[DeTomaso] Rocker geometry problem solved

Thu Dec 30 13:54:47 EST 2010

No, the rockers broke!  The rear arm of the rocker bodies broke away from
the fulcrum.  I'm using high quality 3/8" diameter .083" wall pushrods from
Smith Brothers.  The pushrods were contacting the rocker bodies only about
1/2" down from the steel pushrod cup in the rocker.  For my 8.1" long
pushrods, that's a 16 to 1 leverage ratio helping the pushrod destroy the
rocker body.  The Scorpions are about as good an aluminum forging as you can
get so everyone with aluminum rockers and over about .560 lift should be
watching out for this problem.  If my cam's lift had been only .570 or so
instead of .598, I may have never found out about this problem.  Although
after I discovered this problem I found that I could feel the bind happen
(with the spark plugs out) when I used a wrench to hand turn the engine.

Doug Braun
blue 73L #5505

-----Original Message-----
From: detomaso-bounces at realbig.com
[mailto:detomaso-bounces at realbig.com]On Behalf Of Asa Jay Laughton
Sent: Thursday, December 30, 2010 7:17 AM
To: detomaso at realbig.com
Subject: Re: [DeTomaso] Rocker geometry problem solved

Wow, thank you for the photo, I was clearly confused by your text but
the photo brought it all together. My only question back to the
manufacturer would be, why? If they know the extra machine work is done
by many racers (but it voids the warranty) why don't they manufacture
them that way? I guess I know the answer though, if the warranty is
voided, you'll have to buy new ones if you break one.

I also don't quite understand how the rocker failed. Was it pulled off
the valve stem by leverage of the pushrod against the back of the rocker
arm fulcrum?

Asa Jay

Asa Jay Laughton, MSgt, USAFR, Retired
&  Shelley Marie
Spokane, WA
******************************
http://www.racingagainstautism.com
http://www.teampanteraracing.com

On 12/30/2010 00:37, doug351c wrote:
> Here's a warning for you home engine builders out there to check for a
> rocker geometry problem that bit me.
>
> The #2 and #6 intake rockers on my fresh 393 Cleveland build broke within
> minutes of each other while just easing down the road during the break-in
> period.  RPM's had been kept below 3500 to this point.  The witness marks
on
> the top of the valve stems showed the roller tip wear to be nearly dead
> center on the stems. The pushrods for the broken rockers rolled dead flat
on
> a piece of glass.  A rubber hammer test on the two valve stems gave the
> characteristic "tink" when struck indicating that the valves weren’t stuck
> and were returning to their seats.  A leak down test showed that all of
the
> valves were seated and none were stuck open.  A check of the installed
> height of the beehive valve springs, for the two rockers that broke,
yielded
> 1.903"&  1.916" vs. the Comp Cams recommended height of 1.900".
>
> Contact me off-line if you want to try to guess at this point what caused
> the rocker failure.  If you're stumped, read-on.
>
> I called Scorpion and they had me send the entire set of rockers to them
> which they replaced with a new set under their lifetime warranty (way to
go
> Scorpion!).  Meanwhile, per a Dan Jones suggestion, I purchased a stock
> 302-HO hydraulic roller lifter and machined a brass insert to convert it
> into a solid lifter.  When the new rocker set arrived, I used the solid
> roller lifter to check for valve train binding.  What I found is that the
> pushrods were binding on the back sides of the rockers as they approached
> full lift.  I called Scorpion and had a long talk with Gordon Johnstone.
He
> said that racers grind the backs of their aluminum rockers all the time
but
> it voids the warranty and to shoot for at least 0.010" clearance at max
> lift.
>
> After studying the problem a while longer, I hit on a great way to test
for
> pushrod clearance.  Using my home-made solid roller lifter, I studied one
> intake rocker at max lift and marked the tangent point of the pushrod on
the
> side of the rocker body.  I then removed the rocker and used electrical
tape
> to hold a piece of 0.028" diameter lead-tin solder wire across the back of
> the rocker body at the previously marked max-lift pushrod tangent point.
I
> reinstalled the rocker, turned it through one cycle then removed the
rocker
> to inspect the solder.  The solder was squished from it’s nominal 0.028"
> round cross section down to a flattened mess!
>
> Clearly, I needed more clearance Clarence!  I took the 8 intake rockers to
> the Bridgeport milling machine I have access to at work and with a 7/16"
> diameter end mill, I opened up each rocker's pushrod relief until the
solder
> test showed at least 0.020" clearance.  The same solder crush test on the
8
> exhaust rockers all showed greater than 0.020" clearance due to the lower
> amount of exhaust valve lift of my cam.  All is well now and the engine
runs
> great.
>
> Here's a shot showing the solder taped across the pushrod relief in one of
> the rockers.  You can see how the pushrod slightly squished the solder
(this
> one measured at 0.021" clearance).  It also shows the additional relief I
> machined into the rocker body.
>
> http://www.poca.com/index.php/gallery/?g2_itemId=28305
>
> Doug Braun
> blue 73L #5505
>
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