Reinforced ARP main studs for OPEL VAUXHALL C25XE

The Opel and Vauxhall C25XE V6

ARP lists this kit for the 2.5-litre V6 of Opel and Vauxhall, the C25XE, a V-six. According to the NGK catalogue, it makes 170 hp and powers the Calibra, the Vectra A and the Cavalier Mk III, from 1988 to 1997 depending on the model.

On the stock engine, the crankshaft is held down by caps secured with the manufacturer's fasteners; this ARP stud kit takes their place, in 10 mm diameter.

Reinforced ARP studs for main bearings

On a crankshaft, the main caps are what hold the shaft against the loads passed on by the connecting rods. If their clamping force drops, the cap shifts slightly on its parting line at every cycle: the mating faces brinell, then the bearing bore goes out of round.

This page offers the 209-5402 stud kit in 8740 steel. The studs are first run into the block by hand; the caps then go on and the nuts hold them down. The instructions also mention washers, to be lubricated before fitting, without giving a quantity. Three equal tightening steps, in the manufacturer's sequence. Material: 8740, a chrome-moly steel that ARP specifies at 200,000 psi tensile strength, with threads rolled after heat treatment. Factory-fitted fasteners usually belong to classes 10.9 and 12.9, with 150,800 to 176,900 psi minimum tensile strength. On this six-cylinder engine, the 8740 studs in M10 are rated for 1,400 hp at most (calculated estimate).

Technical specifications

Tightening torque

Fitting the studs: screwed into the block by hand, without a tightening tool. For a permanent installation, the ARP instruction sheet allows a Loctite thread locker, provided the tightening is completed before the product cures.

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

Stronger fasteners change the clamping force: the size and roundness of the main bearing bores are checked after installation, and the bores are align honed if necessary, using the final fasteners and lubricant.

Torque figures taken from the ARP 209-5402 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 209-5402 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.

Advantages over the original bolts

Criterion	Original main bolts	Reinforced ARP studs
Support in the block	The original bolt works in the tapped thread of the block at every tightening	The stud stays seated in the block, the nut receives the tightening load
Cap alignment	No guidance before the bolts engage	The cap slides down along studs already in place
Load path	The tightening load goes through the bolt, which turns in the block	The tightening load goes through the nut, on the cap side

When studs become necessary

• C25XE with output well above stock
• Calibra or Vectra A V6 entered on track or in rally
• Engine opened up to fit forged parts
• Cap housings bored again at the machine shop
• High revs held again and again

Vehicle compatibility

Why is it the most cost-effective solution?

A C25XE pushed beyond its stock output asks more of the crankshaft main bearings. 8740 ARP studs on the caps cost little compared with a block and a crankshaft to replace.