HomeForumsTechnical – GeneralDrivelineBorg Warner M78. All You need to Know

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    Profile photo of rayray07
    Member since: July 19, 2015
    Posts: 41

    I am currently building a 31 spline VL turbo rear end for my VK and I struggled for a while to find detailed info on the Borg Warner M78 Diffs, I found this info on a Yank forum, ThirdGen.org. I thought it might be of some use to the VL-VS Live Rear Axle guys on here.


    All the below info is copied directly from ThirdGen.org.

    The Borg Warner 9 is actually called the M78 Rearend or “78 series”. It has been made since the late 60’s in Australia by a company called BTR Engineering Ltd. BTR was bought out by Borg-Warner under which company name the rears were produced for our cars. In the last few years Borg-Warner was bought out by Dana Corp. and the rears are still produced by Dana today in the same BTR factory under the “Spicer Axles Australia” name. http://www.spiceraxle.com.au

    The rear is offically rated @ 220 kW = 295 HP
    @ 435 Nm = 320 lb/ft
    It can support a GVW of 2740 kg = 6041 lbs

    The above ratings seem to be a bit conservative, as one member I know of has one that has survived the abuse of a 500hp/500+ for a few years now. The 9 bolt offers a few advantages over a 7.625″ 10 Bolt: 4 pinion spider gear design, shorter distance between, and a 7.75″ ring gear diameter adds strength.
    All 9 bolt rears for f-bodies use 28 spline axles.
    Another advantage over the 10 bolt is it uses pressed on tapered axle bearings allowing the axles to be bolted in instead of using c-clips. I’ve heard several people say the 9bolt is stronger than the Dana 44. I do not believe this because in my many years of research, I’ve never seen one shred of FACT to support it. The dana 44 uses bolt in axles and has a larger 8.5″ dia. ring gear versus the 9 bolts 7.75″ dia. ring gear. Most, if not all Posi Dana 44’s built for camaro’s were 4 pinion versions also. Dana 44’s were used by GM as service/warranty replacements for problematic 9 and 10 bolt rears, then offered them thru GM Performance Parts and they were then used on the 91-92 firehawks if that tells you anything.

    85-90 9 bolt equipped f-bodies were available with either standard open differentials or limited slip versions as well as several different ratios including: 2.77, 3.08, 3.27 (G92 w/auto), 3.45 (G92 w/stick), and a few 3.70 ratio’s came in some 85 & 86 Firebird’s w/ RPO Option Codes L69 (305 “High Output” Carb Engine), 5 speed trans, & G92 =Very Rare!

    3.27 and numerically higher gears will not fit on 3.08 and numerically down carriers. If you have a 2.77 or 3.08 rear and want 3.27 or higher you will have to get a 3 series carrier.http://www.9bolt.com.

    The “Limited Slip” or “posi” rears used on f-bodies used what is called the “Seperated Cone Slip Resistant Differential”.
    They also make a better version called the Hydratrak SRD but it uses a different spline count on the axles preventing it from being a direct swap into our rears

    Vehicles originally equipped with the 9 bolt rear:
    Australian made cars: Certain models of Ford Falcons since 1971, Holden Commodores from 1986 thru 1991, Chrysler Valiants since 1971, certain Aussie made Toyota Corrollas & Coronas, Nissan R31 Skylines/Pintaras & certian Datsun’s, Mitsubishi Sigmas, Leyland P76s …1997-up Morgan Plus 8s have the Hydratrak version. In the UK the rears were used on certain Vauxhall models and Jensen-Healey’s.

    I’ve also heard that Ford uses the 9 bolt center section in their Escapes and Mazda Tributes, but I haven’t verified it yet.

    The Nissan Austraila PN for 3.9 gears is 38100-J7100

    3.7 gears were used in R31 Skylines with manual trans, the PN for those gears is 38100-J7110

    4.1 gears were used in R31 Pintara’s (manual or auto) and the PN is 38100-J7000

    Holden’s and Fords got nothing shorter than 3.45 gears AFAIK.

    In 1998 the Australian magazine “Street Machine” did a series of articles on this rear axle, Another member has posted it somewhere in the thread below.




    Here is some info on checking and tightening them up by using shims. This info is from a post originally done by Karl Hunter and info provided by Miles from 9bolt.com.
    The posi units of 9 bolts will wear. One of the most common reasons a 9 bolt fails is due to excessive backlash between the ring gear and pinion (usually due to worn out pinion bearings or improper pinion preload) or between the side gear and the pinion gears (usually due to worn out brake (ie posi) cones). Either of these usually results in broken gear teeth.

    1. Place transmission in Park (or low gear for manual transmission).
    2. Raise rear tires from the ground.
    3. Lock one wheel from rotating.
    4. Measure torque required to rotate opposite wheel.
    5. If torque is less than 35 lb ft unit should be replaced or rebuilt.

