[DeTomaso] Rocker geometry problem solved

[John Taphorn]

Doug

Nice catch.  I am surprised it broke the rocker rather than bending the 
pushrod.  Pushrods are tougher than I give them credit.

Thanks for sharing your experience.

JT
----- Original Message ----- 
From: "doug351c" <doug351c at gmail.com>
To: "DeTomaso Forum" <detomaso at realbig.com>
Sent: Thursday, December 30, 2010 2:37 AM
Subject: [DeTomaso] Rocker geometry problem solved


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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