[DeTomaso] Rocker geometry problem solved

Thu Dec 30 14:10:41 EST 2010

Maybe a high heat-treat in the rocker and enough cycles to cause fatigue 
failure.
Does the fracture surface look like it has a bunch of stripes or 
beach-marks?

http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/anal/kelly/fatigue.html

dave

On 12/30/2010 10:54 AM, doug351c wrote:
> 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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