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Farmall & IHC Tractors Discussion Forum
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valve clearance hot vs cold

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sgtbull

07-31-2007 19:49:02




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Okay, I realize that valves are usually set "hot", on most tractors, because there is a difference in the clearance of hot and cold valves.... but my question is, for an initial setting, should you set the valves a thousandth or so less, or more, to compensate for the dimension change? I know heat expands metal, and it would seem that the "hot" clearance would be less than the cold clearance, making me think if it said, say, .015 hot, I should set it at say, .016 or .017 cold.... is my logic right?

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gene bender

08-01-2007 01:59:29




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 Re: valve clearance hot vs cold in reply to sgtbull, 07-31-2007 19:49:02  
Yes the difference between .014 and .015 is sure not going to make any difference on a 113 cu in engine running 1500 rpms. We aint running at a dragstrip. If its a big deal then we better get a dial indicator set-up and set them running when hot.



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Janicholson

07-31-2007 22:39:35




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 Re: valve clearance hot vs cold in reply to sgtbull, 07-31-2007 19:49:02  
Between the base circle and the lobe on a solid lifter,or roller lifter cam, there is a ramp. This ramp is not really visible as a feature to look at with ones Eye, it is a design feature that takes up the clearance built into the valve train design. It is the height of the valve train clearance at running engine temperature condition of the engine. When the valves are set to the correct spec., this ramp top will be just at the contact patch of the lifter when the valve is spposed to start opening. It will have only provided a slow reduction in slac in the system keeping noise, and impact related wear to a minimum. If too loose on adjustment, the ramp is not able to eliminate the slack, and the valve opens late (as well as the ticking sound of a loose adjustment. If clearance is too tight, the valve starts opening (just a crack) too soon, allowing hot gasses to whistle past the seat and face Providing erosive high temperature damage to seat and face. Thus it is reasonable to be fussy on the adjustment, and if specified as hot, the engine should be at full operating temp then the VC removed and adjustment made as fast as is reasonable to do it. Engineers and cam designers are the ones deciding on the cold adjustment by setting multiple engines hot, then letting them cool to ambient and measuring the actual clearance "cold" then averaging multiple readings to give a spec. A Crane Cam designer discussed this with a group of us in the design department of Crane in 1969, JimN

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Tom Windsor

07-31-2007 23:20:29




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 Re: WOO THERE in reply to Janicholson, 07-31-2007 22:39:35  
1969---you wrote it down? I cannot remember what happened yesterday? :-)



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GordoSD

07-31-2007 20:12:33




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 Re: valve clearance hot vs cold in reply to sgtbull, 07-31-2007 19:49:02  
I read some of the old IH Blue Ribbon manuals on this. In their vernacular 'hot' was in reality 'warm'. As in a nice warm summer day, say 80 degrees, engine soaked at that temp. 'Cold' was a very cold soaked engine, down to as low as 25 degrees F. Just set them at .017 on a nice warm afternoon. Make sure you go all around after the settings and retorque everything. Bet you will find one you missed, and that one will back off and cause you all kinds of problems.

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K.B.-826

07-31-2007 20:36:45




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 Re: valve clearance hot vs cold in reply to GordoSD, 07-31-2007 20:12:33  
Yep, I agree. Most engine manufacturers don't even specify hot or cold anymore for the same reason you mentioned, and because some of these machines require so much disassembly to get down to the valve cover it would pretty tough to do with the engine at full operating temp.



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ScottyHOMEy

07-31-2007 19:58:06




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 Re: valve clearance hot vs cold in reply to sgtbull, 07-31-2007 19:49:02  
The heat will stretch out both your valve stems and your lift rods, and shorten up both sides of your valve system, closing up the clearance. How much I couldn't say, but your thining is on the right track. I shouldn't think that a couple of thousandths would hurt anything for any longer than it would take to warm it up and get them set right.



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Bob

07-31-2007 21:46:31




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 Re: valve clearance hot vs cold in reply to ScottyHOMEy, 07-31-2007 19:58:06  
Don't forget the engine block also expands, pretty much canceling out the expansion of the pushrods, and the head also expands, though not as much as the valves 'cause it doesn't get as hot as the valves. So, actually, the valves "tighten up" very little as the engine warms up, at least in my experience.



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sgtbull

07-31-2007 20:04:24




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 Re: valve clearance hot vs cold in reply to ScottyHOMEy, 07-31-2007 19:58:06  
That's what Iwas thinking, but you know how it's nice to know someone ELSE is thinking the same way... there have been occassions where I completely missed the obvious. A little reassurance goes a long way.



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