Valve Lash - Hot or cold?

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jwicker

jwicker

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I've read many forums on lashing valves on here and almost all of them say to do it hot. I don't necessarily mind doing that, but many old hat mechanics seem to say you can set the lash cold.

Is there any reason this wouldn't work? It seems like the suggestions I've heard for flat tappet cam engines is to:

  1. Use a breaker bar with the correct socket
  2. Spin push rod by hand
  3. Tighten (or loosen) all rockers until there's just a bit of tension against the rod
  4. This should get you to zero lash
  5. Rotate engine 90 degrees, repeat on loose valves
  6. Repeat until nothing is loose
  7. Go to all bolts and do a 3/4 turn tight - this should be the preload
Since this puts the engine at zero lash, I assume you could go back and adjust with a feeler? Or just use a feeler right from the start?

It seems silly to do it hot/running and fight with jumping rockers and oil everywhere when the expansion of the parts should be graphable / known... wouldn't that leave you with a cold feeler thickness and a hot feeler thickness?

(okay maybe I do mind doing the lash while hot, a bit)

Why not just zero lash all the way around?

See this video from VGG :

Welcoming all opinions before I attempt myself.
 
Remember-setting the lash hot doesn't necessarily mean doing it while it's running. Like Halifaxhops said- that's a Chevy thing. Just warm the engine up well and then set your lash.
And use the procedure outlined in the factory service manual (I won't rewrite it here- just look it up)- there's a difference between "just getting it running" and getting it running well.
 
Factory setting. Intake .010 exhaust .020 hot or cold a slant is happy both ways.
I played with different numbers .008 .018, .012 .024 Car always ran the same.
 
I typically set mine cold. 65'

MoparValveLashSetup (1).jpg
 
jwicker said:
I've read many forums on lashing valves on here and almost all of them say to do it hot. I don't necessarily mind doing that, but many old hat mechanics seem to say you can set the lash cold.

Is there any reason this wouldn't work? It seems like the suggestions I've heard for flat tappet cam engines is to:

  1. Use a breaker bar with the correct socket
  2. Spin push rod by hand
  3. Tighten (or loosen) all rockers until there's just a bit of tension against the rod
  4. This should get you to zero lash
  5. Rotate engine 90 degrees, repeat on loose valves
  6. Repeat until nothing is loose
  7. Go to all bolts and do a 3/4 turn tight - this should be the preload
Since this puts the engine at zero lash, I assume you could go back and adjust with a feeler? Or just use a feeler right from the start?

It seems silly to do it hot/running and fight with jumping rockers and oil everywhere when the expansion of the parts should be graphable / known... wouldn't that leave you with a cold feeler thickness and a hot feeler thickness?

(okay maybe I do mind doing the lash while hot, a bit)

Why not just zero lash all the way around?

See this video from VGG :

Welcoming all opinions before I attempt myself.
Click to expand...


It really depends on your cam and what the manufacturer recommends. That video is for a Chevy, no relevance here. When I rebuilt Ford or Chevy engines, I only ajusted hydraulic cams 1/4 turn in. What cam and engine are we talking about. If you are talking about a 60's slant six and a factory cam, the factory procedure is .010" gap (measured with a feeler gage) for intake valves and .020" gap for exhaust valves hot and running. It is a piece of cake using a ratchet and universal socket. You can't get it any better than 360 degrees on the cam lobe and operating temperature for your engine. You can also do it static and checking every once in awhile and hope you are at the lowest point on the cam lobe.
 
You can set them cold but you'll have to set them hot, first, to get a baseline

EOIC Which means as the EXHAUST is starting to OPEN, adjust that intake, and as the INTAKE has opened fully and is nearly CLOSED, adjust that exhaust

Adjust them carefully, engine stopped, as above, HOT. Install covers, warm it up again, RECHECK. Make CERTAIN they are where you want them to be and NOTE that setting

Now let the engine cool FULLY, like, maybe, overnight. Now pull the covers, COLD, and carefully document the settings, average them out, etc, and this will be your COLD setting to obtain the correct HOT setting.
 
