Differential change

[seniorchristo]

I will be changing the differential unit in my '66 Elan S3. It currently it has a 3.9 and I will be changing to a 3.55. I'm not sure what the PCD is of the driveshaft flange but if it is not compatible with the new diff unit is it possible to just swap out the flange/yoke part of the drive shaft? Many thanks in advance to everyone helping! . .
Later
Chris

[seniorchristo]

Sorry, its a 1967 Elan S3 SE

Later
Chris

[rgh0]

Yes you can remove the end flange and yoke from the rear UJ and replace with one with the needed PCD if yours does not match the new diff and you don't want to change the flange on the diff

cheers
Rohan

[seniorchristo]

Thanks Rohan. You are just a wealth of information on Lotus Elans. Off topic. How many Elans do you estimate there are in Austrailia. Just curious.

[rgh0]

Pre-internet days the local lotus club had a register which had well over 100 Elans and plus 2s on it and that would not have been every car in Australia so I would estimate maybe 200 or so cars. There are certainly many hiding in garages and not being used much.

cheers
Rohan

[alan.barker]

3.9 to 3.5 diff is a big change. What gearbox have you got, does it have a high first gear, if so with the 3.5 diff you will need to slip the clutch a lot more
Are you sure the original was 3.9 for the SE model
Alan

[seniorchristo]

Alan,
The original was probably 3.77. I used the method of counting wheel revolutions vs. propshaft revs to arrive at 3.9 so I think it might have been changed at one time. I have to assume it has the semi close gears as standard. I don't know how to check this easily. I'm concerned about slipping the clutch on starts but I also need a more relaxed cruising speed so time will tell.
Thanks
Chris

[Quart Meg Miles]

My wife learnt to drive on a close-ratio box with 3.55 diff so I don't think you will have a problem. I didn't like the combination with the standard engine and changed back to 3.9 but the SE engine should pull it OK.

You can gauge the gearbox the same way as the diff, though turning the engine over a lot of times is a pain, and as the 3rd gears differ by 13% it should be pretty clear-cut. There's only 3% difference between 3.9 and 3.77 so an error there shouldn't muddy the water.

[patrics]

Hi,
Just my opinion but I would make ever effort to fit the small diff flange as the diff is hard enough to fit already

Regards
Steve

[stugilmour]

Alan,
...I have to assume it has the semi close gears as standard. I don't know how to check this easily. ....
Thanks
Chris

Chris, seeing your post a bit late, but a suggestion for checking your existing gearbox ratios is to use David's (msd1107) incredible spreadsheet located here...

lotus-suspension-f42/updated-spreadsheets-t18445.html

Not sure if the link is the most recent version. Using the sheet with tire size & diff ratio entered you can compare RPM and speed in various gears and see which ratio set matches. Also handy for dialing in the speedometer later after you change the diff. I actually used the sheet to adjust my tach, so it is very accurate.

HTH

Stu

[robertverhey]

See my last posting in this thread on lotus cortina forum....may be of interest. Acknowledgements again to mr bean

http://lotuscortina.net/~lotusc7/phpBB2 ... ilit=+Bean

[msd1107]

Yes, that is the latest link.

David
1968 36/7988