Reasoning over Bentley's figures on m30 valve clearance

[Rustam]

Bentley's manual tells the reader that greater valve clearance should be set to HOT engine - .35mm and lesser valve clearance should be set to COLD engine - .30mm...

I find strangeness in these figures for the reason that everything expands in size with greater temperature - the camshaft becomes a bit wider, so do the rockers, and the eccentrics, and the valves, and everything else in nature...

The reasoning tells that therefore, HOT engine must exhibit LESSER valve clearance for the reason of supplanted space by the mentioned parts since they are greater in size. That is:

HOT - .30mm
COLD - .35mm

[rob101]

http://member.rivernet.com.au/btaylo...vesM30E28.html
this should help you out, but i believe you are right smaller clearances when cold is just stupid

[SRR2]

Drill a hole in a plate of steel. Heat the plate. Is the hole larger or smaller when it's hot?

[Rustam]

Drill a hole in a plate of steel. Heat the plate. Is the hole larger or smaller when it's hot?

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...

[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.

[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

[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...

[Bill R.]

also state, .012 cold and .014 thousandths when the engine is warmed up...This motor also doesn't have tappets. It has rocker arms that ride directly on the cams...all this talk of cams expanding etc etc is wonderful except for the fact that the aluminum head that the rocker arm shafts,cams and valve are mounted in grows faster and more when heated than do the steel and cast components... 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.... But rather than argue about what the factory determined a long time ago why don't you set the valves when cold as the factory specifies to .012 inches and then measure the clearance when the engine has warmed up... the factory spec states after the thermostat has opened.

no obviously someone hasn't had to seat a bearing before heat housing, housing gets bigger.
Actually its because the cam expands and is on the other side of the pivot (the rocker) thus the gap closes on the follower and tappet side. i am used to working on cars with different rocker setups where the cam is the same side of the pivot as the tappet.

[rob101]

also state, .012 cold and .014 thousandths when the engine is warmed up...This motor also doesn't have tappets. It has rocker arms that ride directly on the cams...all this talk of cams expanding etc etc is wonderful except for the fact that the aluminum head that the rocker arm shafts,cams and valve are mounted in grows faster and more when heated than do the steel and cast components... 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.... But rather than argue about what the factory determined a long time ago why don't you set the valves when cold as the factory specifies to .012 inches and then measure the clearance when the engine has warmed up... the factory spec states after the thermostat has opened.

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.