Reasoning over Bentley's figures on m30 valve clearance

[632 Regal]

remember it too, there will be a test on this Friday.

Learn something new every day....

[Rustam]

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.

you're concluding that the hole expands based on the argument that circular piece becomes bigger as does the piece with the hole - that is shown by the fact that the circular piece cannot pass through the opening after heating. Well, that tells that the hole acquires smaller radius - correct word for what happens with the hole would be "shrinking" not "expanding"...

the circular piece has expanded albeit not as the hole has... the hole has shrunk.

in fact, if the hole expands, using the experiement that you have provided in the argumentation becomes very hard if not irrelevant...

[Rustam]

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.

I never said that the material with higher coefficient of expansion should not have expanded more. Its quite obvious in fact, that X that grows faster in size will grow to greater extent than Y which does not grow as fast in given amount of time under same heating conditions. That said - I don't see the basis for a conclusion that I should read about the phenomenon of thermal expansion. How I succeded in showing to you that I don't understand this phenomenon totally alludes me.

But what I did mention was that the issue of rate of expansion is irrelevant at the end result which is simple observation of the fact that things in total had become bigger... I never mentioned what happened on the way to heating but mentioned what happens after things have heated - that was my concern...

What defeat are you talking about? I never made statements but inferences which I brought out for a discussion.



Now, Bill's comment did not provide much explanation but did provide further grounds for questioning. Having mentioned what happens at bottom end of the head Bill R misses to mention what happens at top end of the head. Top end of the head expands also, tending to counteract the effect of bottom expansion thereby pushing the valve up towards the rocker, minimizing the space... What Bill R's "explanation" misses to account for is the fact that bottom end of the head being in combustion chamber accepts more heat and expands more than the top end of the head - that being under PCV. Both ends pull the valve, however bottom end of the head now having expanded more tends to pull more - net effect being displacement of the valve down, further away from the rocker...

This serves as an explanation for me.

[88m53453]

seems backwards ,but Bentley is right on this one. .010-.012 on cold and add .002 for a hot engine, in thousandths here of course

[rob101]

I never said that the material with higher coefficient of expansion should not have expanded more. Its quite obvious in fact, that X that grows faster in size will grow to greater extent than Y which does not grow as fast in given amount of time under same heating conditions. That said - I don't see the basis for a conclusion that I should read about the phenomenon of thermal expansion. How I succeded in showing to you that I don't understand this phenomenon totally alludes me.

But what I did mention was that the issue of rate of expansion is irrelevant at the end result which is simple observation of the fact that things in total had become bigger... I never mentioned what happened on the way to heating but mentioned what happens after things have heated - that was my concern...

What defeat are you talking about? I never made statements but inferences which I brought out for a discussion.



Now, Bill's comment did not provide much explanation but did provide further grounds for questioning. Having mentioned what happens at bottom end of the head Bill R misses to mention what happens at top end of the head. Top end of the head expands also, tending to counteract the effect of bottom expansion thereby pushing the valve up towards the rocker, minimizing the space... What Bill R's "explanation" misses to account for is the fact that bottom end of the head being in combustion chamber accepts more heat and expands more than the top end of the head - that being under PCV. Both ends pull the valve, however bottom end of the head now having expanded more tends to pull more - net effect being displacement of the valve down, further away from the rocker...

This serves as an explanation for me.

no it doesn't miss that fact, it doesn't really matter because ultimately a hot head will expand more than a cold one, since the rocker assembly are mounted on the topside of the head and the valve seat is mounted on the bottom side expansion = increased distance.

Just go set your clearances already.

[Rustam]

no it doesn't miss that fact, it doesn't really matter because ultimately a hot head will expand more than a cold one, since the rocker assembly are mounted on the topside of the head and the valve seat is mounted on the bottom side expansion = increased distance.

Yes it does miss that fact - in his message he never differentiates for the variation in temperature between top part of the head and the bottom part of the head. What he does differentiate between is variation in speeds of expansions and extents of expansions of metals involved.

He never mentions the fact that the head expands to different extents at top and bottom.

ultimately a hot head will expand more than a cold one

Pardon me, but cold head doesn't "expand" - it is taken as the datum in this discussion...

Overall I fail to see logic in your message - especially at the arrival to the conclusion.

Just go set your clearances already.

I was not seeking to set the clearances.

[632 Regal]

you get more displacement with a HOT engine than a cold one.

[Rustam]

you get more displacement with a HOT engine than a cold one.

How so? If the metal expands in heat it supplants space making available less of it.

I would say that one gets greater compression ratio with HOT engine...

This message wasn't really meant to be serious was it?

[rob101]

so if you imagine the aluminum head expanding in all directions as it heats up that means the valve seat in the head is going to get closer to the the piston and the rocker shaft is going to get further away from the valve stem

so there you go since the rocker assembly is mounted on the top of the head, and the valve seat is on the bottom of the head an expanding head causes these to move further away. since the ALUMINIUM head expands more than the steel components when you increase the temperature, then the friggin' clearances increase.

I am sorry your logic is so bad newbie, Bill R. said it the right way first time. sorry that you didn't realise the valve seat is on the lower surface of the head and the rocker assembly is on the top.

[632 Regal]

I said displacement not compression...and yes it's a serious post.

How so? If the metal expands in heat it supplants space making available less of it.

I would say that one gets greater compression ratio with HOT engine...

This message wasn't really meant to be serious was it?