[DeTomaso] Rocker geometry problem solved

[cengles at cox.net]

Dear Doug, 

A nice piece of detective work and a problem nicely solved. Thanks for sharing with the amateur engine builders. I will file this in the notebook for the next engine as one more detail to check in the valve train. 

Warmest regards, Chuck Engles


---- doug351c <doug351c at gmail.com> 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
>