Exhaust Manifold Studs

Exhaust manifold and header fasteners clamp a flange that heats and cools by hundreds of degrees on every run, against a gasket that settles with time. Here you will find ARP header bolt and stud kits in stainless steel or 8740 chrome moly, hex or 12-point, plus collector flange bolt kits.

The header flange heats faster and further than the cylinder head, so it grows and slides against the head on each cycle. Every cycle lets the gasket compress a little more, and the fasteners lose clamp load until they work loose or the gasket blows. Studs spare the head’s threads, often in aluminium, from wear at every refit, and they hold the gasket and header in line during assembly.

The kits differ in these ways:

• Bolts or studs: bolts are quicker to fit, while studs protect the head threads and make a header easier to hang.
• Thread: mostly 3/8 in, with M8 and 3/8–5/16 kits.
• Length: match it to the flange. Bolts come in lengths such as .750, .875 or 1.000 in, and some kits are listed for a 1/4 in or 3/8 in flange.
• Head: 12-point heads are smaller for a given thread, which helps between closely packed primary tubes. Some bolts are drilled, usually so they can be lock-wired.
• Material: stainless steel resists the corrosion that seizes exhaust fasteners, while 8740 chrome moly needs protecting from rust in this hot spot.
• Collector flanges: bolt kits are listed by a size range, such as .475–.600 in or 0.600–0.725 in. A few timing stud kits are listed here as well.

Check the header flange with a straight edge before fitting, as a warped flange will not seal however hard it is clamped. Use a high-temperature anti-seize on the threads, especially with stainless, which can gall. Tighten from the centre outwards in stages, then re-check once the first heat cycles have settled the gasket.

Where drilled bolts are lock-wired, wire them in pairs, routed so that any loosening pulls the wire tighter. Soot streaks at a flange or a ticking sound under load point to a leak, so deal with it before the gasket burns through.