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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. Whilst this will fit an MGB, it’s actually completely different to an MGB axle. It was used on a 1992-1995 MG RV8 which was based on an MGB. It’s a Salisbury 12HA axle apparently sourced from a Leyland Sherpa. It has a 3.3 ratio and an OEM Quaife torque sensing limited slip differential (TORSEN LSD). There are a number of other differences as follows:-

  • It’s slightly wider than an MGB steel wheel axle at approximately 52.5” vs 51.5”. This won’t cause an issue with wheel clearance on compression if the axle stays central however might cause minor rubbing of hard cornering. 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. The RV8 has a larger 1310 but has 10mm (M10) metric holes & bolts. These holes are slightly larger than the 3/8” holes on the standard 1310 flange. Luckily my propshaft was built with 1310 UJs with an output flange for an 1140 so I’m able to swap this over for a standard 1310 output flange

  • The axle input flange to axle 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 so with the axle fitted and the 1310 yoke fitted, my propshaft is ~20mm further away from the axle than it should be. A 20mm spacer will make up the difference

  • The wheel PCD is 4 x 4.5” the same as the MGB however the studs are metric M12x1.5. Luckily, the MGB 1/2”x20 studs can be persuaded to fit into the RV8 hubs without too much effort

  • The hubs have a hub-centric 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 custom spigots are desirable

  • 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 half shaft with a large nut to secure it. The RV8 has the hub permanently attached to the half 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. The interface between the cable and the handbrake uses a similar threaded bar, spring and adjusting nut as the MGB however it’s a metric thread. An outer cable securing bracket will need to be added to the transmission tunnel. The RV8 one was too bulky so I made my own slimmer version

  • The flange for securing the axle to the springs is in the same place as on the MGB so it will bolt right up. However it’s missing the fitting for 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. Space in the transmission tunnel is very limited and any excessive vertical movement of the axle will cause the propshaft to interfere. Now that I have more traction, the axle wants to twist more hence causing the nose of the diff to rise. To limit this, I’ve added some anti tramp bars to limit the rotation of the axle & hence the up/down movement of the propshaft.

  • Tyre rub. On hard cornering, if I also hit a bump, the tyre rubs on the wheel arch. This would suggest that the cause is lateral movement of the axle. A panhard rod would prevent this.