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.
Jens Larsen ( snipped-for-privacy@hotmail.com) gurgled happily, sounding much like they were saying :
Yes. There's a good chance - especially if they get *very* tight, that the valves will burn.
A tappy tappet is a happy tappet.
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.
Steve
The valves, especially the exhaust valve, run hotter than the surrounding head and so expand disproportionately.
The clearances are designed so when the engine is running at full tilt the actual running clearances are close to zero.
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.
The car is a Opel Astra 1.7TD, by the way.
And contrary to the laws of physics.
Steve
The message from "shazzbat" contains these words:
Yeh canna change the laws of physics, Captain!
Maybe not entirely...
Some air-cooled OHV motorcycle engines do run with increased clearances when hot - I've measured them!
My reasoning for this apparent contradiction is that the amount of expansion of the head and barrel is greater than that of the valve.
Unlikely to be the case with the OP however :-)
Chris
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.
Chris
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
How the f*ck can they be less than zero?
I bet it was.
Maybe you should get an HNC, then you'd be able to work it out ;-)
Well you got that bit right, anyway...
Explain how a gap can be less than nothing if there is no possibility of the components overlapping?
The message from Conor contains these words:
They're holding the valve open?
Somebody ought to tell the designers, they're getting it wrong, no?
Steve
Thats how I read it.
clearance >0 there's a gap between the tappet and the stem at the top of the stroke.
clearance=0 no gap
clearance
Only if they've not allowed for it when specifying the clearances. Which unsuprisngly they normally do.
But then we're talking about something completely different.
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