ARP 2000 Crank Hub Bolt Upgrade - McLaren 3.8L & 4.0L

Safeguard your high horsepower McLaren with this ARP 2000 Crank Hub Bolt Upgrade.

As we began pushing the envelope of what is possible on the McLaren M838 and M840 engines, we quickly ran into newly discovered weak points.

We started building more cars above the 1,100WHP / 900FTLB mark with upgraded clutches, in environments outside of the UK with more available traction and it was becoming apparent that the crank hub on this car was a major weak point and we lost two engines due to the crank hub slipping during our development phase at 1,100WHP+.

The factory crankshaft is a split two piece design, the crankshaft itself and crank hub. The crank hub mates to the crankshaft using nothing but a diamond coated friction washer and single 12.9 grade stretch bolt. The only thing that is stopping the crank hub from slipping is the clamping force created by that bolt on the diamond coated friction washer.

The crank hub is responsible for spinning the engine oil pump, coolant pump, AC pump, alternator all 4 camshafts/cam phasers, whilst keeping the engine timed via a chain drive. All of these components are applying strain and relying on that clamping force to stop the crank hub from spinning. 

What can make a crank hub spin? Rolling launch control, gearbox kickdown and simply having a setup of over 1,100WHP / 900FTLB in a high traction environment. The McLaren gearboxes also feature inertia push technology and when coupled with upgraded clutches in a high traction environment put a lot of shock on the crank hub during shifts in Track mode. It is not a gradual power increase that makes the crank hub spin, but the immediate and instant torque created by the engine itself and successfully transferred to the tarmac.

What happens when the crank hub spins? If you're lucky and the hub has spun by less than 45*, the engine safeties kick in, the engine cuts out and simply requires retimed in order to be able to start back up. When this happens you will see a crank/cam correlation fault logged on the ECU. This is rarely the case however, with this being an interference engine, the valves on the cylinder head touch the pistons damaging the pistons, valves, valve seats, head and possibly even extending to the bearings and rods. It can cause catastrophic engine failure.

The problem now lies with that factory 12.9 grade bolt. Although a 12.9 grade bolt is one of the stronger fasteners out there on the market, it is still a stretch bolt which is never to be reused. According to McLaren, that bolt is not to be removed under any circumstance. They do not have a torque spec for that bolt. The bolt also cannot be purchased from McLaren. It is not uncommon to remove the bolt to make life easier during engine assembly/disassembly.

Once that bolt has been removed, not only can it not be reused, it cannot be replaced, and even if you decide to reuse or find a replacement, you are left guessing how much nm and torque angle you should carry out on this stretch bolt.

The solution? We contacted the best fastener supplier on the planet to make us an upgraded crank bolt for this application. 

Manufactured to precise specifications, this bolt requires no adjustments to the crankshaft or crank hub. Through collaboration with ARP, we've constructed these bolts from ARP 2000, ensuring a remarkable clamping force exceeding 1,000psi. The best part? You get a real torque spec on a re usable bolt which we guarantee will give you an adequate clamping force on that friction washer. Our confidence in this bolt extends to its exclusive use in our elite Stage 2 crate engines, fitting both McLaren M838T and M840T models.

It should be noted the first engine we lost during development was an untouched factory crank bolt which had never been loosened during disassembly/reassembly, the power level this car was running was 1,163whp with nitrous. The hub spun in 4th gear once the nitrous was activated. The second engine we lost was at 1,300whp+ on a factory crank bolt re torqued down to the spec we calculated using a test to find the yield point. This engine failed during an inertia push upshift. 

We also tried other solutions including pinning the crank and increasing the coefficient friction of the friction washer itself which were unsuccessful. The pin will break and increasing the coefficient friction of the washer itself must be coupled with greater clamping force.

This bolt can be installed without removing the engine from the car. It will be supplied with our suggested torque spec. We recommend this bolt to anyone looking to run over 1,000whp as a mandatory upgrade.