Reinforced ARP crankcase studs and through-bolts for PORSCHE 911 3.6 3.8

The air-cooled Porsche engines of 3.6 and 3.8 L

ARP lists this part number for air-cooled Porsche engines of 3.6 and 3.8 L. The ARP page for kit 204-5406 does not state the model; the instruction sheet describes a universal kit for 3.6 and 3.8 L engines built from 1989 to 1998. The compatibility table therefore reproduces the ARP application as published, with no engine code or vehicle inferred.

The factory part number of the original crankcase fasteners replaced by kit 204-5406 is not published in our sources and so does not appear on this page. Kit 204-5406 is flat crankcase hardware, with fasteners at crankshaft main bearings 1 and 7 and through-fasteners.

Reinforced ARP crankcase studs and through-bolts

On this type of engine the crankshaft is held between two crankcase halves that studs and through-bolts keep clamped together. At every combustion the loads from the connecting rods try to push the halves apart: if the fasteners relax, the main bearing seats wear and the bores lose their shape.

This kit brings together ARP crankcase studs and through-fasteners for air-cooled engines of 3.6 and 3.8 L. The instruction sheet of this universal kit lists: two 6.973 in studs (earlier cases) or two M10 bolts of 6.170 in (later cases) for main bearing 1, one 4.553 in stud for bearing 7 and eleven through-bolts, with stepped washers, nuts and one 12-point nut. All parts are made of Inconel 718: according to the ARP catalogue, this nickel alloy reaches 210,000 to 230,000 psi in tension and is protected against corrosion and hydrogen embrittlement. It takes the place of the original crankcase fasteners and is tightened following the supplied ARP instruction sheet; ARP publishes no strength value for this kit.

Technical specifications

Tightening torque

For installation, the ARP instruction sheet asks for the studs to be screwed in by hand only and leaves the choice of a thread locker for a permanent fit. Studs, through-bolts and nuts get Permatex 133A paste on their threads; the instruction sheet forbids any other lubricant, seizing being the main risk.

Tightening is done in a single pass, to the value in the table, following the sequence recommended by the engine manufacturer: the ARP instruction sheet does not detail it and provides for no intermediate pass.

The torque figures above come from the ARP 204-5406 instruction sheet, revision applicable at publication. The official instruction sheet is authoritative in the event of any difference. Read the ARP 204-5406 instruction sheet

Technical guide

According to the instruction sheet only Permatex 133A paste is suitable, and the torque figures in the table are given with it: engine oil is ruled out. The sealing O-rings for the through-fasteners are not supplied with the kit: the instruction sheet says to get them from Porsche or from an engine gasket set.

The ARP brand

Coming from the aerospace industry, Gary Holzapfel founded ARP (Automotive Racing Products) in the early 1970s to put an end to engine failures caused by weak fasteners in motorsport. The company designs and manufactures thousands of part numbers in California, in several material grades depending on the application.

Advantages over the original bolts

Criterion	Original fasteners	Reinforced ARP studs
Material	Depends on the original crankcase fastener of 3.6 and 3.8 L	Inconel 718, tensile strength 210,000 to 230,000 psi (ARP catalogue), kit 204-5406
Thread lubricant	Depends on the original crankcase fastener of 3.6 and 3.8 L	Permatex 133A paste, the only one allowed by the instruction sheet (204-5406)
Final tightening	Depends on the original crankcase fastener of 3.6 and 3.8 L	Single pass to the value in the ARP instruction sheet (204-5406)

When studs become necessary

• Air-cooled Porsche engine of 3.6 and 3.8 L pushed well beyond its standard output
• Crankcase opened to fit a forged crankshaft, rods or pistons on an engine of 3.6 and 3.8 L
• Crankcase seats and threads to be checked, then restored, before fitting stronger fasteners, on an engine of 3.6 and 3.8 L
• Intensive track, rally or hill-climb use, with an engine of 3.6 and 3.8 L often held at high revs
• Complete bottom-end rebuild of 3.6 and 3.8 L with replacement of the crankcase studs and through-fasteners

Vehicle compatibility

Why is it the most cost-effective solution?

On an air-cooled Porsche engine of 3.6 and 3.8 L that is pushed hard, the loads on the crankshaft rise and the crankcase takes them through its fasteners. ARP kit 204-5406, in Inconel, costs little compared with a crankcase to repair and a crankshaft to replace.