Reinforced ARP crankcase through-bolts for TOYOTA SUBARU 4U-GSE

The Toyota 4U-GSE engine and its Subaru FA20 twin

ARP lists this part number for the 2.0-litre Toyota 4U-GSE engine, a flat-four. It is the engine of the Subaru BRZ under the FA20 code, so the compatibility table brings together the Toyota GT86, the Subaru BRZ and the Scion FR-S, taken from the NGK parts catalogue.

The factory part number of the original main bolts replaced by this kit is not published in our sources and so does not appear on this page. ARP ties this kit to the Toyota designation; the Subaru FA20 designation has its own part number, kit 260-5001.

Reinforced ARP bolts for main bearings

On this flat engine, the two crankcase halves clamp together around the crankshaft, and the through-bolts hold them against the loads passed on by the connecting rods. If their clamping force drops, the halves shift slightly along their parting line at every cycle: the mating faces brinell, then the bearing bore goes out of round.

This kit gathers ARP Pro Series main bolts for the 4U-GSE, in 10 mm and 8 mm, listed in the ARP catalogue among the crankcase through-bolts. According to the instruction sheet, the bolts are started by hand, the underside of the head, the washers and the threads are lubricated with ARP Ultra-Torque, then they are tightened in three equal steps. The chamfered face of each washer goes on the bolt head side: the wrong direction leads to early breakage. The instruction sheet does not give the number of pieces. The material is Inconel 718, a nickel-based superalloy: according to the ARP catalogue its tensile strength reaches 210,000 to 230,000 psi and it resists corrosion as well as hydrogen embrittlement. On this four-cylinder engine, the Inconel bolts in M10 and M8 are rated for 600 hp at most (calculated estimate).

Technical specifications

Tightening torque

Fitting the bolts: through the crankcase, tightened to torque in successive stages after ARP greasing of the threads and head seats.

The bolts are tightened following the order set by the manufacturer for the crankcase halves; every step is given in the ARP instruction sheet.

Moving to reinforced fasteners changes the clamping force on the crankcase halves. After installation, the size and roundness of the bores are checked, then align honed if needed, always with the final fasteners and lubricant.

Torque figures taken from the ARP 203-5002 instruction sheet, in the revision in force on the day of publication. In the event of any difference, the manufacturer's instruction sheet prevails. Read the ARP 203-5002 instruction sheet

Technical guide

Tightening is done with ARP assembly grease: the torque figures in the table were established with it, not with engine oil.

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.

When to upgrade the main bearing fasteners

• Toyota GT86 whose output is well above standard
• 4U-GSE block opened to change the crankshaft, rods or pistons
• Main bearing housings to be checked and brought back into line before fitting stronger bolts
• Intensive track, rally or drift use, with the engine often at high revs
• Bottom-end rebuild with systematic replacement of the main bearing fasteners

Vehicle compatibility

Why is it the most cost-effective solution?

On a 4U-GSE gaining power, the main bearings take loads that the original fasteners were not designed to hold. The price of an ARP Inconel kit stays modest compared with repairing the crankcase and the crankshaft.

Consider the crankshaft main bearings

When the bottom end is open, the bearings are replaced too. Here are the tri-metal crankshaft main bearings designed for these engines: