ARP studs are a way to avoid having to install new head bolts every time your cylinder head has to be taken off the engine. It's a fairly expensive solution. But especially if you mount a separate intake manifold, it is the only safe way to get longer bolts / threaded ends. An alternative of course is to drill the block and head from 9mm to 10mm and use thicker head bolts. But this also costs money.
You can recognize ARP threaded ends by an hexagon hole in the head. The other end is machined flat and usually has a raised edge at the end of the thread on that side.
Additional installation instructions apply for the use of ARP threaded rods for the cylinder head:
- For example, ARP Thread Sealer must be used in the threads in the block. This is to prevent coolant from creeping up when the engine is warm and not running. I wrote an article about it before (thanks to Paul Vanderheijden).
- In order for this paste to work properly, it is highly recommended to thoroughly clean the threaded passages with an M9x1.25 tap. If you can turn the tap in and out by hand, the screw thread is clean enough.
- Then brush out the last bits of oil / grease with a round brush and brake fluid and dry the threads, preferably with compressed air.
- Next the threaded ends have to be thoroughly brushed clean in a warm bath with plenty of soap, rinse with water and rub dry.
- Now coat the bottom half of the wire with Thread Sealer (or an alternative, for example LOCTITE 242 Blue Threadlocker). This should be a thin layer.
Tip: do this with two studs held together and rotate the greased parts against each other so that the thread is filled with a thin layer.
- Ligthly tighten the studs into the block and use an Allen wrench to tighten them just a bit more.
- Remove excess Thread Sealer from the studs and the engine block.
- Clean the deck of the block and the cilinder head with brake cleaner or a similar fluid (do not use benzene or a paint thinner), and put the head gasket in place.
Avoid to touch the cleaned surfaces and place the head.
- Lubricate the washers on both sides with ARP Thread Lubricant put them in place.
- Lubricate the nuts an tighten them by hand. Next, tighten them with a torque wrench. This is done in 3 steps:
For a 9mm stud in highest quality (ARP-3, my definition) the end torque is 65Nm. These steps are, for example, 20Nm, 42 Nm and 65Nm.
This is a bit lower than the recommende torque of 69Nm applicable for an A112 engine. Please note: ARP nuts have a Torx head. I had to use a 7/16" socket for this:
Have a look at the graph I made using data from the ARP manuals