Small Salisbury Diff Rebuild

Drivetrain articles (gearbox, torque tube, diff)
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tobytj
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Posts: 1881
Joined: Tue Apr 10, 2007 3:46 pm
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Small Salisbury Diff Rebuild

Post by tobytj »

Gemini Differential Rebuild

This thread is a compilation of information to assist anyone who wants to rebuild a Holden Gemini RWD differential. There are three differential types fitted to Gemini between 1975 and 1985 – Borg Warner (TX and TC), GMH 178mm small Salisbury (TD to TG Petrol) and Imported Japanese Diff (TE to TG Diesel)

This thread covers the GMH small Salisbury type but the process should apply to other types.

Salisbury Type

The Salisbury design takes its name from … you can google it! It has a central cast steel housing into which axle tubes are pressed and plug welded, the individual diff pieces bolt or are pressed the housing. Recognisable by a cover on the rear of the housing. The other common type is the banjo type where the differential components bolts into the steel housing as a pre assembled unit. E.g Toyota hi lux and early model Holdens.

The GMH differential in a Gemini is a variation of the type found in 6 cylinder Holden sedans and Torana in the 70’s and shares some components with earlier Holden banjo diff centres.

Why Rebuild?

Simple, available used diffs have 30 or 40 years wear on them – we have just become used to the whine and excessive backlash as we jerk through the gear changes.

References

Heaps of stuff on Youtube – most not much good (hillbillies changing out GMC trucks in the dirt with blow torch and a ballpein hammer!) This is one of the best

https://www.youtube.com/watch?v=E-6I0O33HI0

Old Holden workshop manuals have good info but not Gemini specific and describe a method that uses tools you don’t likely have! Also beware of recommended bolt torques.

http://www.holdengemini.net/forums/view ... eb1e44ae41

Books – “Differentials Identification Restoration and Repair” Allen and Layman,2nd edition – interesting but too USA centric, stick to youtube.

Tools

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The GMH small Salisbury is all imperial units. Imperial bearings, imperial bolts, roll pins, etc. So you will need the following sockets/hand tools:
- 1 inch for the pinion nut. Preferably a six sided impact gun socket if available
- 9/16 inch for the axle retainer bolts (14mm will also fit)
- 11/16 for crown wheel bolts
- 5/8 inch for carrier bearing caps
- Pin Punch ¼ inch for side gear shaft roll pin
- Torque wrench
- Dead blow hammer (brass)

Specialist tools

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- Press, to get pinion gear out and press bearing
- Bearing separator, to grip the carrier bearings so you can press them (or else grind and fracture them off you hillbilly you).
- Differential case spreader (easy to make) – optional but makes things so easy
- Pinion special wrench (also easy to make from a bent of discarded torque tube shaft) – again an option but the difference between pain and pleasure.

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- Dial indicator and magnetic base – if you do not have I will discuss a rough way to estimate backlash
- An inch lb bicycle torque wrench or a fishing spring scale
- Rattle gun is handy to have

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All you need to make my pattern differential casing spreader is:
- 2 No. 100 mm x 12 mm Steel Flat 400 mm long
- 2 No. 75 mm x 10 mm Steel Flat 30 mm long
- 1 No. 75 mm x 10 mm Steel Flat 600 mm long to chop up
- 3 No. one half inch UNC or 12mm Stress grade 5 bolt and nut x about 2 ½ inch long (60mm)
- 1 No. 5/8 or 16mm mild steel bolt with at least 3 inches of unthreaded shank
- 1 No 1 /2 half inch UNF bolt Grade 5 bolt and nut (or larger if you like) with at least 60mm of thread. (a long wheel stud would do perfectly)

You need access to a decent drill (preferable drill press) and a stick welder that takes 3.2mm rods or MIG that can handle 10mm steel.

You can make a spreader out of lighter material but if you use the recommended commonly available metal sizes you will have a stiff tool that won’t deflect of buckle under load.
You need a differential with the cover off. Mounted on saw horses is good.

The spreader mounts on two 5/8 inch holes on either side of the housing opening. Cut the 5/8 bolt to make two dowels that fit the holes. Drill matching holes in the 12mm steel flats so that, when the holes line up the edge of the flat aligns with the inner edge of the housing.

