Setting valves cold with hot lash settings
By Jeff Smith
The mechanical cam is alive and thriving – thank you. While Crane offers an outstanding hydraulic roller lifter that we’ve done a story on, where we pushed a big-block Chevy happily to 7,500 rpm with juice lifters, this doesn’t foreshadow the end of mechanical lifter. For the many enthusiasts, lashing lifters is still an important part of the performance plan.
Recently, a friend questioned how much he should change the lash settings on a brand new cold engine so that his final lash numbers would be close once the engine reached its normal operating temperature. Mechanical lifter cam lobes are designed with a short clearance ramp to take up this clearance but the real reason for that ramp is to help compensate for the difference in metal expansion rates from a cold to hot engine.
The information here does not affect hydraulic lifter engines because the lifter preload automatically compensates for the differences between a cold and hot engine. Everybody knows that metal expands when it gets hot. But all metals expand at different rates. So an all-iron engine will not expand nearly as much as an all-aluminum engine.
Think about this for a moment. Imagine you are called upon to design an all-aluminum engine that must start and run in ambient temperatures from -20 F to 130 degrees F. An all-aluminum engine will contract quite a bit as the temperature plummets, much more so than its steel lifters, for example. This means the engine designer must compensate by increasing the diameter of the lifter bore so that the aluminum won’t seize that steel lifter in sub-zero weather. This is why all-aluminum engines must use wider lifter bore clearances.
So now we have a situation where we have a brand new engine with a mechanical roller cam. We can set the lash using the “hot” spec, but it won’t be very accurate once the engine temperature stabilizes. Crane realized this a long time ago and created a spec chart that we’ve included here that compensates for the expansion rates of both cast iron and aluminum engines. As you can see in the chart, there’s a dramatic difference in the clearances when we’re dealing with an all-aluminum engine.
So let’s run through this to make the explanation more clear. Aluminum expands more than cast iron. In fact, we looked up linear expansion rates of both materials and aluminum expands (or contracts) at nearly twice the rate of cast iron. As you can see by the chart, this tends to hold true.
Cold vs. Hot Lash
|Head Material||Lash Adjustment|
With an iron block and heads you would actually add 0.002-inch to the hot lash spec for a cold engine. So if the cam card tells you to set the lash hot at 0.020-inch on the exhaust side of the cam, for a more accurate cold startup, we’d set the lash at 0.022-inch.
As the engine warmed up and expanded, the lash will be very close to that spec. Of course, you still should check the lash on all the valves once the engine is warm, so these specs should only be considered as recommendations. But they will still be very close.
For an iron block engine with aluminum heads, the chart reveals aluminum grows quite a bit more than cast iron, so now we should subtract 0.006-inch from the lash spec. So again, if the spec is 0.020-inch, our initial cold start-up lash setting would be 0.014-inch.
This gets even more aggressive with an all-aluminum engine. The chart suggests a cold setting for a 0.020-inch hot lash spec would be to tighten that by a whopping 0.012-inch – leaving the actual lash at a very tight 0.008-inch. Again, this is just Crane’s recommendation as a starting point.
If your spec is very small, a thin feeler gauge might be difficult to work. You may have to loosen the adjuster and then tighten it down on the feeler gauge to get the proper setting. But even with a thin 0.008-inch feeler gauge, this would not be difficult to achieve. The whole point of this exercise is to offer a reason why the cold spec is significantly different from the hot lash number and that you can expect the lash to change once the engine is fully up to temperature.
This is not a make-or-break issue. Setting an engine cold with hot specs will still work. But if you like working precisely, this is another step toward making your engine happy, even when it’s cold.
Source: Crane Cams, cranecams.com