measuring pushrods on new heads and lifters. 243 heads were surfaced and new johnson 2110 lifters. I'm measuring zero lash, 7.29 intake and 7.27 exhaust. I'll add .035 preload to those to get 7.305 and 7.325. Question is I've read that aluminum heads and block expand approximately .020 when at temp. Do I, or how do I, factor this into my pushrod lengths?

 

You don't. The instruction are for cold preload. As the motor warms up you'll net gain about 010 of lash or conversely you'll lose about 010 of preload. Because the steel pushrods expand less than the aluminum block and heads.

In other words, Johnson already did the math and told you where to set the preload

 

Cool thanks. I figured I was over thinking it.

 

The Coefficients of Linear Expansion are as follows:

Aluminum 0.00001244"/"/degree F (Head)

Cast Iron 0.00000655"/"/Degree F (Block)

Carbon Steel 0.00000633"/"/ Degree F (Push Rod and valve stems)

So assuming from Cam Bearing Center line to Head gasket Deck is 7" and temperature rise is from 70 Deg F to 200 Deg F. Expansion of block would be:

0.00000655"/" X 7" X (200-70 Deg F) = 0.0059"

Push Rod Expansion assuming 7" length:

0.00000655"/" X 7" X (200-70 Deg F) = 0.0059"

The block and push rods expand essentially the same amount. Meaning the push rod would not extend any further above the head gasket surface.

Now consider the aluminum head expansion, which would raise the rocker arm above the gasket surface.

Al head expansion assuming 2.5" from head gasket surface to center of rocker arm pivot:

0.00001244"/" X 2.5" X (200-70 Deg F) = 0.0040"

Now the valve stems will also get longer as they heat up. The exhaust valve stem gets a lot hotter than the intake. So, assume that both valve stems are carbon steel and the intake valve stem heats to 200 Deg F and the exhaust to 400 Deg F:

Intake valve expansion assuming 4.5" length:

0.00000633 "/"/Deg F X 4.5" X (200-70 Deg F) = 0.0037"

Exhaust Valve expansion also assuming 4.5" length:

0.00000633"/"/Deg F X 4.5" X (400- 70 Deg F) = 0.0093"

Now the Cyl head expanded 0.0040" and the intake valve expanded 0.0037", which means there is only 0.0003"= 0.0040-0.0037" increase in valve clearance (for practical purposes 0). On the exhaust valve the clearance decreased by 0.0053" = 0.0093"- 0.0040".

Keep in mind my assumptions for length of valve stem, height of head, and distance from cam center to head gasket surface may be off because I don't have the parts in front of me to measure. So to answer your question of factoring in parts heat expansion into push rod length is really splitting hairs and can be easily adjusted in your valve clearance. So don't worry about it. You should also now see why solid lifter engines generally have a larger valve clearance for the exhaust valve versus the intake valve and that's because the exhaust valve stem expands more because it gets much hotter.