Reinforced ARP cylinder head studs Honda H23A

The engine concerned

ARP designed this kit for the 16-valve inline four-cylinder Honda H23A (2.3 litres), fitted to the fourth-generation Honda Prelude.

On the H23A, the standard cylinder head bolts give way to the studs of ARP kit 208-4307.

Reinforced ARP cylinder head studs

Cylinder head fasteners work constantly: they hold the head tight against the block, crush the gasket to the right value and hold back the push of the gases at every combustion. If they lose even a little tension, the gasket starts to leak.

ARP reference 208-4307 is the stud kit designed for this engine: 8740, 12-point nuts, M12 thread. The instruction sheet has you screw the studs into the block by hand, fit the head, then lubricate threads, nuts and washers with the ARP Ultra-Torque supplied in the kit; tightening then takes three equal passes, in the order recommended by Honda. Material of the fasteners: 8740, a chrome-molybdenum steel that ARP specifies at 200,000 psi tensile strength. Factory-fitted fasteners usually fall into classes 10.9 and 12.9, with 150,800 to 176,900 psi minimum tensile strength. In M12, the 8740 studs hold up to 600 hp on this four-cylinder engine (calculated estimate).

Technical specifications

Tightening torque

The studs are fitted into the block by hand, without a torque wrench. For a permanent installation, the instruction sheet accepts thread locker, with tightening to be completed before it sets.

If the block threads call for it, the instruction sheet provides for cleaning them with the ARP thread chaser before fitting.

When a stud runs through to the water jacket, the instruction sheet requires ARP thread sealer on the part screwed into the block.

Final tightening is done in three successive stages, following the manufacturer's tightening sequence; these torque figures only hold with ARP assembly lubricant on the threads and under the nuts.

The instruction sheet also calls for ARP lubricant on the washers.

The torque figures above come from ARP instruction sheet 208-4307, revision applicable on the day the page went online. The official instruction sheet prevails in case of difference and describes the complete procedure, tightening sequence included. Read the ARP 208-4307 instruction sheet

Technical guide

All torque values on this page are given with ARP assembly grease on the threads and under the nuts; another lubricant would make them wrong.

The ARP brand

ARP (Automotive Racing Products) was born in the early 1970s from the initiative of Gary Holzapfel, an engineer from the aerospace industry, determined to eliminate engine failures caused by weak fasteners in motorsport. The company designs and manufactures thousands of parts in California, in several material grades suited to each use.

Advantages over the original bolts

Criterion	Original head bolts	Reinforced ARP studs
Block threads	Stressed at every tightening	Preserved: studs fitted by hand

When studs become necessary

Standard fasteners remain suitable as long as the engine keeps its original combustion pressure. They reach their limits in the following cases:

• Output exceeds the original figure, whether the engine stays naturally aspirated or gets a turbo
• Compression ratio raised for more power
• Reinforced MLS head gasket, which requires a high and even preload
• Cylinder head that has already lifted or gasket that has already leaked
• Racing or track use, with repeated heat-ups and frequent teardowns

Vehicle compatibility

Why is it the most cost-effective solution?

On the H23A, the standard head bolts are sized for the original output: once it rises, they become the weak link in gasket sealing. ARP 8740 studs give a higher clamping force, for a budget out of all proportion to that of a cylinder head to redo, gasket and labour included.