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Re: [T3] Further research and experimenting material!

On 6 Nov 2001, at 22:37, Hal Sullivan wrote:

> At 08:32 PM 11/6/01 -0600, Jim Adney wrote:
> >The way I see it, however, the biggest contribution comes from the
> >differental thermal expansion of the various parts and the fact that
> >the cylinders and head studs are steel while the pushrods are
> >aluminum (which expands 3x as much as steel.)
> Now *this* I want to check out.  I'm using chromoly pushrods in one of my 
> aircooled VWs --  I'm going to have to start measuring my valve lash both 
> cold and hot, keep a chart of what I find.
> I've heard theories that the reason for the valve clearance is due to the 
> different expansion rate of the aluminum pushrods relative to the rest of 
> the engine.  Following that theory, chromoly pushrods should expand at the 
> same rate (or nearly so).  If that's the case, would the 004/.006 clearance 
> be as necessary in engines that use chromoly pushrods?

I thought one of the things that Berg mentioned in his writing was that there 
was no way to get the steel pushrods to adjust correctly. It is certainly true 
that we wouldn't expect them to need the same adjustment.

The simple picture I painted above is complicated by the fact that the 
cylinders get HOT while the pushrods only get warm. They have oil flowing 
thru them while the cylinder walls are probably the hottest part of the engine. 
Using aluminum in the pushrods is a clever way to make up for the fact that 
the pushrods don't see as much change in temp as the cylinders. This way 
both parts see about the same amount of expansion, thus minimizing the 
change in the clearance in the course of warmup.

Berg mentions that in the course of warmup our engines start out with the 
normal valve clearance, this clearance then drops to zero during warmup, 
then increases back to the original setting when fully warm. This clearly has 
everything to do with the fact that not all parts of the engine warm up 
together; some parts take more time to come to equlibrium than others.

In my mind it seems to me that the clearance should go the other way during 
warmup (increase at first, then decrease), but I haven't done the 
measurements while Berg has. OTOH, maybe I'm remembering this wrong.

I'd like you to look into the settings with the steel pushrods just to see if they 
are actually a good idea for street use, and, if so, what should the valve lash 
be when you use them. The only reason I can see to use them would be to 
support higher RPMs and/or stiffer valve springs. Sounds to me like this 
might be a good thing for drag racers, but not for most of us.

Jim Adney
Madison, WI 53711-3054

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