Someone needs to explain something to me!!

[89 e30]

I've been wrecking my mind about this for a while (valve adjustments). What is so confusing to me is the clearens, everybody says .25mm when cold and .30mm when hot. What i don't understand is why I would use a thicker feeler gauge when the engine is hot. Metal expands when it gets hot which would make the clearens even smaller......or does it get bigger?? I've done a lot of valve adjustments but never thought about this. Someone seriously needs to explain this to me I can't sleep at night anymore

[dnguyen1963]

Remember when the engine gets hot everything expands not just the valves. You adjust the valve clearance a bit bigger so that when you start the engine cold (metal shrinks) you will not have a problem.

[Exodus_2pt0]

I've been wrecking my mind about this for a while (valve adjustments). What is so confusing to me is the clearens, everybody says .25mm when cold and .30mm when hot. What i don't understand is why I would use a thicker feeler gauge when the engine is hot. Metal expands when it gets hot which would make the clearens even smaller......or does it get bigger?? I've done a lot of valve adjustments but never thought about this. Someone seriously needs to explain this to me I can't sleep at night anymore

I... I don't really have an answer for this one.

Here's a logical explanation I found. I'm not one to quote forums for facts, but this does stand to reason.

The head of the valve expands causing it to be thicker and increasing the lash.

[89 e30]

^^ kinda makes sense.. thanks

[vpilarrt]

Here's the real reason behind this - different materials expand, due to heat, by different amounts. Aluminum expands approximately twice that of steel at a given temperature. Using some gross approximations:
The head and the valves are ~ 4" thick/long
Hot temp (190) - cold temp (70) = 120°F
Thermal expansion of alum = 12 µin/in/°F
Thermal expansion of steel = 6 µin/in/°F
So the difference in thermal growth between the alum head and steel valves = (12µ - 6µ) X 4 X 120 = .0029" = .07mm

I knew my Mechanical Engineering degree would be useful at some point - LOL.

[long tall eta]

Here's the real reason behind this - different materials expand, due to heat, by different amounts. Aluminum expands approximately twice that of steel at a given temperature. Using some gross approximations:
The head and the valves are ~ 4" thick/long
Hot temp (190) - cold temp (70) = 120°F
Thermal expansion of alum = 12 µin/in/°F
Thermal expansion of steel = 6 µin/in/°F
So the difference in thermal growth between the alum head and steel valves = (12µ - 6µ) X 4 X 120 = .0029" = .07mm

I knew my Mechanical Engineering degree would be useful at some point - LOL.

too much information. just say
because race car

[Kershaw]

excellent post vpi.

[dnguyen1963]

Now...go to sleep!

[MattAvino]

This reminds me of another question: Is it better to adjust valves when hot or when cold?
Or does it not matter?
Threadjack, sorry.

[dnguyen1963]

does not matter...I prefer cold because I do not have to worry about temp fluctuation. It is hard to keep the hot engine at the same temp unless you can adjust the valves in 15 min.

[89 e30]

Here's the real reason behind this - different materials expand, due to heat, by different amounts. Aluminum expands approximately twice that of steel at a given temperature. Using some gross approximations:
The head and the valves are ~ 4" thick/long
Hot temp (190) - cold temp (70) = 120°F
Thermal expansion of alum = 12 µin/in/°F
Thermal expansion of steel = 6 µin/in/°F
So the difference in thermal growth between the alum head and steel valves = (12µ - 6µ) X 4 X 120 = .0029" = .07mm

I knew my Mechanical Engineering degree would be useful at some point - LOL.

That's the answer I was waiting for, couldn't have explained it better than this ThaNks

[bmwstudent]

As the engine heats up the clearance between the valve tip and the rocker gets smaller. Metal expands when it gets hot it does not shrink as stated in the second post. You provide a clearance or lash, as the engine warms up the valve gets longer and the rocker arm expands a little too. The clearance/lash you provide allows these parts to expand with out contacting each other. If there was no lash when cold, once the motor heats up the valves would actually stay open a little even when the cam lobes are pointing down.

[bmwstudent]

Sorry i reread the second post and it does not state that metal shrinks when hot. My apologies dnguyen1963