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Locate and drill holes that line up with the two cover plate bolt holes either side of the dowel hole. Slot these holes so that there is some ability for the flat to rotate a few degrees around the dowel when ¼ UNC bolts are inserted. You will need some ¼ unc bolts, finger tight, to make sure the spreader is retained and stays engaged with the casing as you load it. The allowance for rotational movement ensures the dowel is loaded not the bolts.

The rest is pretty straight forward.

• Weld the dowels in place (top side only)
• Bolt the 12mm flats to the casing with the retaining bolts so that they are parallel and line up with the inner edge of the differential casing
• Measure and drill the two top sections of 10mm flats
• Drill matching holes in the 12mm flats and bolt up
• Fabricate the bottom section as shown or to your own design.

You only need about 5 turns of the nut to load up the tool and spread the casing. The amount of spread max is about40 thou or less than 1 mm. Any more and the case will yield and won’t preload the carrier bearings properly.

Pinion special wrench

You need to chop up a torque tube drive shaft to make this so try and pick a bent one. Slice of the shaft from the splined cylinder just below the weld with a 1mm cutting wheel. Knock out the remaining bit of shaft that is machine fitted into the cylinder. Use a left over piece of the 10mm flat from the spreader build and the left over bit of shaft to make the handle.
Suggest putting an old socket inside the cylinder as you weld to protect the spline from weld splatter.
Using the shaft as a handle is not an overkill as you will by putting 200- 250 ft lbs of torque onto this tool.
tobytj
Long Term Member
Posts: 1881
Joined: Tue Apr 10, 2007 3:46 pm
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Re: Small Salisbury Diff Rebuild

Post by tobytj »

A correction on my last post – The cover plate bolts are probably 5/16 UNC not 1 /4 UNC.

New bearings

The bearings used are common imperial sizes so they are readily available and cheap. For roller bearings the cone and cup/race are specified individually because the cups can vary in external diameter to suit different applications. In fact the carrier bearings for a Gemini are used in earlier model diffs with different cups listed. Where cones are mated to particular cups frequently they are listed as a “set” in catelogues for quick reference. Front wheel bearing are commonly grouped this way.

- Pinion front SET 309 M86649/M96610 x 1 $14.20
- Pinion rear SET 67 HM88649H/HM88610 x 1 $17.10
- Carrier BLM29749/29711 x 2 $26.40 each

The bearings listed are genuine Timken bearings made in USA and price is from Queensland Bearings in Zillmere, full retail including GST on 3 March 2015.
Add to the budget a pinion seal and a cover gasket/tube of goo.

I recommend use Timken as per originals so there is a better chance the clearances will be as per factory once replaced.
tobytj
Long Term Member
Posts: 1881
Joined: Tue Apr 10, 2007 3:46 pm
Location: Brisbane

Re: Small Salisbury Diff Rebuild

Post by tobytj »

Disassembly

I will assume that the rear axle assembly is out of the vehicle, torque tube removed, oil drained.

- Remove the brake drums and undo the four axle retaining bolts via the access holes in the axle flanges.
- Remove axle. This may require a slide hammer. If no slide hammer fit brake drums on backwards with a few turns of a couple if wheel nuts and use the drum as an improvised slide hammer.
- Remove the wheel bearing cups from the axle tubes. Slide hammer may be needed.
- Pry out pinion seal
- Remove housing cover plate
- Undo pinion nut and washer– with a rattle gun, or jam a piece of softwood into the carrier and use a breaker bar and socket, or use your new home made pinion wrench and breaker bar with socket. The bumper cap pressed on the end of the pinion shaft will come off with the nut.

The next stage is to remove the carrier from the housing.

- Undo the carrier bearing caps checking that paint mark on the right hand cap is still visible. If not mark the caps left and right as they must not be mixed. Remove the caps.
- Check if the carrier is loose. If bearings are badly worn it might fall out on your foot! Or
- If you have a spreader fit it and load the casing a couple of turns and see if the carrier can be freed with a pry bar. Increase the load on the casing via the spreader until the carrier can be dislodged with slight resistance. Don’t spread the casing beyond this point.
- If you do not have a spreader, pry the carrier out as best you can. A slide hammer may help.
- Tag or mark the left hand carrier bearing spacer (or the right) so they do not get mixed up because they are probably different thicknesses.

