diff sizes

[bobcat]

just did a swap on my bimmer,323 into 318 baur, the 323 runs the big diff-318 has the smaller one
(1)are these centres interchangeable -same axles same driveshaft
(2)which ratio is best ,the 318 is 3.64:1,the 323 is 3.45:1
thanks in advance.

[daimlerman]

Yes,diffs are interchangeable,just be careful when laid under the car lifting the larger one in,they are heavy and an arkward shape to lift!
Both the ratios you mention are high,at least by UK standards,I would have expected a 318 to have a 4.1 and a 323 a 3.64...
3.64 will give around 75mph at 3000rpm,the 3.45 about 83mph at 3000rpm.
BTW,you will need to adjust the propshaft length slightly,slacken the collar nut by the centre bearing and slide the rear part forward,large hammer on the diff flange is my weapon of choice!

[gareth]

in addition to the above, the output flanges are specific to each diff size so you'll need to unbolt these, not just slide the whole thing out of the diff with the shaft attached. i say this now as it's bloody hard to do if you've removed the prop / handbrake cables or anything else in the area as you can't stop things turning!

first stage for all diff swaps: get WD40 or similar on all of the bolts and loosen (but do not remove) every input/output shaft bolt. then remove them.

having a functional handbrake and an assistant will save you a lot of crawling too