[DeTomaso] Rocker geometry problem solved

[wkooiman at earthlink.net]

The picture truly is worth a thousand words.  That was a great idea to use a piece of solder.

I've said it before, the 351C is somewhat unique.  It should be considered a requirement to be completely anal about checking for pushrod rubbing.  Mine were rubbing the guide plates where they shouldn't rub.  My rockers were fine (Crane Gold Race).  I didn't notice the guide plate problem until I broke one (a guide plate not a pushrod).

Congrats on fixing it.


-----Original Message-----
>From: doug351c <doug351c at gmail.com>
>Sent: Dec 30, 2010 2:37 AM
>To: DeTomaso Forum <detomaso at realbig.com>
>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
>
>_______________________________________________
>
>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