    First you need to find shims. There is no G.M. part for this, you just have to use whatever is available.

    You need a variety of thickness of shims ranging from .010″ to .050″ in .005″ increments. Some types of diff pinion bearing shims will work for this purpose, but use ones with a MAX OD of 2.5″ and a Min ID of 1.5″

    Mount one axle in a LARGE bench vise with splined end pointing straight up. The vise must be securely mounted to a strong table or work bench.
    Place the differential carrier over the splined end of the axle with the ring gear teeth facing downwards.
    Mark the 2 halves of the housing for alignment during re-assembly.
    Loosen the bolts that hold the housing together. (the ones with the 7/16″ head) It is not necessary to remove the ring gear, however if it is to be removed it should be marked prior to disassembly for proper re-alignment.
    Once all the bolts holding the housing together have been fully loosened remove the upper carrier housing from the assembly. All the internal parts for the differential are now accessible. Each half of the carrier houses a friction cone and a side gear. All parts must be kept in order. Place match marks on the cross shaft and spider gears prior to removal. There should also be a spring pack in the centre of the assembly, which should have 3 springs together. G.M. recommends replacing these springs anytime the differential is apart, but if they are not broken this should not be necessary. If any other parts are broken it is recommended to find a complete replacement for the carrier, however single parts such as side gears or spider gears may be replaced individually in emergency situations. If a friction cone is worn to the point where the end of the cone is contacting the housing at the inner most point (it will be evident by the wear pattern) then the carrier must be replaced.
    Place the friction cone in its respective half of the housing and insert a shim between the side gear and the cone. Place the cross shaft with spider gears on top of the cone and gear in the housing. If the cross shaft seats fully into the housing and there is play between the spider gears and the side gear then use a larger shim. The idea is to remove all play in the gears while still allowing the cross shaft to seat fully in the housing. If too big a shim is used then the cross shaft will not seat properly and the assembly will bind causing the differential to act like a spool or possibly cause premature failure.

    Install brake cones in the differential case. Measure the distance from differential case mating surface to flat surface on brake cone when it is fully seated. This is done to determine the size brake cone shim required.

    Distance Measured In.(mm) Shim Size In.(mm)
    1.155-1.162 (29.34-29.51) No shim required
    1.163-1.167 (29.54-29.64) .005 (.13)
    1.168-1.172 (29.67-29.77) .010 (.25)


    Shim each side until all play is removed and cross shaft seats properly in case, then remove .005 to .010 inch of shim.


    Shim each side until all play is removed and cross shaft seats properly in case and add up to BUT NO MORE THAN .005″ of shim per side. This will provide additional preload to the differential that is not possible unless the gears themselves are loaded. RECOMMENDED FOR RACE USE ONLY. If the vehicle is to be street driven on a regular basis it is not recommended to preload the gears.

    The optimal amount of shim for Street-Race use is the point where all play is removed from the side gears and the cross shaft and spider gears seat properly in the housing. All one wheel peel will be eliminated and the differential will still operate normally.

    RE-ASSEMBLY (This part is easier with 2 people)
    Place components into case half without ring gear and place onto axle mounted in vise. It is important to re-assemble the case using the axles to align the internal components, otherwise the splines on the side gear and the friction cone may not be aligned and it will be impossible to insert the axle when the case is tightened.
    Place cross shaft with spider gears into lower half of case and insert preload springs and spring plates.
    Place other side gear on top of spider gears, then place the correct shim on top of the side gear then place the friction cone on top. All components should now be in place and in correct alignment. Double check to make sure there are no left over parts.
    Place other case half over the assembly. Make sure the alignment marks line up.
    Insert all bolts and hand tighten. The case may not seem to go together at this point, don’t worry, the preload springs have not been compressed yet.
    Insert other axle shaft into upper case half and align friction cone and side gear. Do not remove axle shafts until carrier is re-torqued.
    Tighten bolts little by little in a cross-pattern until the 2 case halves seat together then torque to 29 Lb.Ft.
    Remove axle from upper case half (you may have to hammer it out) and remove carrier from axle mounted in vise.
    Re-install carrier into housing making sure all parts are re-installed in original order. It will be a tight fit, one side bearing shim will have to be GENTLY tapped into place with a hammer, preferably a brass faced, or use a brass drift. Install all components except one side gear shim, then do the remaining side gear shim last and gently tap it into place.
    Install bearing caps in original positions and torque to 40 Lb.Ft.
    Install axles. They may need some persuasion with a hammer to fully seat. Tighten axle retainer bolts to 36 Lb.Ft. Don’t forget the caliper mounting plates


