'74 BMW, Valvetrain Rap When Cold

Joined
Dec 5, 2003
Messages
3,925
Location
New England, USA
Chasing down a valvetrain rapping sound only after a cold start and only with the valve cover on. Started after a valve adjustment and the engine sounds perfect at operating temp. Rebuilt motor has ~2k miles and is fine otherwise. No contact marks under the valve cover. Re-adjusted valves, replaced two of the eccentric adjusting wheels (thought they may have had flat spots) and no improvement. Then I get to the #4 intake valve and find this. The washer for the adjuster had failed and a piece appears to have dropped onto the valve spring retainer. This pic is after I removed the bolt and the larger, upper piece also fell onto the valve. The adjuster bolt, which I removed, is also visible. Cannot really see how this would have caused the sound only when cold, but I am running out of ideas.

The bolt passes through the rocker arm fork pinching the eccentric wheel in the middle. Nut and washers are on the left side of the fork. You stick a small pick or similar into a hole in the eccentric (not visible but usually on top) and rotate it to set clearance.
BMW2002vaw42024.jpg
 
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I am thinking the washer cracked when tightening (I believe it is hardened) allowing the nut to loosen and the eccentric to move causing excess lash which closed up when the engine is up to operating temp. This normal with an aluminum head just the opposite of iron.
 
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