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Checking valve clearances in the cold

3K views 11 replies 7 participants last post by  Toro-8-2-4 
#1 ·
I noticed my exhaust was turning a bit red while running at night. I checked the exhaust valve clearance on a Tecumseh HMSK90. It has been cold out, like in the 20's. The spec is .004 to .008. I measured .003 today. What adjustments should I consider for the cold temps. I am thinking I will lap it for a few minutes and measure it again.....should I am for the high end of the clearance when I go to grind the end of the valve?


This is a fixed jet carb so I can not make the mixture a bit richer. Just in case you were thinking that.
 
#2 · (Edited)
I wouldn't worry about trying to account for cold temperatures, when measuring.

The clearance seems to gradually increase with these, over time. So aiming towards the larger side of the spec is probably not a bad idea. Too-large clearance will make the compression release less effective, so harder to pull. Too-small clearance will make the valve stay open when hot, which is bad, as you've seen.

Just don't take too much off the valve stem! It's a lot harder to put the metal back on :)
Edit: Just checked here:

https://www.engineeringtoolbox.com/linear-thermal-expansion-d_1379.html
If the valve was 4" long, it would be 0.0024" shorter at 20F, vs 70F. So depending on the valve's actual length, maybe it is worth factoring in something for temperature. But as your symptoms do sound consistent with the valve staying open a little, I wouldn't consider your 0.003" clearance to "really" be a 0.005" clearance, for instance.
 
#7 ·
Good post. Just to expand on this a little bit (no pun intended), there are two basic problems that cause valve lash (clearance) to change: 1) the valve is steel while the engine body is aluminum, and they expand at different rates and 2) the valves will get far, far hotter than the overall engine temps.

For those curios, steel expands at the rate of ~ 6.5 millionths of an inch, per inch of length, per degree F of temp. change. For example, a 3" long valve, running at 900F and starting at 70F will have a temp. change of 830 degrees F. So 0.0000065 inches times 3 inches times 830 is ~ 0.016" or sixteen thousandths of an inch. There goes all the clearance and then some. But Aluminum expands at twice that rate so while the aluminum engine will not get nearly that hot, it will expand at twice the rate, and there is a lot more engine length than there is valve length. So the idea is to get as close to zero lash as possible without the possibility of actually getting to zero under the worst operating conditions of the engine. As was already mentioned by the quote below, the amount of lash is not important, what is important is that there is some lash.

Brian
 
#3 ·
Interesting. There is more to it than just the expansion and contraction of the valve. There is a whole stack up of components that determine the valve lash. Some are aluminum and some are steel. But to think practically, it can't be that much. I am going to aim at removing about .003 and go from there.


I had one person tell me some glowing mufflers are normal. Never heard of that on a newer engine. Maybe in a 1970's or older design.
 
#4 ·
You Can Increase the Valve Clearance without Tearing everything apart. Remove the carb, and the valve cover. Then get a file between the valve stem and lifter, (The spring will keep tension on both surfaces) and file away. Or, You can Tear it apart. (Stick a magnet in when you are done to secure filings.)
 
#5 ·
Agreed that the other components will expand and contract with temperature changes, so the net effect of temperature change when setting the lash will probably be less. I wouldn't even want to start to do that complete calculation. I grind to the center of the spec, and focus on getting a clean, square grind on the valve end.

Note that the Tecumseh technician manual says "Valve clearance (between the valve stem and valve lifter) should be set or checked when the engine is cold."
 
#6 · (Edited)
Agreed that the other components will expand and contract with temperature changes, so the net effect of temperature change when setting the lash will probably be less. I wouldn't even want to start to do that complete calculation. I grind to the center of the spec, and focus on getting a clean, square grind on the valve end.
Note that the Tecumseh technician manual says "Valve clearance (between the valve stem and valve lifter) should be set or checked when the engine is cold."
Jack, Funny you made this suggestion. As I was taking it apart I had that exact thought for a few seconds. I was wondering how hard the valve end may be and would my file be able to cut it? I ended up taking it out

and doing it on my bench grinder. Maybe next time.

Paulm,

Right, but how cold? Inspection rooms are suppose to be at 68 deg. F. as it can matter.

Ideally if you could chuck it up in a lathe and take a nice smooth and square cut you could be done fast and you have good control and surface finish.
 
#11 ·
Good post do to all the great replies. I would now definitely try the file method first suggested by Jack. I will have to make sure I have a decent file for that application. if it is double sided you can remove some material off of the valve and lifter. When grinding by hand it can be tough and go. It does not take much to remove too much.



There is some value in taking the head off sometimes. You can inspect the full condition of the valves, guides and the seats. You can clean up any carbon build up and you can inspect the cylinder condition. Sometimes you are just pressed for time and you can do with out the extra work.

Also you risk damaging the head gasket removing the head.
 
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