    Pinion Bearing Preload-Inch Lbs. NEW 12-15 USED 6-7
    Ring Gear Bolts-Ft Lbs. 65
    Bearing Cap Bolts-Ft Lbs. 45
    Axle Retainer Bolts-Ft Lbs. 36
    Rear Cover Bolts-Ft Lbs. 25
    Carrier Half Bolts-Ft Lbs. 29

    Fluid Capacity with stock cover 1.7 qts

    Backlash-.006-.010 inch

    One of the most common reasons a 9 bolt fails is due to excessive backlash between the ring gear and pinion (usually due to worn out pinion bearings or improper pinion preload) or between the side gear and the pinion gears (usually due to worn out brake (ie posi) cones). Either of these usually results in broken gear teeth.

    Set up the gear sets with a little more toe pattern and on the low side for backlash (preferrably between .006-.008). This is a little stronger, but also makes more noise. Also, if the backlash is set too low
    the gears will bind up when hot.


    RATECH http://www.ratechmfg.com
    Great source for individual parts.
    Pinion Shims                        1104*
    Severe Duty Pinion Shims 1133*
    Carrier Shims                       1105
    Crush Sleeve                         3101^
    Solid Pinion Shim               4114^ (Replaces 3101)
    Smart Sleeve                        11004^ (Replaces 3101 or 4114)
    Pinion Seal                           6120
    Pinion Nut                           1508
    RG Bolts                               1303*
    Gasket                                   5124
    Inner Pinion Bearing        Check yours and then check with ratech-they list 3 different ones for some reason
    Outer Pinion Bearing         8001*
    Install Kit (w/o bearings   130K
    * Denotes same as 7.625″ 10 Bolt
    ^ Denotes same as Ford 8″ & Dana 35Reider Racing/Precision Gear- http://www.reiderracing.com
    Best price I’ve found for an overhaul kit.
    They also sell 3.70 Gears

    US Gear/Strange 3.70 Ring/Pinion Set Part # 01-878370BW sometimes you can find one somewhere cheap on clearance.

    Auto Zone has the cheapest price on Timken Axle Bearings (Part # Set-9). You’ll need to replace the seals also, I recommend Timken 710179 for the passenger side and 224255 for the driver’s side.
    They also sell Timken pinion seals part # 7457N. I’ve never found them cheaper anywhere else.

    Randy’s Ring and Pinion sells Yukon Brand 3.70 gears and install or Overhaul Kits. They are the best price for them with an overhaul kit and new axle bearings and seals you’ll find, but I hear the overhaul kit has some mismatched parts.
    Ratech or Reider have competitive prices on their overhaul kits and their parts are all top quality including Timken Bearings and Seals.

    Larry Burd larryburd@mail.com sells shims to tighten your posi.

    And lastly, http://www.9bolt.com – The only vendor here in the US that I know of who sells ratio’s other than 3.70. Somehow he’s got a hookup in Australia and can get the OEM Holden/Ford stuff imported.
    He offers used sets from time to time and offers new 3.27, 3.45, 3.73. 3.91, and 4.10 ratios. They’re high, but they’re OEM parts. He also offers new 3 series carriers. And new internal parts for the LSD unit such as cones, spider gears, and cross shafts.

    If you desperately need small parts for inside the posi carrier and can’t find them locally you may try Dick Craft.
    Craft Differentials
    5 Peel Street, Granville
    NSW, Australia
    Phone 61 2 9637 1973

    -Torco 85w140 RGO Mineral Based Gear Oil (2 required), 1 Bottle of Torco F Type Friction Modifier (Recommended for Cone Type LSD Units).
    -Ratech Smart sleeve instead of traditional crush sleeve.
    -Ratech Severe Duty Pinion Shims 1133. Adds reliability plus makes changing gears easier.
    -Check posi and shim/tighten if necessary.
    -TA Performance offers an Alloy Girdle Cover and ARP Stud Kit for the carrier bearing caps. Highly recommended.
    -Dry film lubricant on ring and pinion as well as internal carrier gears. I used Tech Line brand and applied it myself. Just check with your wife first before using the oven to cure it…..trust me! Seriously though, it’s cheap insurance and you can get it from Jegs or Summit.
    -Another option is Moser Axles. You’ll probably break the gears first, but if you think it’s necessary….they’ll make you a set.
    -Harrop offers a 28 spline Trutrack, and a modified 31 spline unit.




    • This topic was modified 4 years, 2 months ago by Profile photo of rayray07 rayray07.
    Profile photo of TUFGMH
    Member since: March 2, 2015
    Posts: 96

    Cheers RayRay07. They are a great diff, weight and power consumption wise, and can take some punishment unlike the 10 bolts. It doesn’t take much for the whine to kick in.

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