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MG Midget and Sprite Technical Diff rebuild - pinion markings BBS discussion at MG-Cars.net
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MG Midget and Sprite Technical - Diff rebuild - pinion markings
| The last time I rebuilt a diff I used the trial and error method, repeatedly shimming the pinion until the pattern on the crown wheel teeth was correct, using marking blue. The diff has performed faultlessly for nearly 10 years, so I must have got it about right. I now plan to rebuild 2 more diffs which are taking up space in the garage, and would prefer to use the calculation method. This is because pulling the pinion bearings off and on again each time is a pain.
I have the original workshop manual, which says that the marking on the pinion head indicates the variance from nominal. It says this will "always be a minus figure", and gives an example of a marking of -.005". But my 2 pinions are marked .7754 and .7735. This does not make sense. Should I ignore the first 2 digits, making the markings .0054 and .0035? Les |
| L B Rose |
| The markings you need are>>
A on the crown wheel side of the carrier B on the opposite side of the carrier C & D are marked on the diff cage The formula for calculating the shim size is<< Left hand shim, A+D-C+ 0.002 thou. Right hand shim B-D+ 0.006 If there is a framed number on the crown wheel,marked in a square bracket then it should be added into the calculation as follows>> + move from left to right. - move from right to left. ie if there is a +2 marked on the frame, then two must be removed from the left and fitted to the right. There are videos on the net. |
| Dave Barrow |
| Thanks Dave, but that calculation is just for the carrier. I need also to set the pinion at the right depth. Les |
| L B Rose |
| Sorry, I misunderstood, my understanding of the pinion, is that the thrust washer is factory calculated, so the original should be used. That's what I did in my overhaul I wait to be corrected. |
| Dave Barrow |
| The pinion head spacer thickness is affected by three variables -
difference from nominal in each of housing machining, pinion machining, inner bearing thickness. Bearing variation is usually (though not always) minimal. Housing variation depends on the position of two different bearing seats (inner pinion brg and cage bearings) so is arguably the most variable. This makes using the existing spacer as a starting point a good idea. However, pinion head variation is clearly anticipated as per wsm so it may need changing after checking the contact pattern. |
| Paul Walbran |
| It would be nice to find details about rebuilding the BMC final drive but I haven't found those details. This details rebuilding a similar final drive in a Winget dumper, same axle used in my neighbours previous Morgan where we replaced the CWP and set it up, details here http://www.winget.co.uk/document/SERVICE%20%20MANUAL%20SALISBURY%205HA%20AXLES.pdf , likely needs copying and pasting due to the embedded spaces %20 , which gives basic details about what is required. It should give an understanding of what is required. I don't consider it difficult as some do, but then my neighbour and I are engineers and have the precision bits to assist in the set-up. |
| David Billington |
| Just to add- I find bearing blue a pain to use setting up diffs
i mix orange ochre(any light colour would do it) with a touch of engine oil to make it around the consistency of paint and paint it onto a few c/w teeth sparingly,just enough to fully cover the tooth , load it up and turn it over a few times and the mark is really clear Interesting with the markings on your pinion Most other diffs are marked either 0 or range from +5 to -5 as in if you have a plus 5 you sutract .005" of shim I guess if they say all are minus they are just using a different starting point but the theory would still be the same- if you have a bigger minus number you would add more shim to get it right 7754 and 7735 sound more like part nos. to me Are you sure there isn't another number etched on the pinion, on most pinions it's either on the flat end of the pinion or rarely on the shaft between the two bearings Maybe if it has no number it's a '0', some manufacturers only mark = or - and don't mark 0 MGB diffs are marked from 0 to -+5 but that doesn't help you Are your numbers stamped in or etched ,usually set heights are etched |
| William Revit |
| Most of the new cwp units I have seen of late haven't had the setting markings. Would be sooooo helpful if they did. There is usually an ID number, which should appear on both crown wheel and pinion. I think that's the 4 digit number. We did once find a new cwp "set" with non-matching numbers .... only noticed it after having tried to set it up and getting nowhere. Should have looked first! |
| Paul Walbran |
| Thanks Paul, indeed 7754 and 7735 are the ID numbers that match pinions to crownwheels. One of the latter also has 7754 etched on it although I can't see anything on the other one. On closer inspection each pinion has an etched 0 which indicates they are nominal. Sorry for the wild goose chase folks. I'm off to make a bearing puller now.
