FAQs

Cores are to be returned within 15 days or receiving the new unit, unless prior arrangements have been made. Please have the name that the differential was sold to and your order number written clearly on the box being shipped back to us to expedite the return being processed. Please complete Core Return Slip.

Core charge will only be refunded once your existing differential is (1) postmarked to us within 15 days of delivery of the differential purchased from us and (2) Diffsonline finds your existing differential to be of the same type as purchased and in rebuildable condition AS DETERMINED BY DIFFSONLINE'S sole discretion after inspection. Diffsonline's decision as to what is rebuildable/acceptable and what is not is final and not subject to challenge. If we cannot use the case, flanges, limited slip and in some newer examples (the gears) then it is NOT an acceptable core and no credit will be given. If the core deposit requires a limited slip core for full credit, and you return an open core, then you will get the credit that the open core is worth.

We're able to offer rebuilt and performance differentials at a reasonable cost due to the use of cores. A "core" is your original used differential that is returned to our diff builder to be recycled into a new, fully-rebuilt diff. If we did not have cores available, and had to buy new factory differentials from BMW, our performance diffs would be four times the price they are now. To keep this upgrade feasible for the entire enthusiast community we rely on having your good, reusable core returned to the diff builder (Diffsonline) within fourteen days of receipt of your new diff. In order to cover the cost of sourcing another used diff, a separate core charge, in addition to the advertised price, is applied to your order at the time of processing. This core charge can be refunded when you send back your original diff. Only complete and reusable cores will be eligible for a refund (no blown out, abused, or modified cores please). You must return a diff equal in components and accessories to the one sent to you.

We do not accept returns, except for core returns. Please e-mail, or call us regarding any problems with an order for assistance or return authorization number. No return will be accepted and ALL sales are final. Cores which are returned for their deposit are thoroughly inspected before a refund is issued. There is no refund on special order items. There is no refund for any shipping charge.

All differentials purchased through Diffsonline LLC require a 1,200-mile break-in period. During this period, you should keep constant vehicle speeds under 80 mph and drive the vehicle moderately. This will allow the gears, bearings and limited slip parts to break in properly and without excessive heat. After 1,200 miles the fluid should be changed in your new differential with the same recommended fluid. After the initial flush, we recommend changing the fluid every 24K miles in a street driven car.

DO NOT OVERFILL: This will cause excessive heat

To avoid premature wear of gears, bearings, and limited slip componets, all differentials must be properly broken in prior to racing. Drive slowly in the pits making turns left and right to get the oil throughout the differential. This helps to lubricate the bearings and gears thoroughly. It is recommended that after the first event you drain the gear oil to remove all break-in debris and metallic sediment. For dedicated track cars, you will want to flush the differential after the first full track day and change the fluid with every engine oil change. Replace with new oil as recommended; thereafter, drain and inspect the oil and gears as often as you would inspect your engine, and whenever it produces a foul smell from heat.

DO NOT OVERFILL: This will cause excessive heat

For all clutch based limited slips, and including the Diffsonline Race carrier, helical carriers, and open differentials, we recommend using Redline 75w110 fluid. For all builds using the M Variable limited slip, you will need to use the OE BMW XJ+FM fluid. For builds using the OS Giken limited slip, you need to use the OS Giken 80w250. For Kaaz limited slips, you need to use the Kaaz fluid.


For transmissions we recommend using Redline ATF D4 for the gear oil.

For normal street driven vehicles, we recommend changing the differential fluid every 24k and for mixed street/track vehicles, every 12k. On a dedicated track car, you will want to change the differential fluid every other engine oil change at a minimum. Ideally, every oil change is best. The differential fluid lubricates the ring and pinion gears, which transfer power from the driveshaft to the axles. The limited-slip unit requires fluid to keep all of the internal parts healthy and moving properly. Differential fluid changes areas imperative as changing the oil in the engine, and for the exact same reasons. Metal-to-metal contact wears down surfaces and creates heat from friction, which weakens parts and can lead to failure.

REM® Polishing reduces friction, heat, and wear inside your differential. The flash point of where the pinion gear meets the ring gear is where the majority of the heat inside a differential is generated. REM Polishing will reduce these temperatures thus keeping the differential in top working order. Less heat -> longer fluid life -> longer life of clutches -> better performance -> more time between needed rebuilds -> less down time and more time in the seat. Isn't that what we are doing this for in the first place?

We do not include a cover with the differential unless the cover is purchased separately. You can reuse your cover with the new differential and seal it with a proper bead of black RTV gasket maker. Be thorough when applying the bead of RTV to the differential but try to keep as much material from going into the differential when the cover is properly seated as possible.

To avoid premature wear of gears and bearings, all rebuilt transmissions must be properly run in prior to racing. Drive slowly in the pits loading and unloading the transmission in each gear to lubricate the bearings and gears thoroughly. It is recommended that after the first event you drain the gear to remove break-in debris and metallic sediment. Replace with new oil as recommended; thereafter, drain and inspect the oil and gears as often as you would inspect your engine, and whenever it produces a foul smell from heat.

For street cars we recommend changing the fluid after the first 800 miles then 20k miles for each successive fluid change


DO NOT OVERFILL: This will cause excessive heat

All rebuilt products are covered by our 2 year unlimited mileage warranty against workmanship defects. Exception is all OS Giken products or differentials equipped with an OS Giken product are sold without warranty and are intended for off road use only. There is no warranty on individual part sales. Diffsonline is not responsible for shipping (either way) or any labor charges associated with warranty claims.

BMW Diff Rebuilding, while we take in used diffs as cores, what you get is a fully rebuilt and properly set up unit that is often better than what BMW can provide. There is a lot of specialty knowledge required in the diff building industry and Diffsonline has over 20 years of experience in BMW differentials. Nearly all of the major retailers of performance differentials use Diffsonline, even when sold under other brand names. Rebuilding a diff is a very involved process :

• The core is disassembled, cleaned, and glass beaded for inspection
• All parts are run through the heated ultrasonic cleaner ensuring everything is spotless and free of debris
• The case and flanges are refinished with Wurth products
• Any machining of the welded ring gear or custom carrier is done in-house
• The desired gears are selected from the shelf
• The desired limited slip unit is built to the specs of the order
• New OEM bearings (Timkin, SKF, NTN, FAG (no junk) are installed
• Factory BMW seals are fitted
• The entire internal assembly is shimmed to perfect alignment (crucial in avoiding gear whine later on)
• Final assembly and QC checks
• Shipment to your door in special heavy-duty packaging

The packaging can be reused to send back your core. Diffs are shipped without covers and with no fluid inside. Diffsonline recommends Red Line 75/110 fluid for most differentials.

If you want to change your differential specs, get your diff
rebuilt, or don’t want to do a core exchange, we can service your differential.
This will eliminate the core charge for your order. Please print our Rebuild Form and include in the shipment with your contact info and the specs you want
the differential to be built. PLEASE DO NOT order online as web orders require
a core charge. Please email or call us to finalize order (located in bottom
left corner of website).

We also ask that you please email or call us with the
tracking number and payment info. This so we are aware of the shipment and can
get you into queue.  On applications that came with a limited slip from
the factory, the core will need to have the same style limited slip to qualify
for a full core value. IE, if you send an E36 325 diff that was an open carrier
and are buying a differential with a limited slip, you would be responsible for
the difference that we show when you would return an open core vs a core with a
limited slip.

Please be aware that failure to provide the rebuild form
with your diff or not contacting us for tracking number/payment will result in
your diff not entering our build queue.