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BMC 'A' SERIES GEARBOX REBUILDS


Rebuilds are for any car using an 'A' series rib case gearbox. We can make a number of improvements to the gearbox if required.

Unlike modern designs, these gearboxes only have synchomesh engagement on 2nd, 3rd and 4th gears. 1st gear has a crash type of engagement, this means when 1st gear is selected, it is usually felt and heard by a grinding noise. This type of design leads to rapid wear of the 1st gear and the parts it connects with and it becomes excessively noisy and difficult to engage the gear.

The synchronisers and baulk rings on the other gears are small in size, which limits their life, and crunch-free gear changing becomes impossible.

Due to being compact in size, and having an aluminium casing, tailhousing and gearchange that make it light in weight, the gearbox has always been popular for competition use.

REBUILD PROCESS

The gearbox is completely stripped and all parts are cleaned and examined. The rebuild consists of fitting the following new (not good used) parts; front and rear bearings, mainshaft front bearing, layshaft, layshaft bearings, baulk rings, tailhousing bush and bored to size after fitting, front cover with a seal (standard cover has no seal), rear seal and all gaskets.

If other parts are found to be in need of replacementthe customer will be contacted before proceeding.

BMC GEARBOX OPTIONS


  Straight cut gears
  Straight cut gears
OPTIONS FOR COMPETITION USE
We can build a gearbox with straight cut synchromesh gears, the ratios are :

1st gear 2.54 ~ 2nd gear 1.73 ~ 3rd gear 1.25 ~ 4th 1.00


In competition use, the standard synchromesh gear engagement can have rapid wear and breakages. As an alternative we can build a gearbox with straight cut gears and dog tooth gear engagement. This is a total redesign and features a constant mesh 1st gear that allows changing down into 1st gear, just the same as changing down into 3rd or 2nd gear.

We also make a number of other changes to the gearbox to improve the reliability. Also the gearchange has a faster and shorter movement.

  BMC gearbox
Internal clutch release mechanism

To achieve satisfactory gear shifts it is necessary to have the clutch operating correctly. The standard clutch release mechanism, which consists of a fork pivoting on the gearbox front cover and a carbon thrust bearing contacting a pad on the clutch fingers, does not work without problems occurring when operating the standard 1275cc clutch.

If the engine has a racing clutch fitted a ball bearing type clutch release bearing is needed. The standard carbon thrust bearing can be converted to take a ball bearing type, but this is not a satisfactory modification because the bearing does not contact the fingers on the clutch concentrically as the fork moves through an arc, and this results in clutch operational and wear problems. It is vital that the bearing is operated concentrically on a racing clutch. An additional problem is that the fork bends due to the much greater load required to release a racing clutch.

The best method to operate the clutch is with an internal hydraulic release mechanism. The standard transmission front cover, the clutch fork, the pushrod, the bearing and slave cylinder are discarded and replaced with a specially made front cover that has been designed to prevent it collapsing when the very large increase in the release load with some racing clutches is applied.

This mechanism proveds a direct acting and positive clutch operation. Also, the front cover has an input shaft seal to prevent loss of oil from the transmission. Each clutch release mechanism is made to requirements due to the considerable number of different applications of flywheel and clutch being used with the 'A' Series engine.

The mechanism has a fluid supply and bleed adaptor on the cylinder, a bleed valve, a selection of shims to obtain the correct fit of the front cover to the gearbox bearing, a gasket, nuts and washers. We can manufacture the hydraulic lines to requirements.

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