Descrição
This complete automotive drive axle differential assembly came to me a while back as an unknown quantity and with no associated provenance. After a slow but steady process of looking at it and browsing for matching information online, I’m reasonably confident that I now have a good idea of what it is, but I still have no knowledge of its former life, either in or out of one or more relevant vehicles.
Here’s the evidence available from the assembly itself:
• Raised Casting mark on the main housing – BTC793
• Raised Casting marks on the bearing caps – BTC794
• Other casting marks on the housing (inwards, RH side) – 29 A90 and 2A
• Other casting marks on the housing (inwards, LH side, stamped) – 9050
• Teeth on the crown wheel (counted) - 43
• Teeth on the pinion gear (counted) - 11
• Teeth on both gears confirmed by a stamped 11/ 43 on the housing, albeit with the first digit of that 11 hardly visible
• The gear ratio is thus 3. 909 when rounded to 3 decimal places
• Number of splines on the differential wheels - 27 (counted)
• PCD at the pinion coupling - 3. 1/ 8” (by 4 by 3/ 8”)
• PCD at the axle casing banjo flange – 8. 3/ 4” (to be confirmed but certainly by 12 by 3/ 8”)
• Hypoid bevel gears (deduced)
• Stamped across the join of one of the differential bearing housings – ‘RON’
I have now concluded that this differential turns out to be an automotive spare part that seems to apply as follows:
• It’s an example of a BMC design and possibly with Austin, rather than Nuffield, origins
• That axle design was used by BMC for various model ranges where the large car platform (or perhaps sometimes known as ‘C’ series) conventional drive train was applicable
• Usually described as a 3. 9 to 1 ratio, both by BMC and nowadays in the BMC aftermarket parts supply context, as opposed to the 3. 909 to 1 or 3. 91 to 1 ratio that might have seemed a more logical choice to some other manufacturers (e. g. the 3. 909 description that’s often used for Toyota 43/ 11 differentials, for example)
• The relevant BMC axle differential at that ratio seems to have been used as follows:
o Austin A99 & A110, manual transmission
o Princess 3 Litre & Vanden Plas Princess 3-Litre (possibly/ presumably only where manual transmission)
o Austin-Healey 100-6 (hypoid) and 3000 (or at least this ratio was supposedly an available option in both cases and I hope I’m correct in believing that the Big Healey differential is the same underlying unit as the Westminster one)
o Wolseley 6/ 99 & 6/ 110, manual transmission
• This differential unit may conceivably be a quite straightforward conversion or retrofit for some, or all, of the following further models from Austin then BMC (spline shape, size or quantity will, or may, apparently differ, particularly where the original differential unit was a spiral bevel gear one)
o Austin A70 Hereford
o Austin A90 Atlantic
o Austin A90 Westminster
o Austin A95 & A105
o Austin-Healey 100, BN1 & BN2
o Austin-Healey 100-6, BN4 & BN6 (if/ when originally spiral bevel)
o Morris Isis
o Riley Pathfinder
o Wolseley 6/ 90
The following practicalities will apply to any potential buyer
• By my judgement, this unit seems to be in quite good order but it has picked up some surface rusting on the gears here and there, so at least a very good flush out and/ or early subsequent oil change would be advised were it to be installed in another vehicle without any significant overhaul activity taking place beforehand (and it might be a bit noisy in such use anyway – impossible to tell in advance either way, I should imagine)
• The detectable backlash when the pinion is rotated seems to be very little, but I haven’t taken any measurements in that regard, so that again is via my own judgement
• The three most likely ‘change of keeper’ options to apply here, all of which will need to be discussed/ negotiated beforehand, will be as follows:
o Collection from either North Kent (Isle of Sheppey) or South London (Bromley)
o Delivery up to a certain distance - let’s say Kent, Essex, Most of East & West Sussex, most of Surrey, East and South London and perhaps some of Hampshire, Middlesex, Berkshire, Hertfordshire, etc. My time would not be charged for but estimated fuel costs and any relevant road tolls or other charges would be
o Delivery by courier - under such circumstances, I would probably decide to construct a bespoke wooden crate and might charge a small something to compensate for the materials and effort that would be required to get that done to an adequate quality standard
