Reinforced ARP crankcase studs and through-bolts for PORSCHE 911 2.0 2.7

The air-cooled Porsche engines of 2.0 to 2.7 L

ARP lists this part number for air-cooled Porsche engines of 2.0 to 2.7 L. The ARP page for kit 204-5407 gives neither model nor year, so the compatibility table reproduces the ARP application of kit 204-5407 as published, with no engine code or vehicle inferred.

The factory part number of the original crankcase fasteners replaced by kit 204-5407 is not published in our sources and so does not appear on this page. Kit 204-5407 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 2.0 to 2.7 L. The instruction sheet lists two 6.835 in studs on the left half (bearing 1), one 4.553 in stud on the right half (bearing 7) and eleven through-bolts, together with double-chamfer washers, acorn nuts and a 12-point nut with its washer. ARP makes these fasteners from Inconel 718, a nickel alloy listed in its catalogue at 210,000 to 230,000 psi of tensile strength and insensitive to corrosion as well as hydrogen embrittlement. This kit is fitted in place of the original crankcase fasteners and tightened as stated in the ARP instruction sheet supplied with it; ARP gives no strength figure for it.

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.

The nuts are tightened in one go to the stated value, in the order recommended by the engine manufacturer, which the ARP instruction sheet does not reproduce; it provides for no successive passes.

Values taken from the ARP 204-5407 instruction sheet in force at the date of publication; the official instruction sheet prevails should any difference appear. Read the ARP 204-5407 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 many aftermarket suppliers.

The ARP brand

ARP, short for Automotive Racing Products, was born in the early 1970s on the initiative of Gary Holzapfel, an engineer from the aerospace sector, in response to the engine failures caused in racing by inadequate fasteners. Today the brand produces thousands of part numbers in California, offered in several materials depending on the application.

Advantages over the original bolts

Criterion	Original fasteners	Reinforced ARP studs
Material	Depends on the original crankcase fastener of 2.0 to 2.7 L	Inconel 718, tensile strength 210,000 to 230,000 psi (ARP catalogue), kit 204-5407
Thread lubricant	Depends on the original crankcase fastener of 2.0 to 2.7 L	Permatex 133A paste, the only one allowed by the instruction sheet (204-5407)
Final tightening	Depends on the original crankcase fastener of 2.0 to 2.7 L	Single pass to the value in the ARP instruction sheet (204-5407)

When studs become necessary

• Air-cooled Porsche 911 engine of 2.0 to 2.7 L set to make much more power than standard
• Engine of 2.0 to 2.7 L stripped down to fit a forged crankshaft, rods or pistons
• Crankcase threads and seats of an engine of 2.0 to 2.7 L worn by repeated stripping, to be restored before fitting the new fasteners
• Sustained track, rally or hill-climb driving with an engine of 2.0 to 2.7 L held at high revs for long periods
• Full bottom-end rebuild of an engine of 2.0 to 2.7 L, replacing the crankcase studs and through-bolts at the same time

Vehicle compatibility

Why is it the most cost-effective solution?

On an air-cooled engine of 2.0 to 2.7 L that is worked harder, the crankcase takes the crankshaft loads through its fasteners. An ARP kit 204-5407 in Inconel is a small outlay next to a crankcase to be machined or a crankshaft to be replaced.