Reinforced ARP bolts for OEM rods TRIUMPH FM3 FM8 FM11

The Triumph Spitfire 1.3 and 1.5 litre engines concerned

ARP aims this kit at the Triumph four-cylinder engines of the Spitfire, in 1.3 litre (FM3 and FM8) and 1.5 litre (FM11) form, fitted from 1966 to 1980 to the Spitfire Mk III, Mk IV and 1500.

This ARP kit replaces the original rod bolt of these engines with ARP2000 cap screws without washer. Neither the part number nor the diameter of the original bolt appears in ARP's documentation for this kit.

Reinforced ARP bolts for connecting rods

At every revolution, the rod bolt takes the pull of the rod and piston assembly, which tries to force the cap away through inertia. As original equipment, it is sized for stock power and is replaced at every strip-down.

The ARP 206-6004 bolts, in ARP2000, are preferably tightened by stretch, the method ARP recommends, or to the 61 Nm torque in the instruction sheet if no gauge is available. ARP2000 is an alloy steel developed by ARP and heat treated to 220,000 psi tensile strength; its threads are rolled after heat treatment. Standard engine fasteners commonly fall into classes 10.9 and 12.9, i.e. 150,800 to 176,900 psi minimum strength: ARP2000 sits 24 to 46% above that.

Technical specifications

ARP asks you to check, before installation, the chamfer of the hole in each rod cap: it must be wide enough to clear the radius under the bolt head.

Tightening torque and stretch

ARP recommends tightening to a measured stretch, which gives a more accurate preload than torque alone, given as a fallback. Torque and stretch figures assume ARP lubricant.

The values above come from the ARP 206-6004 instruction sheet applicable at the date of publication; it is authoritative in the event of any difference. Read the ARP 206-6004 instruction sheet

The ARP instruction sheet does not require a thread locker for this kit; only ARP Ultra-Torque grease is applied, on the threads as well as under the bolt head.

Once the fasteners are fitted, the ARP instructions call for the big ends to be checked and resized by a machine shop.

Measuring bolt stretch

Tightening to stretch means setting the preload on how much the bolt actually stretches, rather than on a torque affected by friction. With the bolt fitted, the dial gauge is placed on both its ends and the bolt is tightened until the stretch given in the table is read. It is the safest method on a highly tuned engine.

Technical guide

Torque and stretch figures are only valid with ARP assembly grease, applied to the threads and under the head.

The ARP brand

Founded in the early 1970s by Gary Holzapfel, who came from the aerospace sector, ARP (Automotive Racing Products) was born to put an end to fastener-related engine failures in racing. Its thousands of part numbers, designed and manufactured in California for the cylinder head, the main bearings and the connecting rods, come in several materials depending on the application and are recognised for the consistency of their manufacture.

Vehicle compatibility

Why is it the most cost-effective solution?

On a Spitfire whose small four-cylinder is pushed harder, every rod bolt holds the piston against inertia on each revolution; if just one lets go, the rod leaves the crankcase. An ARP2000 set costs little next to rebuilding the bottom end.