Valve Clearance Adjustment on a Hot Engine

[The Critic]

2008 Mitsubishi Eclipse 2.4L (4G69) with 200K.

I went to replace the original spark plugs and found the coils swimming in oil, so I upsold my neighbor a free valve clearance adjustment, valve cover gasket kit and coil boots. Upon consulting the repair manual, I found it peculiar that the valve clearance adjustment must be fine-tuned on a hot engine.

As someone who has only performed valve clearance adjustments on stone-cold engines, what is the rationale for this recommendation?

 

[ka9mnx]

They are saying to adjust the valves cold and then recheck them hot. I wouldn't bother.

 

[Propflux01]

They want the clearance checked at the maximum expansion point. Most Japanese cars of that era and older wanted them hot. My old VW cold. The main difference to me was that, cold setting on the VW was quieter at cold start, louder when hot, and Japanese vehicle was quieter when warm (pertaining to valvetrain noise).

 

[RooflessVW]

They are saying to adjust the valves cold and then recheck them hot. I wouldn't bother.

Same.

 

[Opelman]

Some brands used to want the valve lash checked hot and running. Studebaker V8, Chevy stovebolt 6 come to mind.

 

[PontiacHO]

If valve cover is off and engine is cold, then why not check valve clearance then? Most likely if the clearance is correct when cold it will be in spec when warm. With 200k it’s probably more than has ever been done with the valve clearance-adjustment anyway.

 

[dnewton3]

Cold state indicates .004" for intake; .008" for exhaust.
Warm state indicates .008" intake; .012" for exhaust.
Notice that they don't call out a tolerance (what's the +/- value ???)

I'd set them at the cold value and not worry about the hot values. And for the tolerance, I'd use +/- .001" cold state.

 

[KrisZ]

I’m guessing the rationale here to have both cold and hot specs is to service the vehicles quicker. By listing a hot spec, the clearance can be checked without waiting for it to cool off, which normally would be overnight.

 

[Trav]

Setting them hot is a PITA unless you do the with the engine running like we did on some engines years ago with a modified valve cover or push rod clips. The problem is the engine begins to cool as soon as its turned off, to get all 16 done at the same temp you would have to be greased lightning with the feeler gauges, aluminum begins to contract pretty quick unlike cast iron. Do it cold and forget it.

 

[AdmiralYoda]

If they provide cold and hot...just do cold. And not warm but cold...as in engine temp is the same as ambient, not just cooled down a little bit.

 

[barryh]

As someone who has only performed valve clearance adjustments on stone-cold engines, what is the rationale for this recommendation?

Hard to say. You would think the expansion of the valve train components was predictable enough to calculate what they should be cold. Perhaps it's because the clearances don't change equally for every cylinder between cold and hot. You could set them hot and then measure them when cold to see if that's the reason.

It's not the case that clearance always close up when hot as it will depend on how much the valve train components expand compared to other parts of the engine. There have been examples of clearances that were to be set at zero when cold and presumably opened up a little when hot. There are engines that have been engineered to that the clearances don't change significantly between cold and hot.
I've owned an engine where the clearances were to be set hot and with the engine running at idle. Possibly that was the ideal method but it was sure messy.

 

[The Critic]

I’m guessing the rationale here to have both cold and hot specs is to service the vehicles quicker. By listing a hot spec, the clearance can be checked without waiting for it to cool off, which normally would be overnight.

The "service" instructions stated to set them hot. The instructions for setting them cold was buried in the overhaul manual, which I unfortunately did not discover until after the job was done.

I probably didn't work quick enough for the engine to stay operating temp hot, but worst case, they're probably .002" too loose...which shouldn't cause any harm. It has been about 3,000 miles since the adjustment; the engine sounds fine and has no misfires. I do remember several of the exhaust valves having very little clearance.

Also, is a .004" difference between cold and hot, standard for an iron block/aluminum head engine? I came across this article and was under the impression that the cold/hot expansion rate would be a lot lower:
https://www.enginelabs.com/news/what-i-learned-today-with-jeff-smith-cold-lash-vs-hot-lash/

 

[Brons2]

Setting them hot is a PITA unless you do the with the engine running like we did on some engines years ago with a modified valve cover or push rod clips. The problem is the engine begins to cool as soon as its turned off, to get all 16 done at the same temp you would have to be greased lightning with the feeler gauges, aluminum begins to contract pretty quick unlike cast iron. Do it cold and forget it.

I loved the hot and running method on the domestic V8s with the modified valve cover. Back it off until it clacks then a quarter turn. Worked perfect every time.

 

[twX]

It's not the case that clearance always close up when hot as it will depend on how much the valve train components expand compared to other parts of the engine. There have been examples of clearances that were to be set at zero when cold and presumably opened up a little when hot. There are engines that have been engineered to that the clearances don't change significantly between cold and hot.

Keep in mind that the "hot" clearances can be very different depending on whether or not the engine is running. When an engine is shut off, the temperature of the valves will quickly approach that of the cylinder head. When an engine is running, the valves will be much hotter than the cylinder head.

So for an engine with an aluminum head and steel valves, the "hot clearance" (with the engine not running) will be greater than the cold clearance, since the aluminum head will expand more than the valves.

However, with the engine running, the valves will get so much hotter than the cylinder head, that the clearances will become lower than the "cold clearance", even though steel doesn't expand as much as aluminum does at a given temperature. Maybe this isn't always the case with an engine at idle, but at high load and rpm, the valves will get hot enough for the clearances to tighten. This is why some clearance is always necessary.

You would think the expansion of the valve train components was predictable enough to calculate what they should be cold.

I'd think so as well. I can't think of any reason to check them "hot". It'll just introduce some error since the engine is still cooling off. I'd ignore the manual and only check them cold.

 

[barryh]

Having to set valve clearances is not common anymore but what happened back in the day when your car went in for service, they couldn't possibly afford to wait for an engine to be stone cold so they must have been set warm.

We have great debate on a BMW motorcycle forum with members saying the clearances have to be set stone cold on 40 year old airhead engines but it's not necessary. What I do at an annual service is change the oil and filter, check the ignition timing by which time the engine has cooled enough to set the clearances without getting my fingers burned. And that of course is what a servicing garage would do. To prove the point I once left it overnight to cool and checked them again stone cold with the result that there was no measurable difference.

The exact clearance is not that important anyway give or take a thou as long as all cylinders are the same. In fact the most important result of checking valve clearances is to know how much they have changed since they were last checked because that tells you something about valve train wear or valve recession.

 

[Exhaustgases]

There are some engines that it is impossible to check them hot, in-situ, since there are just too many things in the way, and too many parts involved, to do it fast enough, and can't be run without them.

 

[Oliveoil2]

I’ve always adjusted valves, cold to the max. Never had a problem. Valves, especially exhaust, tighten with miles. This also worked with Mitsu engines that wanted hot engine valve adjustments. After the first adjustment, 80-100k (Honda, Mazda, Toyota) never had to change the setting.

 

[Trav]

The worst valves I ever adjusted was a desmo, the easiest possibly were a Porsche 924 that used buckets with a tapered screw. Both were done cold.

 

[samven]

The last time I did valve adjustments was on a Suzuki motorcycle, they said to set them at room temp approx 70F. Surprisingly they were .004 and .008 too.

 

[supton]

This time of year I'd rather do it on a warm engine. Seems to me, what it's at when hot matters most, cold only matters for start up.

That reminds me, I need to redo it, I wound up with a clattery mess. Guess one has to start somewhere, I did something wrong.