L/89 longer rocker arm studs

[Steve Antonucci]

I'm currently building an L/89 ( L/71 ) 427 that will most likely have a
more radical cam than stock configuration. My concern is over the longevity
of the rocker arm stud bosses. Original big block rocker arm studs have a
standard thread length ( not sure what that is ) and I have heard that GM
offered a longer thread version for the more radical cam applications. This
could possibly have included the L-88, ZL-1, and LS-7 engines.

I have seen numerous sets of original big block aluminum heads that have one
or more rocker arm stud bosses welded due to cracking. Sellers routinely
refer to using rocker arm studs with longer threaded sections. I'd be
interested in hearing from members with experience in this area and possible
solutions to this problem. I have a pair of pristine 3919842 heads for this
project and I don't want to booger them up.

Does such a rocker arm exist from GM? If so, what is the part nuimber?
If they do indeed exist, what factory engines would have been equipped with
them?

Thanks everyone,
Steve

[Clem Z.]

Re: L/89 longer rocker arm studs

the longer in the head thread stud are after market ones,GM never sold any. these were used when using roller rocker because the pivot point is up higher than the stock GM stamped steel ones causing the side load up higher and pulling the studs from the aluminum heads. the same for using higher pressure valve springs and the best fix is a stud girdle the takes the side load off of the studs

[Steve Antonucci]

Re: L/89 longer rocker arm studs

Clem,

Thanks for clearing that up for me.

A stud girdle implies the use of roller rockers - right? This also implies
the use of non-standard chrome big block valve covers for clearance - right?

Steve

[Joe L.]

Re: L/89 longer rocker arm studs

Steve-----

Only 1 rocker stud has been available from GM for Mark IV big block applications since 1968. That stud, which replaces earlier studs GM #3860006 (65-66) and GM #3904395 (1967), is GM #3921912. This stud has an overall length of 2-1/2" and a 7/16-20 thread length of 7/8". It can be purchased individually under that number or as a set of 16 under GM #12495498.

GM advises that whenever the GM #3921912 stud is used on aluminum head applications that a Helicoil thread insert be installed in the head.

The above stud was used for 1968-69 L-88/ZL-1 applications so it will certainly handle any camshaft lift equivalent to that used for those engines. Beyond that, I don't know (I don't know who in their right mind would use a camshaft more radical than the L-88 and ZL-1 camshafts, either).

Aftermarket studs are available from a variety of sources which exceed the strength of the factory studs and have longer overall lengths and longer thread lengths. ARP is one such source. However, ARP studs are not approved for use with self-locking, OEM-style nuts. The GM #3921912 studs have insufficient upper thread length for use with roller fulcrum style rockers and poly lock nuts. So, if using self locking nuts with stock-style rocker arms, you should use the GM #3921912 studs; if using aftermarket style roller fulcrum rockers with poly locks, ARP or other aftermarket studs are best.

As far as stock-style rocker arms go, the best to use is GM #3959182. These were used for all SHP 1969+ big blocks, including L-88 and ZL-1, and are SERVICE for older SHP big blocks. They're available individually under that part number or as a set of 16 under GM #12368082. These rocker arms are stamped "H" and are "long slot" for high lift cams.

Better still, as far as stamped steel, stock-style rockers go, are the Crane Nitro-Carb rockers.

[Clem Z.]

Re: L/89 longer rocker arm studs

if you are going to use close to stock GM springs,rocker arms and cam you should have no problem. the stud girdle does require taller valve covers. if your heads are used and you do not know their history you could get them dye checked for cracks around the stud bosses.

[Steve Antonucci]

Re: L/89 longer rocker arm studs

Thanks for that great information Joe !

OK, now stupid question-time. If GM recommends that the '912 studs be used in
conjunction with aluminum heads and heli-coils, why don't they manufacture the
heads that way from the start?

Steve

[Clem Z.]

heli coil

joe i have never seen a factory BBC aluminum head without heli coils in the stud holes,have you???

[Joe L.]

Re: heli coil

clem-----

No, I have not. However, I think that the very early ones, like the 3904392 and, possibly, the 3919842 did not have them. I think that those are the heads that GM was referring to in their instruction that Heli-Coils be added.

[Steve Antonucci]

Re: heli coil

Clem & Joe,

Thanks for sheding light on this for me.

As I mentioned in my opening question, I was planning to use my set of '842
heads for this propject. I'm almost positive that they are not heli-coiled
as I believe I would have noticed that detail when I purchased them. I do
have a stock set of the later '077 open chamber Winters heads as well.
And now that you mention it, I believe that they are indeed heli-coiled in
the stud bosses. Imagine that !

Thanks again guys !

Steve

[William Holder]

Re: heli coil

Hello Steve,
I think that the reason the studs pull out of these heads is people running parts that do not mix well. It is Very important to measure that you have enough clearance and the proper Geometry in your valvetrain. A lot of people simply buy parts and throw them together. And sometimes even when you get the parts all from the same manufacturer they do not fit together properly because of other mods to the engine. I am talking about measureing the springs for coil bind, the retainer for clearance to the guide at max lift and the rocker arm slot for clearance at max lift. ( There are probably more I cannot think of right now)IF any of these items are not right, you can break the head on an aluminum head engine. On cast iron heads you just break studs, rocker arms and bend pushrods. This is why the aftermarket is so good at seling high dollar "Stronger parts". The stock chevrolet valvetrain will do 7000 RPM for all day long (Well at least a couple of hours) if it is set up properly. There is a book called "How to hod rod Big block Chevrolets" BY HP books that details all I have said and more. I would highly reccomend it for anyone building any kind of BBC. ALso I just built my 435/427 and I just bought a set of Long slot rocker arms from O reiley's Auto parts and there was enough slot clearance for the .595 inch cam I am running. Also I measured the lift on all 16 at every 90 deg in the cam cycle and they were all very close to the same. I have not been able to drive the car yet though so I cannot vouch for their durability.

[Mark #28455]

October 1967 842 has heli-coils

Just checked my NOS 842 heads dated October 1967, they have heli-coils from the factory.

The aluminum heads commonly break as they are used by racers running HUGE roller cams with seat pressures over 200# and open pressures over 700#. Combine that with the roller rockers that add side loading to the studs and they will pop! The stud girdles like the Jomar do keep the studs in line and decrease breakage (but are a pain in the rear).

I have been running the aluminum heads for quite some time with 125# to 135# seat pressure and about 300# open pressure with cams having up to .600 lift with no durability problems at all even with the stock rocker bolts (even with roller rocker arms). But the key is to keep the total lift and spring pressures reasonable. The steel roller rockers even fit under stock style rocker covers (without the drippers) if you use the thicker gaskets, but if the stock rockers work and are about $200 cheaper.

Good luck,
Mark