. I make no guarantees that this information is correct, so you take full responsibility for the results of anything you do that is based on it. This is just a report on my experience with these engines. It is not intended to be a set of instructions for duplicating my work or a recommendation to do it. You're on your own.
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