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To remove the pinion shaft you have to press it. Can’t use an ordinary puller anchored to the torque tube bolt holes because they are irregularly spaced – tried that. A Hillbilly would probably jack up has GMC truck slide the rear axle assembly underneath and lower the trucks weight onto the end of the pinion shaft (could work?)

- The front pinion bearing, large spacer washer and spline gear can be removed from the top of the differential casing when the pinion shaft falls free and bounces around the workshop concrete floor (gravel driveway for hillbillys). Good idea to have rags / blanket under press to catch it.

https://www.youtube.com/watch?v=EKXD__eHGHs


- Remove the crush spacer and check if there is a 20 thou shim under it – unlikely but it means the pinion has been removed before.
- Press off the rear pinion gear and remove the pinion depth control shim that sits under the bearing
- Using a suitable mild steel drift, drive out the front and rear pinion bearing cups from the differential case

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Carrier Disassembly Decision Time – Side gears and axle gears? If your side and axle gears look OK and there is not much backlash/shaft movement there is no need to disassemble the carrier further other than press off the carrier bearings. Otherwise:
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- Remove the crown wheel (which is a slight interference fit to the carrier) as follows:
- Place adjacent marks on the crown wheel and carrier for alignment on reassembly
- Loosen the eight crown wheel retaining bolts a couple of turns
- Use a dead blow hammer or steel hammer and brass/wood drift to loosen the crown by tapping the crown wheel and heads of the bolts.
- Repeat process until the crown wheel is free
- Drive out the 1 /4 inch roll pin retaining the side gear/carrier shaft
- Rotate the side gears on the axle shaft and the side gears will fall out of the big opening in the carrier with their mating thrust washers
- Remove the axle gears and thrust washers.

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tobytj
Long Term Member
Posts: 1881
Joined: Tue Apr 10, 2007 3:46 pm
Location: Brisbane

Re: Small Salisbury Diff Rebuild

Post by tobytj »

It is now time to press off the carrier bearings. These are a cow to get off without a bearing splitter to grip the bearing enough to press it. You may need to shatter the bearing by removing the roller cage and griding or cutting a groove in the bearing shell and striking with a chisel while the shell is supported on an anvil. DANGER WEAR EYE PROTECTION the sheet is likely to fragment and go hand grenade.

Inspection

Crown wheel and pinion

Note that that the crown wheel and pinion shaft have matching numbers meaning they have been lapped (engaged and spun in an abrasive medium) to remove high points. Pinions and crown wheels should never be mixed unless they are relapped. The extra number on the pinion shaft (+3, -3, etc.) indicated the suggested starting size of the pinion depth shim that sits under the rear pinion bearing and controls axial depth of engagement of the pinion shaft into the crown wheel. Different starting shims sizes are recommended for different matched crown wheel and pinion sets to compensate for variances in the lapping process. Some sets require more lapping than others.


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Machining of the differential housing also introduces the potential for variations in pinion depth. This variation is measured using a pinion depth tool and dummy pinion set (if you happen to have one). The size of the pinion depth control shim finally chosen is a combination of the recommended starting size for the lapped pinion and crown wheel +/- variance measured by the pinion depth tool (or at least I think that is how it works?).

Inspect the crown wheel and pinion for chipping and pitting due to corrosion.

The spline gear that connects to the torque tube should be tight on the pinion shaft spline.


Carrier

If you’re a purist you might stick the carrier in v blocks and check runout of the crown wheel mounting face. Otherwise check for burrs and run a smooth file over the carrier face to make sure it is smooth and burr free.

Side and axle Gears

Check for rust and excessive wear. You can still use gears with a bit of pitting and rust because in normal driving these gears do not rotate much.


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The main thing to check is the side gear shaft for wear.


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If a vehicle has done a lot of miles with mismatched rear tyres or lots of wheel spinning the shaft / side gear could be notable worn and sloopy. The more slop, the more unwanted gear train backlash.


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You can still buy genuine GM factory side and axle gears but you need to use the best used shaft you can find. For more info contact Charlie at Diff Lapping and Repairs in Kedron, Queensland. Charlie is a semi- retired differential specialist and looks after vintage spares a helpful advice to enthusiasts (at his kitchen table in my case).


