Think all fasteners are created equal?
Of course not. If you’re an engine builder, you know the importance of using good-quality fasteners. And you’re probably also familiar with the torque-to-yield, or TTY, fasteners that have become more common on today’s engines. There’s a big difference between these fasteners and traditional bolts that make torque-to-yield the factory choice for modern platforms like the GM LS series.
For DIYers, though, there’s still a lot of mystery surrounding torque to yield bolts. Because the LS platform is one of the most popular among hot rodders, we thought we’d cover the basics of TTY fasteners and they apply to LS engines.
What is Torque to Yield?
TTY head bolts are common on engines using aluminum heads and/or multi-layer steel head gaskets. While all bolts experience some amount of stretch during torquing, TTY bolts are engineered to stretch within a controlled “yield zone.” Once they reach this zone, they spring back to provide a precise level of clamping force over the entire area of the gasket. This stabilizes the load for a head gasket when the engine is both cold and then as it warms up. This is key with aluminum heads where the expansion rate of the material can stretch typical bolts past their yield point and cause them to snap.
Once the torque-to-yield fasteners are torqued, go through different heat cycles, and are subjected to stretching due to material expansion, they never return to their original length.
For this reason, you should never reuse a TTY head bolt.
This video from Fel-Pro illustrates TTY bolt stretch and why you can’t reuse a torque-to-yield cylinder head bolt.
Which LS Fasteners are TTY?
On a GM LS engine, the cylinder head bolts are TTY and cannot be re-used.
The connecting rod bolts are also TTY, but can be tightened up to three times for bearing checking.
The balancer bolt is also a torque-to-yield fastener.
The intake manifold, rocker arm, and rear cover bolts can be reused.
The vertical main bolts can also be re-used, but the side bolts are often replaced because they have sealant on them. These can be re-used if sealant is re-applied.
How to Install a Torque-to-Yield Fastener
It is important for TTY bolts to reach their controlled yield zone for the most consistent clamping force, so the installation procedure becomes critical. Torque-to-yield bolts require extra steps—and extra care—to achieve proper installation.
To get the TTY bolts into the proper yield zone, you need to tighten them to a certain torque spec, and then turn them an additional number of degrees. You should use a torque-angle gauge to ensure that you turn the bolt the exact, specified amount of degrees. Of course, if you upgrade to aftermarket bolts like ARP bolts, then that torque-angle spec cannot be used since the ARP’s fasteners are made from a much stronger steel. ARP supplies a specific torque to use instead.
When installing TTY-style bolts, follow the proper torque sequence and specifications as follows:
First pass all M11 bolts (1-10) in sequence: 22 ft.-lbs.
Second pass all M11 bolts (1-10) in sequence: 90 degrees.
Final pass all M11 bolts (1-10) (2004 and later blocks): 70 degrees
Final pass all M11 excluding medium bolts (1-8) at front and rear of each head (pre-2004 blocks): 90 degrees
Final pass all M11 medium length bolts (9-10) at front and rear of each head (pre-2004 blocks): 50 degrees
M8 inner bolts (11-15): 22 ft.-lbs.
Before you install the bolts, always clean the threads where the bolts pass through, since any corrosion will create added friction. This can yield a false torque reading and prevent you from reaching optimum clamping force.
This graph illustrates the importance of reducing friction.
Fel-Pro recommends lightly oiling the threads and under the heads to prevent binding, but warns against over-oiling the bolts.
By following the proper torque specs and sequence and adhering to the tips above, you’ll ensure maximum performance from your TTY bolts.