pros and cons of a 3.7 diff

[hotrodder13]

what are the pros and cons of a 3.7 diff, my dad seems to think it will lower top speed and higher the revs, will i still be able to go on the motor way

thanks tom

[linearaudio]

It will do just the opposite! (if you can find one, that is). It may not increase top speed, but will lower the revs at any given speed, by about 13 percent. Could be a bit too much for a standard 1098, but good for a 1275. The 3.9 diff is much more common, though still hardly thick on the ground, and will give useful results (about 9 percent lower revs).

[aupickup]

will give you better top end cruising but less acceleration

[hotrodder13]

thanks i might try and get one then and it will be mated up with a 1275, whats a good price for one with light surace rust on teeth.

thanks tom

[linearaudio]

Wheres that piece of string gone!!!

BMC can find them for about a fiver each ( like most other things!), but the rest of us will pay £40- £80 depending on demand at the time. I didn't make a big thing about the acceleration drop, as you have a 1275 so the effects will be lessened.
There's a 3.7 on EBay at the moment, 1 day left and only bid up to £118 so far

[mike.perry]

It is a trade off between acceleration and relaxed cruising revs. I find that the best compromise for a 1275 is a 3.9 diff which still gives a reasonable acceleration but cuts the revs at motorway speeds compared to the 4.22 diff which gives plenty of acceleration but revs the nuts off the engine.
Also relevent is whether you have a 5 speed box which will give you a high 5th anyway and you will not need a high diff.
It also depends on your local terrain, if you live in a flat area you will probably go for the higher diff, but if you live in a hilly area you will want the hill climbing ability of low gearing.
I have not tried a 3.7 diff so I cannot comment.

[bmcecosse]

My 'warmed up' 1098 goes really well on the 3.7 diff, yes - I have to pop down into 3rd sometimes, but the speeds in the lower gears are a real advantage. A 1275 will be even better! I wish I could get a 3.7 for a Fiver!!!!!!!! I had to pay a LOT more for mine, but it is a good investment - 3.7 prices seem to rising faster than the FTSE these days! If the diff has rusty teeth - it's very probably useless - so worth nothing.

[mogimad]

Just got my 3.7 diff, arrived today.
It's in superb condition, no sighs of wear on the crown wheel or the other gears.
Paid £150 inc delivery, but as i said it's like new.

BMC, i read somewhere that you posted the tpm speedo that's nearest to give an accurate road speed reading, but cant find it again, could you share your knowlage again,
Many thanks,
Derek.

[bmcecosse]

1248 is absolutely bang on both for speed and miles in my car - 155 X 14 tyres and 3.7 final drive. Don't pay more than £5!!

[linearaudio]

............... Don't pay more than £5!!

[bmcecosse]

I think mine was £3 or so from ebay - and a couple of £ for postage........

This one http://cgi.ebay.co.uk/CLASSIC-MINI-CENT ... 27affd3387 is 1376 tpm - so no use for you!
And this one http://cgi.ebay.co.uk/CLASSIC-AUSTIN-MI ... 4a9f31ae8a looks like it 'may' be 1248 - I can't quite make out the numbers. But it is a bit expensive anyway!

This one - http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 2761.l1259 has been conformed to me as 1248, and comes with extra gauges!

This one is also now confirmed as being 1248 - http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 0540237245 but v expensive and quite unjustified postage!

This one also now confirmed as 1248 tpm - http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 0478556455 and much better postage charge!

And - this one too is confirmed at 1248 tpm - http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 0544523335 - with reasonable post costs.

This one also is now confirmed as 1248 tpm - http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 0498413194 it's very 'clean' - but also very pricey!

So - there are LOTS of these speedos out there!

[Pyoor_Kate]

It occurred to me that I should mention that I had a 3.7 diff that went off for reconditioning which had light surface rust on the teeth - I was advised by the reconditioner that there was no point doing it as the light-surface-rust destroys the hardening, so the teeth wear very quickly...

So now I have a different 3.7 Diff

[mike.perry]

Can you recommend anyone to service a diff, my 3.9 whines and I have several Series MM axles which need repairing.

[bmcecosse]

I don't think it's too difficult Mike - just follow the manual. But of course if the gears are worn into each other, with years of running - it may not be possible to completely eliminate whine without buying new gears - even if they are available!

[cormorant]

Yep I have a 3.7 diff in a standard 1098 and the 1248tpm speedo is spot on. Don't really notice the reduced acceleration as the 1098 engine is a good old slogger. When I get round to it I will wire the extra green warning light at the top of the speedo as an extra indicator warning light though you could use it for anything you want. The mounting ears are a different size on the mini speedo so i put the mini innards into the standard Minor housing

[bmcecosse]

Ahh - didn't want to disturb anything (or spoil the Minor speedo) - so I just bent the ears!

[mike.perry]

I have several Series MM axles which need repairing.

I don't think it's too difficult Mike - just follow the manual.

I have read the manual, that is why I asked!