I must say the official workshop manual is very difficult to follow, especially how to use the dial gauge. It doesn't mention the ID numbers at all, hence my confusion. Les |
| L B Rose |
| I can't for the life of me remember how I removed the pinion front bearing last time. Maybe I didn't and only adjusted the crownwheel. Anyway I am doing this properly now and have welded together a puller, along the lines of the official 18G 285 tool. Blimey the bearing is tight. The puller lugs are bits of angle iron and they just bend. Does anyone have any tricks for getting this thing off? Or know where I can beg borrow or steal the proper tool? Les |
| L B Rose |
| Sorry I meant pinion rear bearing. |
| L B Rose |
| A small press is good. They are cheap enough and make quite a few other jobs easier |
| Paul Walbran |
| Paul, do you mean a hydraulic press? Can you give me an example? |
| L B Rose |
| Les, Where are you based in Wiltshire as I have a few presses you could use and I think my neighbour may have a suitable puller from when he rebuilt his Morgan final drive. |
| David Billington |
| Thanks for the offer David, I am in Salisbury. I don't mind a run out to wherever you are. |
| L B Rose |
| Les, Contact me at lbr(at)metal(hyphen)arts(dot)co(dot)uk . I'm near Bath so about an hours drive. It's only about 35-40 miles but invariably takes an hour or a bit more. I know as I used to commute through there every couple of weeks and occasionally have to go to the spinal unit at Salisbury hospital. |
| David Billington |
| I have now got one of the pinion bearings off (thanks Dave!) so I know the washer thickness. But I need one more bit of data. The workshop manual shows a dial gauge setting block with 3 steps, marked A type, B type, and C type. I think these are for the 3 axle types. The manual doesn't say what the dimension of the A type step is. I suppose the purpose is to bring the dial gauge into range. However it would be a great help to know what this dimension is. I can't find anything on the web that helps. Les |
| L B Rose |
| Les If you are doing a basic bearing replacement the pinion shim should be correct and not need changing It's measured up when initially assembled to adjust for differences in housings, If you keep the shim that's matched to a housing it should be ready to go |
| William Revit |
| Today I tried installing the pinion. I got the nut up to 140 lb/ft without any sign of the crush spacer collapsing. The bearings remained loose. Took it all apart and the spacer is actually slightly longer than another new one I have had for some time. Before I complain to Moss where I bought it, can I be doing something wrong? Les |
| L B Rose |
| "Possibly" the wrong spacer- Have a try with the other one |
| William Revit |
| Progress report. Both diffs are back together now. The crush sleeves eventually collapsed but I had to use a 3 foot jack handle on the socket wrench to do it. This was way over 140 lb/ft. I had to back the torque off a bit to get the bearing preload correct. Fortuitously it came out at close to 140, or maybe it's designed to do that.
Now having fun with the backlash. On one of the diffs the crownwheel is marked 0.008 but the backlash is twice that. I can't move enough shims to get it right, so I suspect the wear is too far outside spec. Odd though that the teeth on crownwheel and pinion both look excellent. I could get it in spec by lapping a few thou off one cage bearing and shimming the other one more, but is that too much of a bodge? 8 thou is a lot to take off. William, I forgot to answer your earlier point. The workshop manual says that the pinion depth has to be reset if the bearings are changed, which is why I was planning to do that on one of the diffs at least. However the pinion shims are not available from any of the usual suppliers, so I have drawn a veil over that idea. Les |
| L B Rose |
| Les, What value did you get for the rotational torque of the pinion after getting the spacer to crush. IIRC on the Morgan final drive I helped rebuild that was about 15 lbf-in and the pinion nut torque to achieve that similar to the spridget. |
| David Billington |
| Dave, I made up a lb/inch torque wrench. An ally tube graduated in inches, with a 1lb weight on it. The book says 11-13 lb/in and I got it to 12. I checked that again after adjusting the torque on the nut and it was correct. I checked the mesh with marking blue and it was perfect. Les |
| L B Rose |
| Les, Pinion shim looks pretty simple to make, mainly just turning but finishing to the required thickness would be best done by surface grinding IMO. I have a surface grinder but it's not up and running just yet. What manual do you use for your final drive set-up as I haven't yet found a comprehensive manual for the build of a BMC final drive. |
| David Billington |
| Les If your contact pattern is perfect as you say, then you have won A perfect contact is the whole aim of the game When you say perfect do you mean -- -A full tooth coverage -An incomplete coverage but central -Or a central pattern biased towards the toe Just curious as to what your literature suggests as being perfect Love to see a pikky of the pattern |
| William Revit |
| Dave
