on an iron head yes. on an aluminium head not. Aluminium expandsa more than iron when heating up.
Do valves clearances get smaller or larger when engine is hot?
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I agree but part of "spec" is adjusting valves on a cold engine. It does make a difference or the manufacturers would not spec "cold engine" when adjusting valves.
It is true the exhaust clearance cold is greater than the intake, initially it needs to be even on aluminum engines and heads.
In the case of aluminum the valve gets hot long before the head reaches operating temp so yes the exhaust needs more initial clearance but once the head reaches temp clearance will increase on aluminum and decrease in iron.
I have seen this many times where someone set the clearance on the "loose" side of spec on aluminum heads and the engine was dead silent when cold but had a noticeable tick at operating temp. This is common on Honda vehicles.
I set aluminum ot the tight end of the spec and iron on the loose end, the iron head may have a slight tick cold but goes away at operating temp, just the opposite of aluminum.
www.hotrod.com
In the case of aluminum the valve gets hot long before the head reaches operating temp so yes the exhaust needs more initial clearance but once the head reaches temp clearance will increase on aluminum and decrease in iron.
I have seen this many times where someone set the clearance on the "loose" side of spec on aluminum heads and the engine was dead silent when cold but had a noticeable tick at operating temp. This is common on Honda vehicles.
I set aluminum ot the tight end of the spec and iron on the loose end, the iron head may have a slight tick cold but goes away at operating temp, just the opposite of aluminum.
Setting Valve Lash - How to Check and Set Your Lash the Right Way - Circle Track Magazine
In this tech article CIRCLE TRACK shows you how to check and set your valve lash to ensure your race engine will be trouble-free for the entire season - Circle Track Magazine
The key piece is that you don't want to have any valves with zero clearance hot or cold. The cold temp adjustment specs take this into account. They know what the operating temperatures will be. Zero clearance means the valve potentially isn't closing fully, and can't dissipate its heat to the seat. If you are going to error, it is better to error on the loose side. Ticking when hot is not a bad thing, I would be more concerned if an engine with an aluminum head was silent when hot.
The video I posted is an aluminum head
A service manual from... 50 years ago? That engine certainly does not use the same materials as today. Nobody is using iron heads and stamped rockers anymore.
I have owned a nine second, naturally aspirated small block Ford making about 680 hp, with a solid roller. Cold lash was always set at 4 thousandths tighter when cold. I checked it hot a few times and it was always close enough that I stopped doing it.
The video I posted was just an example, and is a re-uploaded television show. The guy hosting it has been building engines for 30 years... He's not wrong.
Again, we each have sources that we trust more than others. If you have a source that you trust, that’s all that matters.
I don't believe it's always the case for every engine that exhaust valve clearance decreases when hot. I have one that doesn't, it's a pushrod motorcycle engine with alloy heads. There are even examples where clearances are specified as zero when cold because they increase when hot. This can be achieved by careful choice of pushrod material.
An alternative reason for exhaust clearance being larger than inlet is that it provides more safety margin to allow for the greater valve seat wear of exhaust valves vs inlets.
forgot about pushrods, haven't worked on them for 15 years now, and even then it wasn't a car engine...
Didn't have time to watch it then, but will tomorrow
Steel valves in iron heads have reduced valve clearances when hot, period.
Hundreds of thousands or cast iron heads being produced currently btw, albeit with cast rockers/bushed pivots.
I don't see Cummins going to an aluminum head with the current design. Head's too long to be reliable in alloy.
Hundreds of thousands or cast iron heads being produced currently btw, albeit with cast rockers/bushed pivots.
I don't see Cummins going to an aluminum head with the current design. Head's too long to be reliable in alloy.
Watched the video now. Pushrod engine, so the bigger head expansion increases valve lash when hot (fulcrum of the rocker rises).
Basically, you need to know what you are dealing with, or follow the recommendations if available.
