[Fot] Valve Lash While Engine is Hot

[Michael Porter]

On 4/11/2012 8:04 PM, Tylerpthompson at yahoo.com wrote:
> My engine has a Paeco TR-6600 cam.  The cam card calls for a .014 intake/.016
> exhaust valve lash.  A little looser than stock, but not by much.  I know
> folks running larger clearances on the hotter cams and wonder if these
> clearances may be a bit too tight.
>
> That said, In talking with a fellow racer tonight who will remain nameless (I
> don't want to name him in case I get a lot of back "lash" from this question)
> he thought I should ask the FOT group if anyone has measured lash on a hot
> motor after a race.  We expect it to be tighter, and perhaps there is an
> optimum clearance we should be shooting for.  Just a thought in case my valve
> lash is too tight.
>
>

I'm kind of surprised Larry Young hasn't weighed in on this one.  Here's 
his Tilden Tech page on valve lash:

http://www.tildentechnologies.com/Cams/Tip_Lash.html

Larry does make a point about the difference between opening quickly and 
acceleration (i.e., the two are not equivalent).

As for lash hot/cold, it's a lot easier to get uniform settings doing it 
cold, if only because one has to move like the bejesus to get (in the 
case of a 6-cyl.) twelve valves adjusted while the engine's still hot, 
and there's always the worry that the engine will cool enough that some 
will end up being too loose. It's fine to /check/ them hot against the 
cold settings, and calculate the best cold setting.  As for any uniform 
rules, engine to engine, about them getting tighter or looser with 
temperature, or by how much, I doubt it.  When I was doing a lot of 
air-cooled VW engines, it occurred to me that a stock cold lash of 
0.006" didn't seem like much, given how hot the exhaust valves ran 
(thinking that the stems would grow quite a bit).  Not so.  The heads 
got a good deal taller, and the hot lash was ~ 0.016".  All-iron 
engines, on average, probably grow less, because the coefficient of 
expansion is ~ 1/4 that of aluminum.

Another tip on the lash is to have a really critical eye about the 
rocker noses. Even though they're hardened, they have a tendency to 
pound down after a while, and if there's even the slightest cupping, 
you'll find it nearly impossible to get the lash right with an ordinary 
shim stock gauge.

Cheers.


-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....