The following are useful tips for technicians and
engine specialists who spend any time sealing up engines.
1. Make sure all the head bolts are in perfect condition with clean,
undamaged threads. Dirty or damaged threads can give false torque
readings as well as decrease a bolt’s clamping force by as much as 50%!
Wire brush all bolt threads, carefully inspect each one, and replace
any that are nicked, deformed or worn. If a bolt doesn’t thread into a
hole smoothly with finger force alone, there is a problem.
On aftermarket studs or bolts, never use a die to recut the threads.
Most have “rolled” threads, which are stronger than standard threads.
Use a thread chaser to clean up the threads.
2. Dirty or deformed hole threads in the engine block can reduce
clamping force the same as dirty or damaged threads on the bolts. Run a
bottoming tap down each bolt hole in the block. The tops of the holes
should also be chamfered so the uppermost threads won’t pull above the
deck surface when the bolts are tightened.
Finally, clean all holes to
remove any debris. If the block deck has been resurfaced and you are
using studs rather than head bolts, chamfer the top of each hole
3. For head bolts that screw into blind holes, lightly lubricate the
bolt threads as well as the underside of the bolt heads with engine
oil. Make sure the bolts do not bottom out or hydrolock because of oil
trapped in the blind hole.
For head bolts that extend into a coolant jacket, coat the threads with
a flexible sealer. Failure to coat the threads may allow coolant to
leak past the bolt.
4. Because TTY head bolts are permanently deformed once they are
tightened, they should not be reused. Reusing TTY bolts will cause them
to stretch further, which increases the risk of breakage. A stretched
bolt also will not hold the same torque load as before, which may cause
a loss of clamping force resulting in head gasket leakage.
5.
Check bolt lengths. Make sure you have the correct length bolts for the
application and for each hole location (some holes require longer or
shorter bolts than others). If a bolt is too short and only engages a
few threads in the block, it may pull the threads out of the block.
Bolts should also be measured or compared to one another to check for
stretch. Any bolt found to be stretched must be replaced because 1) it
may be dangerously weak, 2) it won’t hold torque properly, and 3) it
may bottom out when installed in a blind hole.
6. When installing head bolts in aluminum cylinder heads, hardened
steel washers must be used under the bolt heads to prevent galling of
the soft aluminum and to help distribute the load. Make sure the
washers are positioned with their rounded or chamfered side up, and
that there is no debris or burrs under the washers.
7. Resurfacing a cylinder head decreases its overall height, so be sure
to check bolt lengths to make sure they won’t bottom out in blind
holes. If a bolt bottoms out, it will apply little or no clamping force
on the head, which may allow the gasket to leak.
If a head has been
milled and one or more head bolts may be dangerously close to bottoming
out, the problem can be corrected by either using hardened steel
washers under the bolts to raise them up, or by using a copper head
gasket shim in conjunction with the new head gasket to restore proper
head height.
8. Always look up the specified tightening sequence and recommended
head bolt torque values for an engine before installing the head
gasket. Never guess. Complete cylinder head torque specifications for
domestic and import vehicles can be found in service manuals and torque
tables published by gasket manufacturers.
9. Use an accurate torque wrench to tighten standard-type head bolts in
three to five incremental steps following the recommended sequence and
torque specs for the application. Tightening the bolts down gradually
creates an even clamping force on the gasket and reduces head
distortion. It’s a good idea to double check the final torque readings
on each head bolt to make sure none have been missed and that the bolts
are retaining torque normally.
If a bolt is not coming up to normal torque or is not holding a
reading, it means trouble. Either the bolt is stretching or the threads
are pulling out of the block.
With TTY head bolts, use a “Torque-To-Angle Indicator” gauge with a
torque wrench to achieve proper bolt loading. Don’t guess on the angle.
Be exact.
10. If a head gasket requires retorquing (most do not), run the engine
until it reaches normal operating temperature (usually 10 to 15
minutes), then shut it off. Retighten each head bolt in the same
sequence as before while the engine is still warm. If the engine has an
aluminum cylinder head or block, however, don’t retorque the head bolts
until the engine has cooled back down to room temperature.
On some applications with retorque-style head gaskets, it may be
necessary to retorque the head a third time after a specified time or
mileage interval due to the design of the engine. Follow the vehicle
manufacturer’s recommendations.
Did You Know? In an engine with 4” cylinder
bores and peak combustion pressures of around 1,100 psi, each cylinder
exerts about 13,827 lbs. of pressure against the cylinder head at full
throttle. In fact, head bolts may have to handle loads of more than 5
tons per bolt at wide-open throttle!
