quote:Originally posted by George P: Even though its quoted "hot", the lash of any camshaft can be set cold too, using these guidelines:
(A) Iron block and iron heads: Cold lash = hot lash setting plus 0.002”
(B) Iron block and aluminum heads: Cold lash = hot lash setting minus 0.006”
(C) Aluminum block and aluminum heads: Cold lash = hot lash setting minus 0.012”
I think that’s a pretty good rule of thumb and pretty consistent with what I’ve seen. It’s a pretty well known phenomenon for pushrod engines and is caused by the difference in the coefficient of linear of thermal expansion between the metals over the distance of the cam centerline to the tip of the pushrod. The cam, lifters, and pushrods all being steel and then the block and head material differences and corresponding differences in thermal growth, which will always grow more that steel cam/lifter/pushrod combo.
If you look up the coefficient values and measure the distances for the various materials, then use the difference between fluid temperatures and ambient temperatures as a proxy for temperature change, you will get very similar values for change in lash.
On alloy block and head pushrod engines the increase in hot versus cold lash at valve tip will also be accentuated by higher rocker ratios and longer cam-PR tip distances. For iron/iron engines it’s almost a non-event but on ally/ally it can actually be a limiting factor on how tight you can run and is a major reason (among others) why you’ll see tight all alloy track engines staged with preheated block and oil sump heaters before being started so as not to risk bending a pushrod or damaging a roller bearing as they often can be at/near zero lash cold depending upon conditions.
On my 9.5 deck all alloy roller the difference between hot and cold lash is .011” (1.73 rocker ratio) measured after full soak dyno runs and then cold measurement in the morning. It’s only necessary to check the difference on a couple cylinders as it will be very consistent. Just start conservatively cold and get a couple good readings on an easily accessed cylinder for the hot read and you can set lash cold from there on without worry, or if you’re already on the road, get a good hot read on a convenient cylinder and measure same cylinder(s) cold and you’ll know.
Best,
K