I started putting in new eccentrics into my M20 last night (I read that old eccentrics will have a dimple wear in the spot where the valve stem makes contact, causing improper clearances) and ran into some questions:
From the Bentley Manual:
Over 85 C (hot engine) – 0.012 inches
Under 35 (“cold” engine) C – 0.010 inches
I thought, that can’t be right, you’d expect the clearances to shrink when the engine is warmer due to the valve stems growing. However, the manual is right, I last adjusted the clearances in October to 0.010” and yesterday night (-7 C in the garage), I couldn’t fit my 0.010” feeler in there before I loosened the eccentrics to swap in new ones. The only explanation I could think of is that since the head is aluminum and the valve stems are steel, the aluminum would shrink more than the steel (aluminum has almost twice the thermal growth coefficient of steel), effectively making the valve stems longer with respect to the head, making the clearances shrink.
So then the next question, what do I adjust the clearances to when working on a “very cold” engine. Assuming linearity, you’d expect that if at 85 C the clearance is 0.012”, 35 C the clearance is 0.010”, -5 C the clearance should be 0.008”, right? Anyone else run into this when working on a “very cold” engine?
From the Bentley Manual:
Over 85 C (hot engine) – 0.012 inches
Under 35 (“cold” engine) C – 0.010 inches
I thought, that can’t be right, you’d expect the clearances to shrink when the engine is warmer due to the valve stems growing. However, the manual is right, I last adjusted the clearances in October to 0.010” and yesterday night (-7 C in the garage), I couldn’t fit my 0.010” feeler in there before I loosened the eccentrics to swap in new ones. The only explanation I could think of is that since the head is aluminum and the valve stems are steel, the aluminum would shrink more than the steel (aluminum has almost twice the thermal growth coefficient of steel), effectively making the valve stems longer with respect to the head, making the clearances shrink.
So then the next question, what do I adjust the clearances to when working on a “very cold” engine. Assuming linearity, you’d expect that if at 85 C the clearance is 0.012”, 35 C the clearance is 0.010”, -5 C the clearance should be 0.008”, right? Anyone else run into this when working on a “very cold” engine?
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