“What I Learned Today” With Jeff Smith

With the popularity of modern hydraulic camshafts (both flat tappet and roller) for street performance engines, it seems as if the mechanical camshaft is becoming a thing of the past. Despite that trend, there are still plenty of opportunities for mechanical lifter camshafts in the market. This brings up the issue of where to set lash, especially on a brand new engine.

All camshaft companies list the operating clearance — or lash — in the specs. Usually, those specs are for a hot engine, or one that has reached its normal operating temperature. The reason for this is that all engines grow or expand as operating temperature rises. Conversely, a cold engine will contract. This temperature change only accounts for a minor swing in lash specs, but when assembling a new engine with a solid lifter, you want to be as close as possible to the desired hot specs once the engine comes up to operating temperature.

Thermal Expansion Table

Because the engine dimensions will change with the heat of operating temperature, Crane Cams came up with a chart that we’ve reproduced here that compensates for cold lash settings compared to the published specs for an engine fully warmed up:

Block Material	Head Material	Lash Adjustment
Iron	Iron	Add 0.002”
Iron	Aluminum	Subtract 0.006”
Aluminum	Aluminum	Subtract 0.012”

These specs will change depending upon block and head material. This is based on something called linear rate of expansion, which is an engineering term that specifies how much a material will expand with a given increase in temperature. As an example, aluminum expands at nearly twice the rate of cast iron or steel. This is important since an all-aluminum engine will expand a much greater amount than a completely cast-iron engine. The Crane chart reveals this in the different specs.

As you can see, if the engine is all-aluminum, it will grow by close to 0.012 inch. You might want to check the piston-to-valve clearance at both specs and make a decision based on your findings. The cold lash setting will be the clearance when the engine first starts and then will increase with the wider clearance once the engine warms up.

Where this gets interesting is with tight-lash camshafts requesting a hot lash clearance of 0.014- to 0.020-inch on a hot engine. If that cam is used on an all-aluminum engine, this comes very close to producing a zero lash package with a cold engine. It’s important that the lash not be too tight once the engine cools down or it may leave the intake or exhaust valve open. That can lead to potentially burning a valve on a cold start, as the valve does not have the chance to cool between lift cycles. Just something to think about especially with cold engines.

Setting the proper lash on a cold engine will likely not change on an iron engine. But with aluminum heads or an all-aluminum engine, a tight lash mechanical cam can substantially close up the clearances.