|The MGA With An Attitude
OIL FLOW RESTRICTORS - CS-101
in the Crankshaft Rod Journals
At 09:34 PM 3/19/04 -0500, Tony Cox wrote:
>"I have just had the crank reground. During the process a brass plug with a hole in it came out of one of the connecting rod journals. These parts are not available but do show on the original factory parts books. Any idea why they are there and are they absolutely necessary."
The rod journals in the MGA crankshaft are cross drilled, so there are two holes coming out of opposite sides of the journal. At high speed the centrifugal force wants to push most of the oil out through the outside hole, while the hole closer to the central axis of the crankshaft would pass very little oil. The brass part is a small orifice intended to restrict flow through the outer hole to force a more equal amount of oil to flow through the hole closer to the central rotational axis.
I have never known exactly how important it is, but I have always been conservative enough to replace any missing part. The brass bits are not difficult to make. Engine shops are always making press fit brass plugs to close off oil gallery drillings in the block. The only difference here is that the plug has a small hole drilled through it. Make the plug 0.001" to 0.002" larger than the hole to be a tight press fit. Tap it into place using a punch to drive it below the surface. Use various drill bits to gauge the size of the hole in one of the other restrictors. The hole in the new part can be drilled before installation, but should also be chased out after installation (in case a burr was raised in the process which would decrease the bore or the restrictor).
I once had one of those restrictors come loose while the engine was running. I have no idea how long it had been loose, but it was quite evident when the engine was later disassembled (for some other reason). The brass part had the end worn off to conform to the contour of the bearing, and the bearing had a groove worn in it all the way around equal to the width of the restrictor, through the white metal but not through the copper underplating. The rest of the surface of the bearing was still in good condition, and there was no damage to the journal on the crankshaft.
These engines are generally quite tolerant of abuse. I have know them to occasionally be assembled with no restrictors in the crankshaft, and they don't seem to suffer any ill effects. My best guess is that it is more important when the engine gets old and worn with larger clearances in the bearings. But I never let the rod bearings get very loose anyway. Whan an engine gets down to 20 psi oil pressure at idle I install new bearings (well before it would knock), and all is well with the world.