The pinion shim seems quite hard material, presumably to withstand the compression. I have a lathe but too small for this job, and I'm a biologist not an engineer! Worth thinking about for someone skilled in the art, as these things are no longer made. I did find some NOS in the US, but expensive. I have the BL manual AKD4021, 11th edition, which I picked up at an autojumble in 1972. I'm happy to scan the relevant pages for you if you like. William Here is a pic of the contact pattern. Looks OK to me. I used the instructions at: https://www.differentials.com/technical-help-2/installation-instructions/ I just managed to get the other diff's backlash in spec. I was being a bit hamfisted with the dial gauge before. But there are no shims on one side so I count myself lucky! Les  |
| L B Rose |
| Les, Thanks for the offer to scan the manual but I had a quick look and found the 12th edition available to download so have done so and will read the pertinent section later on. |
| David Billington |
| Les, Having had a quick read of the information I picked up this "Drop the magnet on the surface plate and zero the clock gauge to the small gauge block on its step marked 'A'. (This is the thickness of the standard bearing.)" page 137 (H.7) . This would seem to indicate if you can identify the bearing nominal thickness then you know the A step size. I couldn't see any indication of the other steps being used so maybe they're other applications with larger bearings. |
| David Billington |
| Les, A bit of a search for the diff cage bearing which I believe is 2K5943 turned up this page https://de-de.facebook.com/237368740337813/posts/rear-axle-upgrade-adding-a-39-ratio-cwp39-ratio-for-the-morris-oxford-mo-rear-ax/237370357004318/ which seems to indicate it's a 7207B angular contact bearing which would make it 35x72x17mm so that would seem to suggest the A step is 17mm high. I'll have to go out in to the workshop tomorrow and verify the dimensions to see if that is correct. |
| David Billington |
| Les,
Before locking up the garage for the evening I just checked the final drive I have apart on the bench and the diff carrier bearings measure up at 35x72x17mm . You might actually be able to use one of the bearings to set the DTI as the bearing races are pretty accurate so you could use one and measure it accurately and compensate for any deviation if required. Just pick an inner or outer race to use as a setting standard, the set-up noted in the manual deals with any deviation of the bearing inner to outer race from nominal so you can compensate with shims to get the correct pre-load. |
| David Billington |
| Les
Thanks for the pic, but I can't quite see the marking fully The marking we need to see is the drive side of the crownwheel tooth which is just out the top of your pic As far as what you have put up there , the pinion coverage looks fine and the coast side of the crownwheel tooth down there is ok but just can't see enough of the drive side up the top there Another pic. ---? From what I can see the pinion height looks fine but the crown wheel probably needs moving in a few thou another pic of up the top there will tell |
| William Revit |
| Dave I thought exactly as you have, but looking at the list of service tools I am not sure it's the same setting block. One is 18G191 and the other is 18G191A. The list shows 2 setting blocks. Quite a coincidence if the bearing thickness (which I knew is 17mm) is the same as step A on the 18G191 block. Les |
| L B Rose |
| Les ,any chance of that pic---
There are some basic rules for setting up diffs An old saying which it pays to remember is-Toe and heal move the wheel Face and flank move the crank Your face/flank marking looks nice and central on both the drive and coast sides of the tooth so your pinion height must be correct--Another sign that will show if your pinion height is out would be the toe/heel markings in different positions on drive/coast sides--eg- If the contact pattern was towards the heel on one side and towards the toe on the other then that would mean the pinion height needs adjusting Your pinion height looks fine to me, I wouldn't touch that But depending on what the heel of that drive side marking is doing (can't quite see in the pic ),the crownwheel might need adjusting in a tiddle On doing that, a question-- How much preload is on your side bearings at the moment--- When you fit your carrier into the main housing does it just slide in or is it a light push in fit or firm push or do you really have to push hard or tap it in willy |
| William Revit |
| I found this which may help clear up the 18G191 and 18G191A difference. https://www.ebay.co.uk/itm/Churchill-BMC-Tools-18G191-Bevel-Pinion-Differential-Gauges-Morris-MG-18G191A-/254321661312 click on 'see original listing' |
| David Billington |
| William, here is another photo. Shows both drive and coast sides hopefully. The bearing needed a firm to hard push. I judged that to be about right as the bearing pinch is 0.002" and is taken into account in the calculation. Les  |
| L B Rose |
| Dave, here is the page from the manual. The kit on eBay clearly says PINION SETTING GAUGE, while the manual shows two different types of block. 18G191A says it is the differential bearing gauge not the pinion one. Les  |
| L B Rose |
| Les, Yes I just saw that in another manual I found online http://www.spridgetguru.com/AKD1775/AKD1775H.pdf . |
| David Billington |
| Is this image of any help? Diagram of markings on the crown-wheel ("drive gear") is from the Jaguar Mk1 WSM. I will post another page which might be useful. Bob  |
| R L Schapel |
| The other page. Bob  |
| R L Schapel |
| Thanks for the pic. les------------Bob
