Head studs hold your head and block together. As you make more power than your engine was designed to hold from the factory, the head and the block push apart from each other, stretching the factory bolts, and causing various failures. ARP Head Studs are stronger than the factory fasteners, and are designed to keep your race engine together under any circumstance.

WHAT IS CUSTOM AGE 625+?
You may have heard of the popular L19 upgrade for Head studs, Main Studs, Rod Bolts, etc. The L19 material is stronger than the ARP 8740 or even ARP 2000 materials, however L19 is difficult to work with. You cannot touch L19 with your bare hands or the oils from your fingers will corrode the studs. CA625+ material is just as strong, offering an increase in clamping force, without the difficult-to-deal-with issues that L19 has. These CA625+ studs will allow you to run even more boost without the head lifting, and will ensure the longevity of your engine even more. If you are looking for the best way to do it, this is the way to go.

Factory fasteners were not designed for performance use. The material chosen by the Original Engine Manufacturer for your engine's factory fasteners was chosen to meet the criteria of a normally driven consumer vehicle at the provided stock horsepower and rev limit. The material is not optimum for a high reving, high heat, high power, race car. 

The factory fasteners were also not designed to be reused. Every time you torque a fastener, it stretches to perform the proper clamping force. If the fasteners stretch too far, they will not longer provide the required clamping force, and can lead to an engine failure. When you take your factory engine apart to build it into your race engine, discard your used factory main bolts and switch to a brand new set of ARP Head Studs to provide your engine with the proper clamping force required to keep all of your parts together in the harshest race environment.

We have seen heads lift and blow head gaskets, pressurize the cooling system, and worse. ARP Head Studs are a very inexpensive part that couldn't be more important to your engine build. Consider them a must.