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BMC B Series
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BMC B Series
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ARP-206-6003 ARP rod bolt in 8740 for OEM MG BMC B-Series 18GB and 18GF rods (MGB 1964-1968).
ARP aims this kit at the 1964 to 1968 BMC B-Series engines coded 18GB and 18GF, the ones fitted to the MGB GT and the MGB convertible of those years.
This ARP kit replaces the original rod bolt of these engines with a 3/8 in 8740 steel cap screw without washer. ARP does not publish the part number of the original bolt.
Few parts of the bottom end work as hard as the rod bolt: at every revolution, it is what stops the cap from pulling away under the inertia loads of the rod and piston assembly. The original bolt is designed for stock power and is single-use.
The ARP 206-6003 bolt, in 8740 and 3/8 in, is preferably tightened by stretch, the method ARP recommends, or to the 61 Nm torque in the instruction sheet if no gauge is available. These bolts are made from 8740, a chrome-moly steel brought to 180,000 psi tensile strength by ARP's heat treatment; the threads are rolled afterwards. For comparison, classes 10.9 and 12.9 of original fasteners have a minimum of 150,800 to 176,900 psi. The 8740 bolts hold up to 8,750 rpm (calculated estimate).
| Part number | Brand | Grade / material | Head | Thread | Under-head length |
|---|---|---|---|---|---|
| ARP-206-6003 | ARP | 8740 steel | Not published | 3/8 in | Not published |
ARP asks you to check, before installation, that the hole of each rod cap has a chamfer wide enough to clear the radius under the bolt head.
| Reference | Tightening torque | Stretch |
|---|---|---|
| ARP-206-6003 | 61 Nm | 0.165 - 0.178 mm |
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.
Source of the torque and stretch figures: ARP 206-6003 instruction sheet, in the revision applicable at publication. The manufacturer's instruction sheet prevails in the event of any difference. Read the ARP 206-6003 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.
According to the ARP instructions, the big ends must be checked and resized with the fasteners in place; plan a visit to a machine shop.
Stretch measurement supplements torque tightening and gives the most accurate preload. It is done with the bolt in place: the gauge, fitted with a dial indicator, bears on both ends of the bolt and shows the stretch achieved, to be brought to the value in the table above. This avoids the variations caused by friction, which matters on a highly stressed engine.
Tightening is done with ARP assembly grease, on the threads and under the head: the torque and stretch figures in the table were established with it.
ARP (Automotive Racing Products) came into being in the early 1970s, when Gary Holzapfel, who came from the aerospace industry, decided to tackle the engine failures caused in motorsport by inadequate fasteners. Today the brand produces thousands of part numbers in California for the cylinder head, the main bearings and the connecting rods, in materials suited to each application, with manufacturing known for its consistency.
| Brand | Model (chassis) | Engine code | Power | Years |
|---|---|---|---|---|
| MG | MGB GT | 18GB, 18GF | 95 hp | 1964-1968 |
| MG | MGB Convertible | 18GB, 18GF | 97 hp | 1964-1968 |
On an MGB that is revved higher, every rod bolt holds the piston against its inertia loads on each revolution; if one lets go, the rod leaves the block. An ARP 8740 bolt costs little next to rebuilding the bottom end.