Bearings

Don’t bother, they are nearly 40 years old, ditch them.

Cleaning

At this stage you have a grottie axles and bucket full of greasy gear parts. Grit and grot destroys bearings and gears. Set up a clean work area and move your bits in to it only after they have been cleaned (I use kero for internal bits).

A pressure clean of the axle /differential housing is a good start but I also grit blasted and painted the housing so it was spotless for the rebuild. If you grit blast make sure you wash and scrub out all the grit.


Rebuild

Generally speaking, if you are just rebuilding a diff – same housing, same carrier, same crown wheel and pinion, and you are using Timken bearings the rebuild is likely to come out close to the original factory specs.

Lubricate all bearings and moving parts with a small amount of 90 Grade HP gear oil as you assemble – enough to coat the part. But not the gear faces of the crown wheel and pinion or it will mess up your contact pattern test. It is essential that bearings are oiled before checking pre load.

Pinion bearing shells

Drive the new bearing cups into the differential housing. These are big cups and easy to miss align if you are just using a hammer and soft steel drift. If they start “ringing” and not moving – stop, drive them out and start again.

Carrier

Press on the carrier bearings. A tip – take the old bearing inner shell, hone or grind the inner surface with a dremel so it can slide on and off the carrier by hand pressure and then use it as a drive dolly for the new bearings.

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Insert one side gear, with thrust washer on the back of the gear and rotate the assembly 180 degrees. Insert the second gear and thrust washer and rotate the assemble 90 degrees. The side gears should now line up with the shaft holes. Line up and drive in the roll pin.

The crown wheel is an interference fit and one trick is to immerse it in boiling water to heat and expand it so it slides easily onto the carrier. I found that lining up the crown wheel marks then using the crown wheel bolts to progressively pull the gear on works. Torque bolts to spec in a star pattern and two stages.

Checking pinion depth

To check the pinion depth you:

• install the pinion without the crush sleeve
• tighten the pinion nut to the correct bearing preload value
• Install the carrier and crown wheel with the preload spacers
• Check carrier bearing preload (change spacer total thickness if necessary to get the preload right)
• Check the back lash – (change carrier spacers if necessary to get the backlash right but keeping the same total thickness)
• Run a pattern check and adjust the pinion height shim (see chart on website)

http://www.chevynova.com/reference/tech ... eplacement

It can be a pain if you have to change the pinion depth shim as it involves pressing off the new bearing. If this is just a rebuild and using same components and original depth control shim then the first check should come up OK. If however you are introducing new parts to the mix you might want to consider making a set-up pinion bearing. A set-up bearing is the old rear roller bearing or another new bearing (preferred) that has been honed out to fit more easily (tap on and tap off) on the pinion shaft and speeds up determining the correct pinion depth. The set up bearing cannot be used for the final build.

- Place pinion height adjustment shim on pinion than press on the rear pinion bearing
- Insert the pinion , without the crush sleeve, into the differential housing
- Fit the front bearing, spacer, drive spline and washer
- With luck you should get the nut to start but if not leave off the washer until you get the front bearing pulled down on to the shaft a bit.
- Tighten the pinion nut until you get 12 to 15 inch pounds of rotational resistance.

Tips: If you have not made a pinion spanner as per my previous post a rattle gun and block of wood to jam up pinion gear will work for this stage. A Sidchrome sliding T socket wrench (set and taped to 5 inches long) and spring balance works well. 2.5 pounds force = 12.5 inch pounds. If the pinion nut is over tightened, loosen the pinion hut and use a hammer and brass drift to relieve the preload on the bearings - you can’t do this with the crush sleeve in the final assembly.

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Write down the preload value – you will need it later for the carrier preload check.

Install the Carrier but leave out the left shim/spacer. Once the carrier is in position install the spacer using light taps of brass or nylon hammer. It is not that hard to bend, burr or distort the spacer if you hit it too hard. If you have a spreader then place a small amount of load on the spreader, start to install the spacer then wind up the spreader until the spacer taps in with token resistance). If you do not have a spreader there is a special tool to drive the spacer which has a semi-circular cradle to support the rim of the spacer/shim as it is driven. Fabricating this tool would be simple using the spacer as a template. Otherwise apply great patience.

