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MG MGA - Crown wheel and pinion

I recently ordered a new 4.3 ratio crown wheel and pinion fom a supplier in Sydney who imported it from England. Not sure who his supplier is.
My old crown wheel and pinion have 43 and 10 teeth respectively. The new crown wheel and pinion have 39 and 9 teeth. This still equates to 4.3.
I have not heard of this combination. Has anyone had any experience with this combination.
I presume it will work. The crown wheel has identical diameter to the old one but the pinion appears to be a couple of millimetres narrower.

Brendan
B Sullivan

Rather than 4.300:1 it is 4.333:1. Less than 1% difference, but still, this is s gear ratio I've never heard of before. Are you sure it's a matched set? Or are you sure it's for the Austin B-series rear axle. So far I've only known the 9-tooth pinion to be used with the 41/9 (4.556:1) gear set, and the 39-tooth ring gear to be used with the 39/8 (4.875:1) gear set. If this is real I would like to know the supplier so I can add it to my list of available gear ratios. See here: http://mgaguru.com/mgtech/rearaxle/ra201.htm

Barney Gaylord

Barney

The guy I bought the parts from tells me that he obtained them from Moss Motors in the UK.
Their website shows the tradiional 43/10 combination with no mention of the 39/9 that I received. Unless its a recent development.


Brendan
B Sullivan

I am always suspicious of undocumented parts that are not original specification. It may might be a mismatched set that would never fit together. I don't know why anyone would cut gears with this strange tooth count, or what application they had in mind. Even if it is legit, you may have the only gear set in the world like this. You get to be the field tester for new parts that may or may not work. Who pays for the labor to install and test them? Who pays for the labor to do it all over again if they don't work? Are these the only parts now available for this application? How desperate are you to get the car immediately assembled and on the road? The parts are obviously not the specification that you ordered, so surely you can send them back if you like.
Barney Gaylord

Such gears are produced on very special (co$tly) machines. If you have a machine setup to produce this ratio/tooth count, at the right pinion offset, then it is relatively easy (cheap) to adjust blanks so that you get a "standard" gear that fits a "special" application.

The 4.333 ratio is common, and the range of acceptable/probable pinion off sets is small, so this is not an unlikely scenario. But it certainly should be documented/explained. It does lead to possible availability of other novel ratios as well, if you find out who and where. A good guess is that it is a development of the Hindustan "A55" production and its successors. That may well be a simple new design in an old envelope. And, it is said that Hindustan's gear machines and procedures are the best in the world, to the point that they produce "zero-zero" gears, so that replacement involves no setup for mountain village mechanics.

FRM
FR Millmore

I just noticed a note on the Moss Motors UK website where the parts came from. It acknowlges that their 4.3 crown wheel and pinions are indeed 39/9 and not 43/10. They say that it is a quiter running gear set and they get them from"Ivor".

Brendan
B Sullivan

Hard to believe that fewer teeth makes quieter running. My guess is that they sell the 39/9 gears because the found a cheaper source than the 43/10 gears, and no other reason. That seems to me to be a lame excuse for incorrect substitute parts. It ranks right up there with selling short life fiber thrust washers for the differential when the original parts were long wearing bronze.
Barney Gaylord

This thread was discussed between 20/07/2012 and 21/07/2012

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