Reasoning over Bentley's figures on m30 valve clearance

[dacoyote]

expansion of ice is not important in this topic. neither is it pertinent...

It's a good worthless triva post...

[rob101]

drilling a hole and heating is unnecessary - given that metal expands at higher temperature its quite obvious that the hole will be a bit smaller after heating...

no actually holes get bigger after heating, its why you head the housing and cool the bearing if you are seating a bearing. Bill R is right in this case, it is because aluminium expands alot more than steel. slightly less than twice as much to be exact.

[SRR2]

Except that your opinion is exactly wrong, which was my point. The simplest thought experiment to prove this is to consider what happens to the plate without a hole. Imagine a circle drawn on the plate before you make the hole. Now heat the plate. The circle is larger because the entire plate expands uniformly. When it cools, remove the material inside the circle. Heat both pieces. The circular piece expands as does the piece with the hole, but when they're the same temperature, the circular piece will drop right into where it came from in the piece with the hole. Thus, the circular piece has expanded as has the hole it came from.

The same principle applies to valve gaps.

[Scott H]

warm/hot/whateveryouwanttocallit ??

Practicality takes over here....

Now regarding various rates of expansion of aluminum and steel. The manual distinctly addresses the state of the engine as HOT it does not say "on the way of heating up". So the premise is of having an engine, warmed up to operational state - that is things are warm and expanded maximally. The issue of faster expansion of aluminum is irrelavant - its not what is addressed. Whats addressed now is the state of things after they have finished expanding - that is when the engine is hot.

[rob101]

Except that your opinion is exactly wrong, which was my point. The simplest thought experiment to prove this is to consider what happens to the plate without a hole. Imagine a circle drawn on the plate before you make the hole. Now heat the plate. The circle is larger because the entire plate expands uniformly. When it cools, remove the material inside the circle. Heat both pieces. The circular piece expands as does the piece with the hole, but when they're the same temperature, the circular piece will drop right into where it came from in the piece with the hole. Thus, the circular piece has expanded as has the hole it came from.

The same principle applies to valve gaps.

who's opinion? there are engines that have hot clearances smaller than cold AFAIK

[SRR2]

But he's wrong. Once ice has formed it exhibits a positive thermal coefficient of linear expansion = 51x10^-6 until the temperature falls to approximately 80K at which point, for which there is no agreed-upon explanation, the coefficient passes through 0 and turns negative (i.e. it expands as it falls further in temperature toward absolute 0). The commonly observed expansion-on-cooling property of ice is only at the point of liquid-solid phase change where the solidified volume is higher than the liquid volume due to crystal-like ordering of the water molecules. Once it's frozen it behaves like most other solids with respect to thermal expansion -- as it cools, it shrinks -- until it hits the aforementioned 80K.

[dacoyote]

But he's wrong. Once ice has formed it exhibits a positive thermal coefficient of linear expansion = 51x10^-6 until the temperature falls to approximately 80K at which point, for which there is no agreed-upon explanation, the coefficient passes through 0 and turns negative (i.e. it expands as it falls further in temperature toward absolute 0). The commonly observed expansion-on-cooling property of ice is only at the point of liquid-solid phase change where the solidified volume is higher than the liquid volume due to crystal-like ordering of the water molecules. Once it's frozen it behaves like most other solids with respect to thermal expansion -- as it cools, it shrinks -- until it hits the aforementioned 80K.

Learn something new every day....

[rob101]

Now regarding various rates of expansion of aluminum and steel. The manual distinctly addresses the state of the engine as HOT it does not say "on the way of heating up". So the premise is of having an engine, warmed up to operational state - that is things are warm and expanded maximally. The issue of faster expansion of aluminum is irrelavant - its not what is addressed. Whats addressed now is the state of things after they have finished expanding - that is when the engine is hot.

perhaps you should read about the phenomenon of thermal expansion, if two materials expand at different rates, then at a higher temperature (which is constant) the one with the higher coeffecient of expansion will have expanded more, that is how this works, Bill R is right, perhaps you should admit defeat and go set your clearances. as the steel components (cam, rocker valve) expand less than the aluminium head they sit in just as bill said.

[rob101]

But he's wrong. Once ice has formed it exhibits a positive thermal coefficient of linear expansion = 51x10^-6 until the temperature falls to approximately 80K at which point, for which there is no agreed-upon explanation, the coefficient passes through 0 and turns negative (i.e. it expands as it falls further in temperature toward absolute 0). The commonly observed expansion-on-cooling property of ice is only at the point of liquid-solid phase change where the solidified volume is higher than the liquid volume due to crystal-like ordering of the water molecules. Once it's frozen it behaves like most other solids with respect to thermal expansion -- as it cools, it shrinks -- until it hits the aforementioned 80K.

I am throwing in the towel, Bill R's post should have been the end of this arguement. bye bye

[Bill R.]

cold. I knew i had seen this a few times in the past myself. Most auto manufacturers don't give you specs hot and cold. Most give you one or the other depending on how they want them set.. A quick look through some of my old books shows a opel 1966 1100cc engine having a gap that shrinks as it heats up. I know that some of my datsun factory manuals also gave specs hot and cold but i'm not going to dig them all out. Off the top of my head i would venture to say that any that get smaller as they warm up are going to be iron block,iron head pushrod motors, not overhead cam motor.I seem to recall a number of american engines that meet those specs that have ironblock,ironhead, aluminum pushrods with steel ends and the pushrods expand more than the block/head so the clearances tighten up as they warm up.

I meant to correct myself bill, this was after looking at haynes and realising that 2 workshop manuals can't be wrong, The justification I gave was wrong too, but thats life sometimes your brain just stops working. So no i don't think the BMW factory is full of idiots, if I did I wouldn't be driving one would I? There are cars that have hot clearances that are smaller then cold clearances aren't there? I swear i've seen that somewhere before.

PS: I have deleted that second post as it is a load of ****, I don't know what the hell was going on with my brain last night none of what i said makes sense lol.