Core Returns

Core Returns

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 IN it'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.

Diff Rebuild Process

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.