[DeTomaso] Rocker geometry problem solved

Thu Dec 30 13:30:01 EST 2010

I too would have thought the push rod would have bent.  I would double check them ...just in case.  I figure the arms broke between the rocker stud and the pushrod seat?  Right?  Bill 1362
-----Original Message-----
>From: Asa Jay Laughton <asajay at asajay.com>
>Sent: Dec 30, 2010 10:17 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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