1098 Gearbox Conversion

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1098 Gearbox Conversion

Postby SUE482 » Fri Jul 03, 2015 10:05 am

This was a thread I started in the Mechanical Section but I have edited the chit chat out and added some addition bits. It may be of some use to someone who wishes to do the same.

After many miles of putting up with my old and seemingly reliable 803 gearbox I have decided to “upgrade” to a 1098 and use the old tail extension in order to keep the interior look of the car as per original.
After much searching and advice from all on this forum and from Declan in sending me details of previously published conversions, I decided to to start from scratch and do it myself although I must say the article by T. K. Marshall was most helpful in pointing out the problem areas.

This is the result which I hope, IF ALL IS SUCCESSFUL, will assist others in doing the same modification.
I am going to ignore the ratios, mounting differences and clutch/flywheel problems of the three boxes, i.e. 803, 948 and the 1098 as this has been covered in many previous threads and just concentrate on the mating of the 803 tail piece extension and operation of the modified gearbox.
Whilst I am at it the 1098 gearbox is having a strip and full examination, the procedure of which is in the Workshop Manual or my Haynes manual although it's for the 948 box, it's basically the same.

This is the 803 gearbox

This is the 1098 gearbox with the extension removed

Here we have the the two extensions 803 on the left and the 948/1098 on the right, the 948/1098 extensions appear to be identical except for the bearing recess for the 3rd motion shaft.
and the two faces of the extension pieces again, 803/948 on the left and 1098 on the right

And this is one of the reasons why there is a need to have a plate between the gearbox and the extension, on the 1098 the 3 motion shaft bearing and circlip protrude above the face, and on the 948 gearbox the circlip is flush.


803/948 bearing recess

and the 1098 bearing recess

I did consider machining out the 803 extension to fit like the 1098 extension but as you can see there would be insufficient material left to safely house the bearing. On the 1098 gearbox it is much more meatier in this area I suspect for that very reason.

For the removal of all the parts, just follow the Workshop or Haynes manual. I found it OK up to the point when the 1st motion shaft is removed, instead drive the shaft out as it says but by only just enough to enable the circlip to be removed from the bearing and then drive it back into the gearbox and remove the assembly from the inside, the lay-gear can then be removed but measure the end float before you do. Measure the end float and allow 0.003” clearance if it's out of spec calculate and order a new rear thrust washer, if you cant obtain an exact size carefully grind to size using emery paper making sure the side you have ground down is not the face which mates up to the lay shaft. All this is in the pdf outlined below.

A good clean up is now required, I brushed Gunk on the casings and let it soak for ½ hour and then jet washed it and finally lots of blowing out with an airline. Gears etc. were soaked in diesel and thoroughly cleaned and examined. I found that all those parts you would expect to replace, lay-shaft and needle bearings, 1st and 3 motion shaft bearings including the needle one for the 3rd motion shaft, and a new rear thrust washer to control the end float on the lay-shaft, extension end oil seal and speedo oil seal, there was additionally the reverse gear that required a new bush although the shaft was fine, and a replacement baulk ring as one was quite worn.

I am going to use biomed32uk excellent pdf for assembly:
https://dl.dropboxusercontent.com/u/509 ... ebuild.pdf

Now while I'm waiting for the parts I might as well get the spacer plate ready, I am using an old 948 extension by sawing off the end bit as shown.

This now needs to be machined down to the correct width and altered to fit over the bearing. Referring to the two articles supplied by Declan, Juergen Feye-Hohmann indicates a 6.2mm (0.244) thickness whereby T. K. Marshall says 3/16” (0.188), I made it 0.158.

I have just measured the length of the selector rods from the face of the gearbox, and the difference between the 803/948 and the 1098 is 0.158, interestingly it corresponding to my plate measurement calculations. I am going for that.

By my assumption the wider the spacer plate the more packing shims are going to be required on the end bearing.

With the rebuild nearly complete, now is the time to measure the spacer plate requirements, and as you see it corresponds to my previous measurement before the rebuild, of 0.158.

This means when the plate has been fitted the top of circlip on the bearing will be flush, as per 803/948.
While we're at it the speedo gear needs to be moved back by the same amount, here it is done.

A couple of things to consider at this point, firstly the speedo gear requires replacing as I intend to change the original diff of 5.3 with a 4.2, then the speedo gear from the 1098 box will be OK if I change the speedo to a 1376TPM, lots of threads available on this subject, and secondly the reverse gear selector rod requires modification to prevent accidentally selecting reverse. This is achieved as outlined in T.K. Marshall's explanation, by welding in a step of metal so that to select reverse, the gear leaver has to be lifted over the step, like this.

Now if we go back to the reverse gear bush, removal of the old worn one is done by pressing out using a vice, and replacement again using the vice after the gear has been in the oven and the bush in the freezer for an hour. It then requires reaming out to the correct size.

Speedo oil seal and extension end oil seal are replaced in the same way by putting the extension and speedo oil seal in the freezer and the extension oil seal and speedo drive housing in the oven, not too hot though, the speedo oil seal just slipped in but the extension end oil seal had to be gently hammered home.
Note that the 803/948 speedo bushes were brass and 1098 plastic.

While I am waiting for my spacer to be machined to size I thought I'd measure up for shim requirement for the 1st motion shaft bearing, again follow the pdf or the Haynes Workshop Manual to get the correct size.

Here is the spacer plate back from the machine shop and measurements taken to establish the correct shim thickness, one other small matter is to mark and drill the hole for the extension locating peg, seen here protruding above the plate, the 803 and 948 didn't have one or at least mine didn't.

Back to the conversion, gaskets and spacer plate in position for the tail piece, but then another problem became evident, when the tail piece is fitted there is insufficient clearance with the reverse rod selector and the extension pushes the rod in so that no gears can be selected, at least that shows that the interlocking system is working. If the spacer plate had been 0.050” thicker, no problem. You live an learn.

I could think of 3 solutions, thicker plate, grind the end of the rod down or machine the offending piece to give additional clearance. I chose the later like this, you can just see the small recess to accommodate the rod.

This gives sufficient clearance on the reverse rod and all now is fine, gears select OK and the box is ready for it's tail piece.

All done and dusted, completed conversion, if I've missed anything let me know.

So to sum up then,
Spacer plate: T. K. Marshall says 0.188” I would advise 0.2” even though you will require
additional shims, alternatively grind the end of the reverse rod by the same amount, less shims.

Speedo Gear: Spacer by the same amount 0.2”, change gearing as required if altering rear axle ratio.

Reversing Rod: Weld in as outlined above.

Good luck.
Should be ready for tea time.

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