Are there any negative aspects of smaller valve clearances than specified? I suppose the camshaft runs more smoothly if it's closer to the valves. The valves are open a little longer, but is that noteworthy?
The valve clearances on my diesel engine are 0.05 mm, but they are supposed to be 0.15 mm.
The clearance exists to allow for expansion as the engine reaches normal working temp, if the cold clearance is too small the hot clearance is zero and then you can get a loss of compression becuse the valves don't fully close,and as Adrian said, burning of the valves.
Apart from the possibility of burning the valves, incorrect clearances alter the valve timing. Usually to the detriment of performance. Power, fuel consumption, etc. Mike.
OK, but how do you explain this: Before I got the valve clearances adjusted for the first time, they were 0 mm or less (you understand) on a cold engine. I've never measured them on a hot engine. When the engine was cold it was a slow starter. I had to crank the engine. When it started it would shudder for the first seconds. Lots of black smoke was produced. After approximately 15 seconds it settled on normal idle with no smoke from the exhaust.
When hot it was always a good starter. After valve adjustment it was good starter even when cold.
To me it sounds like the valve clearances increased and the compression improved when the engine got hotter, contrary to what you say.
If steel valves are used in alloy heads, or steel pushrods with alloy barrels, when everything has come to the same equilibrium temperature then yes - because the expansion coefficient for aluminium is higher than steel. However, generally valves run a lot hotter than the head and so the clearance should reduce in a running engine.
Indeed. Another factor with an OHV engine is that the distance between the cam and the rocker is much greater than the length of the valve, so the amount of expansion will be greater.
Only if everythings the same temperature, it's not unknown for inlet valve clearances to increase & exhaust valve clearances decrease simultaneously on a stressed engine
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