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MG RV8 Axle

While the original 3.9 axle ratio is tolerable with the Vauxhall 5-speed gearbox, it’s not ideal. I’d often find myself pulling away in 2nd gear. I’d prefer the final drive ratio to be a bit taller. In pursuit of this, I won an auction on Ebay for an MG RV8 axle at a price that I could not refuse. This is actually completely different to a standard MGB axle. It was used on a 1992-1995 MG RV8 which was based on an MGB. It’s a Salisbury 12HA axle apparently originally found in a Leyland Sherpa. It’s a 3.3 ratio and has an OEM Quaife torque sensing limited slip differential (LSD). There are a number of other differences as follows:-

  • It’s slightly wider at approximately 52.5” vs 51.5” for the MGB steel wheel axle. I don’t think this will cause an issue with wheel clearance on compression if the axle stays central. I was planning on fitting a panhard rod at some point anyway so perhaps this has moved up the agenda

  • The 4 cylinder MGB input flange is a Hardy Spicer 1140 and the v8 is 1310. The RV8 is also a 1310 but has 10mm (M10) metric holes & bolts. These holes are slightly larger than the 3/8” holes on the standard 1310 flange so I guess they’d need to be drilled out if a metric flange yoke can’t be obtained. Luckily my axle was built with 1310 UJs with an output flange for a 1140 so I can swap this over for a standard 1310

  • The axle input flange to centre line distance is similar but not the same. An MGB is about 9.75” (248mm) whilst the RV8 is slightly less at about 9.25” (235mm). This places the input flange slightly further away from the propshaft. A standard 1310 yoke is 6.6mm shorter than my “1310 with 1140 face” yoke which makes the RV8 prop ~20mm further away than it should be. I’ll use a 20mm spacer to make up the difference

  • The wheel PCD is 4 x 4.5” the same as the MGB however the studs are metric M12 x 1.5. Luckily, the MGB 1/2”x20 studs will fit into the RV8 hubs without too much persuasion

  • The hubs have a hub flange at 73mm for hub-centric wheels. The MGB hubs do not have hub-centric flanges but my LE alloy wheels do at 78mm so I’d probably choose to use spigots

  • The way that the half-shafts fit into the axle tube is entirely different. The MGB has a separate hub that fits onto the end of the shaft with a large nut to secure it. The RV8 has the hub permanently attached to the shaft without the hub nut on the end. It reminds me more of the Ford cars that I’ve owned in the past. The bearing retainer is simply a circular disc that’s bolted into threaded holes in the axle tube rather than bolting through with a nut on the back like the MGB. This would make a disc brake conversion somewhat more complicated

  • The handbrake mechanism for the drums is also very different. The RV8 axle has two separate cables that run all the way to the handbrake lever where they join via a small compensator. It looks like the interface between the cable and the handbrake uses the same threaded bar, spring and adjusting nut as the MGB. An outer cable securing clip will need to be added to the transmission tunnel

  • The flange for securing the axle to the springs is in the same place as on the MGB and looks to be entirely compatible. However it’s missing the fitting where you attach the droop straps. The RV8 had telescopic shocks so I guess they were used for that purpose instead

After fitting the axle, it became apparent that I have a few more challenges to address:-

  • Torque effects. My propshaft is quite a bit fatter than the RV8 propshaft. This leaves little room in the transmission tunnel for the handbrake mechanism which has now changed from a single cable to two cables and a rather large bracket. Space is very limited and any excessive vertical movement of the axle will cause the propshaft to interfere. To limit this, I’ve added some torque control arms to limit the vertical rotation of the axle & hence the up/down movement of the propshaft.

  • Tyre rub. On hard cornering, the tyre rubs on the wheel arch. So far I’ve not noticed this on compression of suspension, only on cornering which would suggest that the cause is lateral movement of the axle. A panhard rod would prevent this.

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