So I am rebuilding my engine and bought Chromoly pushrods from Raby. I have read that you set the valve clearance to 0.000 instead of the .006 that you would with an alumium rod. I am just wondering what are the physics behind that difference? It just seems that as things expand I would end up cooking valves. Hoping someone much smarter than me (not hard to do) can explain the reasoning behind this.

Aluminum expands a whole lot when heated. Chrome-moly steel only expands a little bit when heated. You set the valve clearances cold, and the clearances shrink as the pushrods expand. When you have aluminum pushrods, you are compensating for their expansion when you use the 0.006" clearance. When you have chromo pushrods, you don't have to compensate for that, so you use 0 clerance.

At least, that is my understanding of the reason.

--DD

^^^ Correct. Rate of expansion is much different.

Thank you.

To get those pushrods to expand .001" requires a temperature that will turn your heads into a pile of molten aluminum. The stock pushrods expand 4x more than ours.
All the testing we have done to come up with proper clearances has always supported the fact that zero lash was required.

The wood pushrods in my 356 engine don't expand much with heat either ;-)