Actually the clearance decreases when the engine gets hot. That's why you have to wait to do all the measurements & adjustments when the engine is cold. If they loosened when the engine got hot you would do all measurements when just coming back from a ride, or just pulling into the shop, or just plain warming the engine up. That's why you freeze the crank & heat the bearing to install cranks in dirt bike engines for example. Metal expands when heated, not shrinks. It's just physics...nothing else to say about that. As long as they are within spec after the adjustments are made you will be ok.

 

From my understanding, metal normally expands as it heats up, shrinks as it cools.

That said, the reason to work on the engine while cool... it is stable.
Depending on how long/hard the engine has been run and how long it takes
to cool, the engine temp can be air temp up to 240 degrees. If you work on it,
while it's in such a state, the tolerances are changing and you're chasing a
moving target. It's the same logic used for only checking tire pressure when they are cold.

That said, IDoDirt's logic on how to short-cut the process still makes sense
and might save a second pulling of the cams for further adjustment. Wish
I'd thought it through, but I was thinking the cams might actually be forced
tight and need more then just a calculation from zero as the starting point.

Hence my logic, that it would take two trips through to get there. Once to
get a measureable clearance, and then a standard adjustment to spec.

The back and forth I get from these forums, is what keeps me here.
Constantly learning/refining and re-thinking my picture of the processes
involved in wrenching on these beasties.

Thanks, to all who take the time, to contribute to these discussions, Ern

 

+1

The logic is good. There is still a chance of having to do a second pull of the cams though.

 

Thanks guys I appreciate all the tips. Ill let you know what I do and how the process unfolds. Hope to have some time soon to work on her.

 

You and MadHattr059 are correct in that metal and other materials expand when heated. You also have to keep in mind that you're dealing with different types of metals here too. The valves themselves are made of steel. The cylinder head is made of aluminum. The clearance issue comes into play based on how these compare with each other. The cylinder head being made of aluminum expands at almost 2 times the rate that steel does. So as the head is expanding, it has the effect of pulling the valve seat in a direction away from the camshaft. That valve seat takes the valve with it, effectively increasing the clearance between the cam lobe and valve stem.

I'm an old school mechanic and worked on many cast iron V8's as I was growing up. Adjusting valves was something we did all the time. Cast iron and steel have nearly identical expansion rates and the gap would close up when they heated. We also weren't working on overhead cam engines either. Working on aluminum engines has opened up my eyes to new possibilities.

 

Allright I've got the measurements for the shims using dirts formula and will hopefully get them ordered soon. I am assuming that when the clearances are close to the larger side, that would be better. In thinking that the gap will get smaller over time by the valve hammering into the seat. ( this was very confusing at first I was thinking how can they get longer) but after looking at it I guess by the valve hitting the seat thats where the wear happens therefor making the stem close up the gap. And i am assuming that once the cam is touching the the bucket that no more wear on the seat can happen so in that case the method dirt mentioned by subtracting clearance from the existing shim should work. Thanks for all the help and i will update as I get it back together.