Install carrier retainer caps and bolts and tighten to 3/4 full torque spec. Don’t mix left and right caps.


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Check the backlash using a dial gauge set at 90 degrees as shown in the photo. Somewhere around 8 thou (0.20mm). For a rough and dirty check, 8 thou is the gap on a gemini exhaust valve. If you have a spare head handy you can put it next to the diff, get the feel of 8 thou movement in your fingers and use the same, now calibrated, hand to rock the crown wheel. If the backlash is way out, and it is just a rebuild, consider the possibility that the left and right spacer got mixed up.

Run a pattern by smearing Prushian blue or similar on the coast and drive side of 6 to 8 teeth. Rotate the pinion in both direction for at least 4 complete turns of the crown wheel. You need to apply as much resistance as you can to the carrier turning using a bar or lump of wood while turning to squeeze out the Prussian blue from the contact zone and get a nice clear pattern. The back lash must be in spec first or the pattern test is a waste of time.

Check the patterns against the chart (see link above). What you are looking for is not too shallow and not too deep. Pinion depth must be adjusted to get the depth right. The pattern location between heel and toe is less critical.

Carrier Preload

Clean the Prushian blue off the teeth and lubricate the crown wheel and pinion with some 90 grade oil. Redo the preload measurement for the combined pinion and carrier preload. Subtract the pinion load and multiply the answer by 3.9 to get the carrier bearing preload. Check against spec.

Some references are bit casual about carrier preload in Salisbury diffs. A common rule of thumb is to insert the carrier and a left hand spaced only that gives 000.5 thou backlash (i.e virtually none) then pick a snug right spacer(about 010 thou bigger than the gap) that when inserted gives 008 thou backlash and the preloads will take care of itself.

Final Build


If/when the preloads, pattern and backlash are all good then tear it all down, press out the pinion and do the whole process over with the crush sleeve and 020 thou shim installed. Make sure the shim does not foul the bearing. Mine, as pictured, was a bit wide and needed some filing.
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The shim cam be made from a jam tin or any metal. You could probably go down to a 010thou shim if you like although 020 thou is recommended by early Holden manuals.
This time when the pinion nut is tightened you will hit a big ramp up in resistance. My first attempt was with a 600 ft lb rattle gun and it was no go because I could not jam the pinion gear enough with a lump of wood to resist the torque (whatever you do, do not put your hand on the pinion gear).

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If you make the pinion spanner it is just a question brute force. Use a socket and breaker bar fixed in a length of pipe and push down with all your weight on the pinion spanner anti clockwise. Stop and check pre load every 1/64th of a turn after the play in the pinion vanishes. It only takes the smallest turn to add a few pounds of preload. If you over tighten then you need to start again with an additional shim (up to a max of 2 x 020thou shims).

Carrier goes in as before with lock tight and full torque on retaining bolts. Do a final backlash, carrier bearing preload and pattern check.
Without a home made pinion spanner you will probably need to insert the carrier and jam it to tighten the pinion nut. In that case you will use the total preload reading (pinion plus carrier) as the point to stop tightening the pinion nut.

Insert axles and check that side and axle gears move freely.

Install a new pinion seal using sealant on the lip and grease on the girdle spring to prevent it falling off the rubber lip during installation. Check that the girdle spring is in place after install.

Rear cover

Have you noticed that this sucker is heavy thick steel? That’s because it is structural and when bolted up, with correct torque on the specially designed cover bolts, stiffens the differential case.

Specs

Cover bolts 25 ft lbs (dry)
Carrier retainer bolts 55-58 ft lbs with locktite
Crownwheel bolts 45-48 ft lbs with locktite
Pinion bearing preload 12-15 inch lbs
Carrier bearing preload – probably 20-30 inch lbs because larger diffs are commonly spec at 25-35 inch lbs. I would not worry about anything under 40 inch lbs.
F3ARED
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Re: Small Salisbury Diff Rebuild

Post by F3ARED »

Moved to Workshop - Driveline. Thanks Toby

Nick-
Previously GeminiCoupe/GeminiRollingShell/SCAR3D/Jonnoisac#nt

78 TD Gemini Sedan - R.I.P
81 TE Gemini Sedan
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