I hate bearing blue-------!!!-personal choice Bright ochre for me Looking at the pic, the 2nd tooth clockwise from the end of the crownwheel bolt hole at 1 o/clock looks perfect then the 3rd and 5th teeth as it goes around and the blue is getting thinner shows that it 'might' need the pinion in a few thou closer maybe It looks pretty good though and even on drive / coast sides bias confirming that pinion height is close to correct On the 3/5 teeth where the marking looks a tiddle if anything,, up towards the face of the tooth--looking at it the other way the mark is quite good as far as toe and heal distance goes If that was mine, I'd be tempted to leave it and run with it as is But The perfect marking is one that is dead centre face and flank wise and with the toe / heal, if you have a full tooth ,good, but if it's just short of covering the full tooth it needs to be biased slightly towards the toe as when the load is applied to it during operation the load point will actually move out towards the heal a bit If you really really want to keep fiddling with it as a project-- I'd move the pinion in a couple of thou This would result in the mark going down into the flank of the tooth a tiddle but really hardly worth the hassle as it's so close already It would move the mark just a bit but wouldn't really change the backlash much at all, maybe a thou I'd leave it as is I use yellow/orange ochre for diffs just mix a little bit with some oil till its probably about like toothpaste or a bit thinner and paint both sides of a crownwheel tooth with it till it's lightly covered,just enough so you can't see metal then swing that down near the pinion and get a ring spanner c/w bolt size and holding the pinion flange for a bit of load swing the c/w tooth back and forth through the pinion teeth5-6 times the wind it back up top and have a look Leaves a magic ,easy to read marking willy |
| William Revit |
| Thanks Willy, I think I'll leave it as is after so many hours of work. I'll post pics of the other one shortly. And thanks Bob - those pics are similar to the ones I used. Les |
| L B Rose |
| Just out of interest, there's a magic oil from Castrol that I tend to poke in most diffs now It was developed for the hob cut C/W and pinions in the high performance Holden/Falcon diffs that are really noisy if run on ordinary diff oils but quiet as a mouse on this stuff It's got all the goodies already in it and ok for lim/slip diffs If you have to go and get some oil it's worth considering Castrol SAF-XA 80W140 |
| William Revit |
| I have sold the first diff on eBay. A bit disappointing as there was only one bidder, so it went for much less than I hoped. Here is what the other one looks like. I fear that the wear on the CW and pinion is too much. I have no shims on the right side and the backlash is still 0.010" when it should be 0.008. Looking closely at the blue patterns there seems to be a bit of scoring on the drive side. Also the contact is too far towards the toe to my eye. Les  |
| L B Rose |
| I reckon that was a good price Les. You can still pick up 4.22s out of rusty Morris Minors with next to no wear on them for well under a hundred, sometimes as low as 50. |
| David Smith |
| Les, I think I found it and not a lot of money. Doing a collection only auction can have quite a limiting effect on customers, I've got some real bargains that way. I know of one case of an antiques restorer that had a collection only auction and it didn't sell, so he relisted it with worldwide shipping and got an enquiry from Japan so he got a quote for crating and shipping and that person won the auction at £600 plus the shipping so everyone one seemed happy. |
| David Billington |
| Didn't realise you were in this for money Les |
| William Revit |
| It wasn't collection only, I just offered collection as an option. Item number 193157579396. Willy, it seems reasonable to clear out unwanted parts and make them available to other enthusiasts, while getting some small compensation for my time. I'm not running a business, and I'm grateful for comments, which I hope will help other readers. Les |
| L B Rose |
| Les, I found the wrong completed auction then, the auction number you gave wasn't recognised. |
| David Billington |
| ebay playing up again - I also got a 'no match' when searching on that item number yet it IS correct! Just search on (midget,sprite)(diff,differential) and tick Sold Items only, it's a couple of days ago now. |
| David Smith |
| Yes, it doesn't give a result when searching on the number, but when adding it to the usual URL, it finds it. https://www.ebay.co.uk/itm/193157579396 This NOS diff sold for £134 https://www.ebay.co.uk/itm/143405832912 |
| Dave O'Neill 2 |
| The NOS one doesn't look new to me, with all those oil stains. The CW teeth look very good but there is a wear pattern. I don't think that's from the factory lapping, as the ends of the teeth are darker from oil. Sorry this getting close to sour grapes! |
| L B Rose |
I thought the NOS final drive looked to have been used so I took a couple of pictures of the NOS 3.9 I have on the shelf after cleaning some of the protective coating both NOS 3.9s came covered internally with. The main assembly below.
 |
| David Billington |
A better close-up of some of the crownwheel teeth. The protective goo is fairly adherent and could do with a good soak in a mild solvent to remove before use. It seems to coat all the internals.
 |
| David Billington |
This thread was discussed between 17/09/2019 and 28/10/2019
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