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STUDS vs. BOLTS...
A TECHNICAL DISCUSSION
ARP's factory Tech Representatives are often asked which is
better, cylinder head studs or bolts. The answer, invariably,
depends on the installation. On many street-driven vehicle,
where master cylinders and other items protrude into the engine
compartment, it's probably necessary to use head bolts so that
the cylinder heads can be removed with the engine in the car.
For most applications, however, studs are recommended. And for
good reason. Using studs will make it much easier to assemble
an engine (especially a racing powerplant which must be serviced
frequently and quickly!) with the cylinder head and gasket assured
of proper alignment.
Studs also provide more accurate and consistent torque loading.
Here's why. When you use bolts to secure the head, the fastener
is actually being "twisted" while it's being torqued
to the proper reading. Accordingly, the bolt is reacting to
two different forces simultaneously. A stud should be installed
in a "relaxed" mode - never crank it in tightly using
a jammed nut.
If everything is right, the stud should be installed finger
tight. Then, when applying torque to the nut, the stud will
stretch only on the vertical axis. Remember, an undercut shorter
stud will have a rate similar to a longer, standard shank stud.
This provides a more even clamping force on the head. Because
the head gasket will compress upon initial torquing, make sure
studs and bolts are re-torqued after the engine has been run.
It is for
good reason that virtually every top professional engine builder
relies on ARP Pro Series head studs for their all out competition
powerplants. Simply stated, there's not a better stud setup
on the market today.
For openers, ARP uses a premium grade 8740 alloy that is rated
far superior to "aircraft" quality. Each stud is placed
vertically in special racks and precisely heat-treated to 200,000
psi. This procedure ensures complete heat penetration and the
results are far superior to those lesser quality studs from
other manufactures who just dump pieces in a packet and hope
for the best.
Following heat-treat, each stud is centerless ground to make
it as close to perfectly concentric as possible. This procedure
involves about ten very slight cuts and results in an exceptionally
straight part. It's important to note that lesser quality studs
are not even centerless ground - the material is thread rolled
in bar stock form (mostly before heat-treat, when the material
is easier to machine). Because ARP studs are manufactured to
such exacting tolerances, you will note that gaskets and cylinder
heads literally glide into position and are perfectly aligned
- something that won't happen with inferior quality head studs.
ARP studs are thread rolled after heat-treat, which gives them
about 2000% (that's twenty times) better fatigue strength than
those studs that are threaded prior to heat-treat (a very common
industry practice). It costs a lot more to do it this way, because
it's tough on tooling, but the results are well worth the extra
You will also note that ARP offers specially undercut studs
for several engines. This procedure (done only to the shorter
studs) more equalizes the "stretch" of both studs,
which makes for a more consistent clamping force - and one that
compensates for head gasket compression when the cylinder heads
are installed. This helps prevent blown head gaskets, and assures
optimum engine sealing!
Premium quality heat-treat 8740 chrome moly steel head stud
kits are available for most every domestic and import applications.
You won't find a better quality stud on the market from any
other source. Look for ARP stamped on each stud as your assurance
of quality. Clearly, they are the best on the market today,
and the favorite of leading professional engine builder in all
forms of racing.