You have to set them cold before start up. You do that manually rotating the crank and making sure you have enough clearance. Do not average anything. the gap within .001 of an inch should apply to each and every gap between the rocker tip and valve stem. Usually cold settings are .002 larger than hot settings on a cast iron head. Maybe less on a slant six.
 
Why not do it per the Factory Service Manual?

It was written for a reason.
 
I adjust them cold. Have no problem. I don't like getting burned.
 
I have set them cold and it usually works ok… but the engine seems quieter and smoother when you set them hot. I’ve never tried it with the engine running, just one at a time going through the firing order.
 
First few times I did it cold and then with the engine warmed up but shut off and my engine always ran rough.

Used slantsixdan's procedure and my engine ran much smoother. IMO it's easier to do it this way over the engine not running. People think they're going to get oil everywhere until they try it and realize that oil doesn't spritz out at all when the engine is idled down.

 
If there is a difference between how the engine runs [ smooth or rough, post #13 ], it means the lash is different between both methods.
I suspect with an all iron engine, there will be little or no difference between the hot/cold setting.
 
Bewy said:
If there is a difference between how the engine runs [ smooth or rough, post #13 ], it means the lash is different between both methods.
I suspect with an all iron engine, there will be little or no difference between the hot/cold setting.
Click to expand...
Exactly. People who say otherwise have wild imaginations.
 
Bewy said:
If there is a difference between how the engine runs [ smooth or rough, post #13 ], it means the lash is different between both methods.
I suspect with an all iron engine, there will be little or no difference between the hot/cold setting.
Click to expand...
On a V8, it is about .002 larger when cold. That is with small block cast iron heads and factory rockers. Will it make a difference, who knows. On a six, probably less change in gap. If I'm going to run a solid cam, and I always do, why set the gap just close enough? I'm not running "Stock Class", I'm going to set it how the cam card tells me to.
 
Last edited:
Bewy said:
If there is a difference between how the engine runs [ smooth or rough, post #13 ], it means the lash is different between both methods.
I suspect with an all iron engine, there will be little or no difference between the hot/cold setting.
Click to expand...
suposedly .004 on a cast iron head, .006 on an aluminum head= cold
I have been setting my solid roller cam/alum. heads at .006 tighter for 12ish yrs. , the one time I checked them they were right on , have had no problems ...
 
Argh this again.. comes up about every other month. I've always set mine "close" cold and started them up and adjusted them hot and running for the "dial in". Never been burnt by slinging oil, a stock ish /6 could always be idled down around 500 for purposes of this adjustment, which cuts down further on the slosh which is less than many would think to begin with. Takes a little getting used to wrenching on moving parts but in this case not all that bad.but you do what you want
 
Alright - I set the lash with the engine running and hot, and my idle is now able to lower, and is much smoother than before. I still have a bit of a blowby issue; e.g. seeing very light smoking from the crankcase, oil smells like gas - but I'm going to do an oil change as the oil is older than my ownership, and hope it improves. I need to redo my compression test now that valves are correct. Thanks to all for the help!!

Also, I should note, it's about 100x easier to do while running as you don't have to rotate the engine over, and you're only holding the wrench on the rocker for a moment at a time.
 
Also often overlooked, is you can usually idle those sweet 6s down to 500ish rpm, to make adjustment a little easier while running.
 
supposed
Bewy said:
If there is a difference between how the engine runs [ smooth or rough, post #13 ], it means the lash is different between both methods.
I suspect with an all iron engine, there will be little or no difference between the hot/cold setting.
Click to expand...
supposed to be .004 ...research it /cast heads....006/alum. heads-------------cold...
 
famous bob said:
supposed

supposed to be .004 ...research it /cast heads....006/alum. heads-------------cold...
Click to expand...
On the slant 6, it's about a .002" difference from cold to hot. Not enough to affect it one bit.
 
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