Compression Spring. Engine Timing Chain Tensioner.
An oil pressurized Tensioner that keeps the Chain that runs from the crankshaft to camshaft or the Chain that runs from camshaft to camshaft, from becoming loose and jumping a tooth or more causing Engine damage or out of sync crankshaft to camshaft Timing. Spring.
Engine Timing Chain Tensioner
Fits 535i, 635CSi, 735i, 735iL, M6