The MGA With An Attitude
OIL COOLED PISTONS, MG B-Series Engines -- OF-101D

From a prior article you know that oil escaping from the connecting rod bearings is thrown around the inside of the engine, which lubricates some other parts.
connecting rod with oil hole
Additionally there is drill hole in the connecting rod big end, shown above with a wire through it. This will throw on the underside of the piston. This function is a little tricky. There is an oil hole drilled in the crankshaft at an angle from the main bearing journal to the connecting rod bearing journal which supplies oil to the connecting rod bearing. When this hole is aligned (momentarily) with the dill hole in the con-rod big end, there will be a little squirt of oil from top side of the big end. This happens about 20 to 15 degrees before top dead center. Due to angle of the drill hole and direction of travel of the big end at that moment, the oil is thrown almost straight upward.

With downward stroke of the piston a fraction of a second later, underside of the piston collides with the oil jet. A small rinse of oil is therefore applied to the underside of the piston with every rotation of the engine. This oil splash serves to cool the piston. I have been informed that the A-series engines (AH Sprite and MG Midget) do not have this hole in the con-rod. For general cooling of the engine heat must be removed from the oil. Much of that heat escapes through the oil pan by conduction and air cooling. Otherwise an oil cooler may be installed to improve cooling of the oil.

On 4/6/2017, Michael Anderson in Australia wrote:
"The Twin Cam conrod does not have that drilling. Also the Twin Cam conrod has an oil catchment depression and drilling at the top of the conrod small end. This lubricates the bush of the fully floating wrist pin, which is not clamped like the one on the pushrod engine".
Twin Cam engine connecting rod without oil hole
Twin Cam engine connecting rod without oil hole

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