The Stovebolt Site Forums The Shop Area Engine Shop Brain twister valve adjusting question.

Okay, you go by the book and adjust your valves on your 235 spot on, sounds simple enough. Then let the engine cool down and check the gaps with a dial indicator.

Now, explain to me why you can't just adjust your valves cold next time to the reading you just got.

"It's not supposed to be done that way" is NOT an acceptable answer. I want to know the flaw in my logic.

I imagine you could do that. In fact, lots of heavy duty diesels (NTC Cummins and John Deere) set their valves cold.

The thing that would prevent me from cold setting the valves is the small lash amount (.006?) on the intakes. With that small of a gap I would prefer to have the engine warmed up to specs so that the adjustment would be perfect.

The cam is ground so that the valve leaves and returns to the seat gently. Once the lash is taken up the cam profile becomes much steeper and quickly opens the valve to the maximum, and the reverse is true when the valve closes. If the valve lash is too great, the valve will tend to 'slam open and slam shut'. This is VERY hard on valve train components. If the lash is insufficient, the valve will not fully contact the seat and transfer it's heat to the seat. A burned valve quickly results.

I think before I completely trusted the cold setting method, I would have to see VERY consistant results between hot and cold lash readings.

I'd wonder just how cold "cold" really is, compared to the pretty invariable "operating temperature", are you gonna set it cold in Jan or July? .... I believe there is a spec for cold setting in the shop manual, but they point out that it's important that all the metal as well as the oil should be operating temp for accurate setting to the hot specs

Bill

Once you know what the cold setting is, I see no reason why you can't check them on a cold engine and re-adjust to that cold setting again. The temperature variance on a cold engine would be the difference in atmospheric temperatures, say from 65 to 85 or 90 degrees. With a hot engine is it always going to be ran up to the same temperature?Then when you start checking, you could have some cooldown before you have finished, which could allow for some variance. I ran supercharged engines in modified pulling trucks for 25 years, and always adjusted my valves cold.In all that time ,I can't say that I ever had breakage because of improper valve settings.

Most likely, Bill has the answer: what exactly is a cold temperature vs what is your warm-engine temperature (after you have warmed up, it is basically a constant temperature determined by your thermostat)?

This logic also would be an argument in favor of adjusting the valve lash with the engine running (because the engine temperature would most likely increase for a while after turning the engine off).

Okay, for the sake of argument let's say we have a climate controlled garage where the temp is always 70º.
Why couldn't you set your valves cold?
How much difference is there for every 10º of temperature?

for the sake of argument? why argue with the design engineers?

I'd imagine that, given the variety of shapes involved and associated masses, the change in a setting of 0.006 could be very different from cylinder to cylinder .... give it a try and let us know next time you adjust, check every valve hot and cold, 6 mo later, recheck'em cold, warm it up and recheck'em hot

Bill

Every OHC engine I've ever worked on has you set the valves cold, since you can't set them hot without burning yourself, but since they don't have pushrods there is less to go wrong. The SOHC and DOHC engines also have much tighter lash than an old stovebolt.

I think some of it is going to be how much the differential expansion between metals is going to be between cold and hot lash, and some of it is going to be how much the oil film on the lifters and cam bearings differs in thickness between cold and hot.

You ought to be able to get it really close cold if you precisely measure the lash on your particular engine after you have adjusted it hot and let it cool down to a reference temperature, and then make sure it is that temperature if you are going to adjust them cold. But really, how hard is it to adjust them hot? Its not like you are adjusting a flathead or a volkswagen, these adjusters are pretty easy to get to.

I think there is more error sneaking in there by making sure you are exactly on the base circle of the cam than the error you get knowing the cold numbers for your particular engine.

Specifications? We don't need no stinkin' specifications! Mechanical lifters need to be set with the engine stopped, and up to normal operating temperature. Once the thermostat opens, there's enough hot coolant in the block to keep the block and head at a pretty steady temperature long enough to get an accurate adjustment.

For a cold setting, get the cylinder at TDC on the compression stroke, tighten the adjuster to just eliminate the slack, back up 1/4 turn, and lock the nut. Run the engine to operating temperature, and do a hot adjustment. Argue about it, rationalize all the cold-setting methods you want, but you'll end up doing it right eventually.
Jerry

". . . there's enough hot coolant in the block to keep the block and head at a pretty steady temperature long enough to get an accurate adjustment . . "

I now adjust valve lash on mechanical lifter 216/235/261 engines using the engine off method. However, my engine does not stay at "steady temperature" when I turn the engine off. The temperature usually rises for a while after I turn the truck off.

Very true,when ya shut it off all the coolant quit's circulating and the heat has no place to go but up and out through the block.If you are setting them in 20 below weather mabe you should have tuned it up last fall! Tom