1.5 rocker to 1.75
 

Any one out there explain why you will break or pull a 3/8" rocker arm stud going up to a 1.65 rocker when all Chevy six cylinder engines survive quite well with 3/8" studs and standard factory equipped 1.75 rockers with a cam that has about the same intake lift and duration of a 350 Pontiac 2bbl ??

Don

If I am not mistaken, Chevy studs do not neck down on top to the threaded section like Pontiac studs, and that`s where all Pontiac studs snap.
Some years ago I had a Ponti 350 that I installed a .440" lift cam with 1.65 rockers and I snapped a stock stud in the thread area and I had plenty of installed height and guide to retainer clearance.

To run 1.65 my heads needed a lot of work to fit the pushrods. A higher ratio than that might need too much cut out of the head. I think there are very few pontiac heads that can use 1.65 without moding.

It's such a PITA to clearance for 1.65's in the pushrod hole/guide plate area. Seems like a big mistake with all those recomendations in the magazines for decades saying.. "Just install 1.65's" BS ! Easier to keep 1.5's and buy a hotter cam IMO.

Thanks for the reply Steve, you are correct that the chevy stud does not neck down but chevy studs still use the same lock down nut as a Pontiac. That means the shaft is even less diameter than Pontiac where it necks down and then goes down and into the head.

D.

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A little off to my original topic but I used 1.65's on my 455 with #15 small valve heads and now I'm using a set of 7K3's on the same engine with no problems with pushrods hitting the head.
I just want to know why Chevy's with a I-6 and 1.75 rockers don't break rocker studs.



D.

There is an advantage to running 1.65 vs. bigger cam. The 1.65 gives you fast opening like the more modern cams do. Fast opening and large duration/high lift cause more stress on the cam. 1.65 is a way to get higher lift faster opening with less friction at the cam/lifters than that larger cam would have.

This is a tiny bit off topic, but when Chevy brought the LS style engine into nascar in the form of the SB2 the cam diameter was literally double the size of the small block chevy. This allowed the cam to be fast opening radical without the threat off the lifters wanting to "jump". (Nascar does not allow roller yet) Running a bigger ratio rocker is kind the same type of advantage.

FWIW Chevy pinned the 3/8 studs in the hi perf 409s. Ran a pretty big cam in mine with 1.75 rockers and no issues with studs .580/.590 lift 255/265 @ 0.050 solid.

Well some of it may be tracked down to different alloy steel between Chevy and Pontiac?

In regards to fast cam rates and HP, in the last 3 years or so Pro Stock motors have gone to very high cam placement blocks in order to run shorter push rods.
This has let them (1) run less valve spring pressure (2) turn more RPMs (in many cases close to 11,000) (3) run rocker ratios over 2 to 1!!
Top teams are now making over 1400 HP out of 500 CID and have cut valve train related failures by a large %.

Smokey Yunick lobbied for non stock rocker ratios in NASCAR engines, didn't get anywhere, back then, at least. It's somewhat easier to make valve lift at the rocker arm, that at the cam/lifter, especially w/ a flat tappet cam.

Keeping the lifter on the cam, and all the details associated w/ that relationship in a "real" racing engine (as in 7000-9000 RPM's), is something that's still being working on to improve....There are associated risks w/ increasing the rocker arm ratio, but not as much as the lifter/cam situation...

Running a some what mild cam w/ high ratio rocker arms, makes sense to me, for reliability reasons....

I just recieved an article on this topic by email.

http://www.streetlegaltv.com/tech-st...etter-rockers/

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Early Super Duties ran a 3/8" PINNED stud too , and with 1.65 rockers so it must be the metal.
What is needed in aftermarket is a conversion kit to make a standard head carry the cam like MacKellers engine. This would not be hard to do. Even better would be some of these specialty Pontiac aftermarket people to step up to the plate and make the whole head this way.