ARP head studs vs head bolts: what to fit and when

Head studs are the cheapest insurance on a boosted engine and the most argued about part in the shop. Studs beat bolts on the bench and that bit is not in dispute. The useful questions are whether your engine needs them yet, which kit suits it, and what else should go in while the thing is apart.

The short answer

• Any turbo engine that has been turned up past factory boost should be on head studs, not factory bolts.
• If ARP list two kits for your engine, the ARP2000 version is the one for a build that will see serious cylinder pressure.
• Buy the assembly lubricant with the kit. ARP quote their torque figures with it, and without it the number on your wrench is not the clamping load you think it is.
• Do the main studs while the block is bare. Doing them later means stripping the bottom end a second time.

What a stud does that a bolt does not

A head bolt gets screwed down into the block and tightened, so it is being twisted and stretched at the same time. Some of the torque you put through the wrench goes into fighting friction and wind up instead of clamping the head down, and that varies from hole to hole.

A stud goes in the other way around. You wind it into the block finger tight so it sits home without pulling on the threads, then the nut does the work on top. The stud only sees a straight pull, so the clamping load ends up more even across the deck. That evenness is what keeps a head gasket alive when boost climbs.

There is a practical bonus. When the head comes off next time, the studs stay in the block and the threads in the block are not disturbed again. On an alloy head engine that has been apart a few times, that alone is worth the price.

The other half of the story is what you are replacing. Many factory head bolts are designed to stretch as they are tightened, which makes them a one use part. People reuse them anyway, and that is where mystery head gasket failures come from.

ARP or ARP2000: reading the kit names

ARP is American, out of Ventura, California, and has been making racing fasteners since 1968. The naming trips people up because some engines get listed twice.

The Nissan twin cam RB family is the clearest example. There is the standard head stud kit ARP 202-4301 covering RB20, RB20DET, RB25, RB25DET and RB30, and there is the ARP2000 version, ARP 202-4309. Same fitment, different material grade, and ARP2000 is the stronger one. The RB26DETT kit ARP 202-4207 is an ARP2000 kit as sold.

You will also see thread sizes and nut types in the titles. The 4G63 Evo kit and the SR20DET kit are both M11, the Barra XR6 kit is M12, and the 3SGTE kit uses 12 point nuts. That last detail matters when you are working out what sockets you need before the head goes on at 9pm.

Which head stud kit for which engine

The ARP head stud range covers most of what gets built in NZ garages. Toyota six owners want the 1JZ and 2JZ kit ARP 203-4205. Subaru builders want the EJ20 and EJ25 DOHC kit ARP 260-4701. There are separate kits for the 4AGE 16 valve and the 20 valve, and a kit for the Mazda Miata BP and B6.

Order the studs with the head gasket, not after it. A Cometic gasket in the right thickness and a set of studs is one job, and splitting the order across two weeks is how a car ends up in pieces over a long weekend.

Main studs, rod bolts and the rest of the ARP catalogue

Head studs get the attention, but the parts that stop a bottom end coming apart sit lower down.

Main studs clamp the main caps and keep the crank tunnel round under load. The range covers the RB kit ARP 202-5403, the 1JZ and 2JZ kit ARP 203-5405, the 4G63 kit ARP 207-5401, SR20, 3SGTE, 4AGE, 4B11 and the Barra. These go in while the block is at the machine shop, because the mains usually get line honed with the new studs fitted.

Rod bolts are the ones people skip and regret. On a reused factory rod the bolt is normally the first thing to let go at high rpm. There are kits for the 2JZ and 1JZ, the SR20, the 3SGTE, the 4AGE and the early 4G63 M9. If you have bought aftermarket rods, check what they came with. ARP also sell replacement bolt kits for Eagle rods in more than one length, so measure before you order.

Flywheel bolts round it out. Piece counts differ by engine, so the RB26 kit is six pieces and the 4G63 Evo 4 to 9 kit is its own part number. There is also an ARP 2000 crank bolt for the 1JZ and 2JZ, which is a sensible upgrade on anything that has had a harmonic balancer off and on a few times.

Fitting them so the torque figure means something

Clean the threads in the block properly and blow the holes out. Oil or coolant sitting in the bottom of a blind hole will hydraulic the stud and give you a false reading, or crack the block if you are unlucky.

Wind the studs in finger tight. They are not meant to be torqued into the block.

Use ARP assembly lubricant on the threads and under the nut face, because ARP publish their torque figures with that lubricant. A bottle of ARP 100-9910 is enough for several engines. Then torque in stages, in the pattern in the instructions, and go around again after the first heat cycle if the instructions ask for it.

Where NZ Performance fits

We keep the common ARP kits on the shelf in Palmerston North rather than ordering them in per job, so a head does not sit on a bench for three weeks. The ARP fasteners page and the full ARP catalogue cover the wheel studs, cam sprocket bolts and odd bits that never make the build list until you need them.

Building a whole engine rather than replacing one gasket? The RB rebuild kit guide and the JZ and 3SGTE rebuild guide show where the fasteners sit in a full package. For the boost side, head studs, gaskets and pistons for big boost is the one to read next.

FAQ

Are ARP head studs better than head bolts?

For a boosted engine, yes. A stud is threaded into the block by hand and the nut does the clamping, so the fastener is pulled straight rather than twisted as it is tightened. That gives more consistent clamping load across the deck, and you can lift the head later without disturbing the threads in the block.

What is the difference between ARP and ARP2000 head studs?

They are two material grades. ARP list some engines in both, such as the RB twin cam, where you can buy the standard kit ARP 202-4301 or the ARP2000 version ARP 202-4309. ARP2000 is the stronger of the two and is the one to pick when boost and cylinder pressure are climbing.

Can I reuse my factory head bolts?

Often not. Many factory head bolts are designed to stretch as they are tightened, which makes them a single use part, and a stretched bolt will not clamp to the same figure again. If the head is off and the car is boosted, studs are tidier than gambling on used bolts.

Do I need to use ARP assembly lubricant?

Yes, if you want the torque figure to mean anything. ARP quote their torque specs with their own lubricant on the threads and under the nut face, so swapping to engine oil changes how much clamping load you actually get.

Do I need main studs and rod bolts as well?

Not on every build. Main studs are worth doing while the block is bare and the mains are out, which is why they go in during a rebuild rather than after. Rod bolts are the one to sort if you are reusing factory rods at higher rpm, because the bolt is usually what lets go first.