Diff and Gearbox Upgrade

[SUE482]

SUE482 is a 1955 2 door saloon and is original in terms of gearbox and rear axle, the remainder of the car was completely rebuilt, which included the engine that was in at the time I acquired her, a 1098.
I have confirmed that the diff ratio is 5.375, the tyre size is 155x80x14's and that the speedo is a TMP1000. By my calcs I should be travelling at 39mph at 3000rpm. Am I right?
Now I know a 3.9 or even a 3.7 ratio axle would be great but since I don't do a great deal of miles, and the price of course, I have calculated that if I change to a 4.22 ratio and upgrade the gearbox to a ribbed which has I am led to believe has a 2.6 speedo drive I need a 1376TPM speedo which I understand is available in gold faced, the speed would increase to 49mph at 3000rpm.
Any confirmation or otherwise and any comments or suggestions would great please.
Cheers, Ian

[chesney]

I'd say go for it.
Do bear in mind that the gearbox crossmember is different for the '1000' gearbox, so you may want to find one.
Plus depending on whether you want to keep the 803 long lever or keep the remote housing on the rib case box, you'll need to fit a later gearbox cover panel, and change the handbrake I believe.

[bmcecosse]

Your calcs are near enough correct. You don't have to change the box of course - just the final drive ratio to achieve the rpm drop, but as you have judged - getting a speedo to show correctly may be tricky if you keep the old box.

[Declan_Burns]

You can always use one of my custom built corrector gearboxes on the back of the speedo!
Regards
Declan[frame][/frame]

[SUE482]

Well thanks for the advice and comments.
Silly me I intend to keep the 803 tail piece which of course has the speedo drive in which is I believe 4.5 so I am now back to 795TPM. Not standard I don't think, but if the correction ratio is the right way round I suppose I could make use of Declan's gearboxes. 1000/1.25 = 800 near enough. Is that right Declan?
Failing that is it possible to change the speedo drive ratio on the 803 tail piece?

[mike.perry]

If you have a 1098 engine then the normal diff ratio is 4.22 :1 and the corresponding speedo TPM is 1408 depending on speedo drive ratio.
beware of fitting too high a diff ratio, 3.9 or 3.7 as you may find that the acceleration, hill climbing and top speed may suffer

[liammonty]

It's absolutely possible to change the speedo drive, BUT only if you change the gear that drives it on the 3rd motion shaft. They're a straight swap, sII and 1000 are different numbers of teeth so won't mesh. You can imagine how I know this!!

[Declan_Burns]

Well thanks for the advice and comments.
Silly me I intend to keep the 803 tail piece which of course has the speedo drive in which is I believe 4.5 so I am now back to 795TPM. Not standard I don't think, but if the correction ratio is the right way round I suppose I could make use of Declan's gearboxes. 1000/1.25 = 800 near enough. Is that right Declan?
Failing that is it possible to change the speedo drive ratio on the 803 tail piece?

I usually ask to check the ratio by driving the car at say 50mph with the correct tyre pressure and compare with a sat nav (GPS). Then I pick the best ratio possible and build the gearbox accordingly.
Regards
Declan[frame][/frame]

[IslipMinor]

Ian,

Keeping the 803 gear lever with a 1098 gearbox is not straightforward - the details are in the archive. Using it with a 948 gearbox is a straight swap, but not 1098.

If you go the 1098 route and do the mods to keep the 803 lever, then swap the speedo drive pinion from the 1098 remote housing into the 803 non-remote housing. That way with a 4.22 diff, the standard 1098 speedo will be the right tpm of 1408.

[liammonty]

Ian,

Keeping the 803 gear lever with a 10988 gearbox is not straightforward - the details are in the archive. Using it with a 948 gearbox is a straight swap, but not 1098.

If you go the 1098 route and do the mods to keep the 803 lever, then swap the speedo drive pinion from the 1098 remote housing into the 803 non-remote housing. That way with a 4.22 diff, the standard 1098 speedo will be the right tpm of 1408.

It isn't actually quite a straight swap with the 948 box, if you want to do it properly at least, as the reverse selector needs changing over, otherwise you will have no 'gate' to prevent engaging reverse instead of 4th. Also, as mentioned previously, the drive gear for the speedo on the 3rd motion shaft needs changing in order to use the SII speedo drive 'head' ~(i.e. the bit that the cable engages into in the gearbox). That entails a bit of dismantling. Otherwise, granted, it's far simpler than the 1098 alternative.

[SUE482]

Thanks for all the info and advice, I am now deliberating.
Since I also have a smooth case gearbox, presumably a 948, then that looks favourite. Only problem is that I want to keep the gold faced speedo so more thought needed.

[liammonty]

If you change the speedo drive gear in the gearbox as described, you can keep the gold faced speedo and it will work correctly. My SII was an early one, so I had to do that, as I couldn't fit a different speedo.

[814XUW]

Just keep an eye out for a 1376 tpm gold faced speedo, these are easily distinguished from the earlier models as they go up to 90mph as opposed to 80. Pop up on eBay from time to time, or try auto jumble/national rally.
They were standard fitment to 'dual personality' minors - that is, those that retained early features such as pull switches but had later drivetrain (aka 1098) fitted, from '62 to '64 roughly.

[SUE482]

OK I'm now confused, I have a ribbed and a smooth case gearbox side by side on the bench with both remotes removed, the only difference I can find is that the ribbed has a 1st motion shaft which protrudes 3.5mm further than the smooth, the speedo gear on the 3rd motion shaft is 4mm closer to the 3rd motion shaft bearing and an extra hole in the case at the top of the bell housing. The outline of the face and hole positions to be mated with the engine backplate seem identical, so if I am correct in assuming that a smooth case will mate with a 803 backplate why wont a ribbed leaving only the speedo gear to be realigned?
I know its been said that you cant, but what am I not seeing.

[bmcecosse]

The selectors won't line up - it has been done - with some machining.... The 'official' method involves a spacer plate - it is detailed here somewhere in an earlier thread.

[SUE482]

Thanks Roy and yes I can get my head around that, but I am sure I have seen somewhere where it suggests a need to replace the engine backplate as well, or have I dreamt it.

[bmcecosse]

That's more to do with clutch and release bearing travel.....nothing to do with gearlevers ! Simplest is to use a 1098 backplate and flywheel (with clutch) and 1098 release bearing - with a 1098 ribbed gearbox casing!

[SUE482]

Whats the backplate got to do with that, different thickness?

[bmcecosse]

Yes - it's thicker than the 948 backplate, so moves the gbox back slightly... . Also different thickness flywheels and the clutch too, Some other combos will work - but the one I list will definitely work!

[mike.perry]

Thanks for all the info and advice, I am now deliberating.
Since I also have a smooth case gearbox, presumably a 948, then that looks favourite. Only problem is that I want to keep the gold faced speedo so more thought needed.

If you are using a 1098 4.22 diff then you can use a gold face 1408 speedo from the early pre facelift 1098 Minor (1963-4)
0 -90mph