[DeTomaso] Stupid question

[Asa Jay Laughton]

Nice Questions Will, I'm sure Dan Jones will set us all straight.

In a nutshell, thermal expansion.

In a solid lifter arrangement, the cam grows, the lifter grows, the 
pushrod grows, etc.  If you are set to zero lash, then if just one part 
grows .001 your valve will now be open when it should be closed, causing 
an intake leak or an exhaust leak, take your pick, either one is bad.

In a hydraulic lifter arrangement, the lifter is designed to take a 
certain amount of preload, because the center of the lifter is built to 
move.  Thus when all the parts heat up, expand and grow, the pushrod 
simply rides a little further down in the lifter without keeping the 
valve open.

Why do hydraulic lifter cam lobes wipe?
Because someone adjusted the rockers too tight, typically bottoming out 
the lifter.  This is why it is so critical to properly prep hydraulic 
lifter prior to installation, AND to pay very close attention to the 
preload.  Here is the typical problem:

351C stock rockers are bolt-down for use on hydraulic cams.
You take the heads to a local shop to have a valve job.
They don't pay attention to valve stem installed height.
You bolt the rockers back down (not knowing some valve stems are now 
taller than they should be).
You (inadvertently) have now taken all the collapsible distance out of 
the lifter (you've bottomed it out cold, essentially making it a solid 
lifter with zero lash).
You start the engine and it all runs fine, until the heat starts 
expanding everything and then it starts wiping the lobe on the cam 
because there is no clearance anymore.

Why do some cams require different lash than others?  It depends on a 
lot of factors, but generally if it's the same engine, lash should be 
about the same.  Now if we start talking about radical difference in cam 
profiles, some of the lash may be calculated for specific moments of 
inertia to help prevent valve float.  (That momentum thing you were 
dreaming of).  This is where my experience and knowledge base gets 
fuzzy, so I'll let better experts chime in here.

Hopefully this helped answer some of your questions.  Don't let them 
keep you up late at night.
:)

Asa Jay

Asa Jay Laughton, MSgt, USAFR, Retired

& Shelley Marie
Spokane, WA

1973 Pantera L 5533
[ASASCAT]
    
******************************     
http://www.asajay.com
http://www.351c.info
  



Will Kooiman wrote:
> I have a stupid question that has been bugging me lately.
>
>  
>
> Why does a solid lifter camshaft require clearance (lash)?  In other words,
> why can't you install it at zero lash - or at least near zero?
>
>  
>
> Of course, I know you can't, but why?  If anything is going to move around
> at high rpm, it should create too much lash, not too little, right?  For
> example, you're not going to hit 7,000rpm and suddenly have your pushrods
> grow 0.250.  They might flex a little, or you might have valve bounce, but
> that would increase the lash, not decrease it.
>
>  
>
> We all know, a hydraulic lifter camshaft has zero lash.  How can it get by
> w/out lash, but a solid lifter camshaft can't?
>
>  
>
> Is a solid lifter camshaft installed with lash, because it can be?  (sorta
> like the joke, why does a dawg lick. nevermind)
>
>  
>
> The reason it has been bugging me is because I was thinking about wiped
> camshafts.  How many of these are hydraulic, and how many are solid?  All of
> the incidents I can remember have been hydraulic.  Maybe solid lifter
> camshafts are less prone to wiping because of the lash.  It makes sense
> anyway.  Hydraulic camshafts get rubbed 360deg by the lifters.  Solids only
> get rubbed about 140-150deg  (duration @ lash / 2)
>
>  
>
> A bonus question is, why do different solid camshafts require different
> amounts of lash?  I'm guessing it's because that's what they were designed
> to have.  Okay.  Why?  Why not grind everything for a lash of 0.020?  What
> advantage would there be to grinding a cam for 0.025?
>
>  
>
> Another idea I had was to allow the lifter to build up some momentum to act
> on the valve, but that's starting to sound silly.
>
>  
>
> Like I said, they're stupid questions.  Now you know what I'm thinking about
> on the way home from work.
>
>  
>
> Will.
>
>  
>
> _______________________________________________
>
> Detomaso Forum Managed by POCA
>
> Archive Search Engine Now Available at http://www.realbig.com/detomaso/
>
> DeTomaso mailing list
> DeTomaso at list.realbig.com
> http://list.realbig.com/mailman/listinfo/detomaso
>
>
>