# Engine Crankshaft Seal Replacement in Upland > Front or rear main seal leaking? Crankshaft seal replacement in Upland starts with a dye trace, so you know which seal is leaking before approval. Source: https://route66autorepairshop.com/engine-services/engine-crankshaft-seal-replacement Updated: 2026-09-17 Business: Route 66 Auto Repair, 1775 W Foothill Blvd, Upland, CA 91786, (909) 608-7699 Hours: Monday to Friday 8 AM–5 PM, Saturday 8 AM–2 PM, Sunday closed **TLDR:** There are **two crank seals**, and the **rear main** needs the transmission removed. A **dye trace** confirms which one leaks before you approve work. Engine crankshaft seal replacement at Route 66 Auto Repair in Upland stops oil leaking where the crankshaft exits the engine. There are two seals: the front seal behind the harmonic balancer, and the rear main seal between the engine and transmission. The labor is very different for each, so we confirm which one is leaking with a dye trace before quoting. ## Front seal or rear main seal: what is the difference? The front crankshaft seal sits behind the balancer pulley at the front of the engine. A leak there flings oil onto belts and the underside of the engine. Replacing it usually means removing drive belts and the balancer, and on some engines the timing cover. The rear main seal sits at the back of the engine where the crank meets the flywheel or flexplate. A leak shows as oil dripping from the seam between engine and transmission. To reach it, the transmission comes out. That is a lot of labor for a small part, so confirming the source matters. ## How do we trace a crank seal leak? Oil from a leaking valve cover, oil pan gasket or oil pressure sender can run down to the same spot a rear main leak appears. We clean the area, add UV dye to the oil, drive the car, and follow the glow to its highest point. The PCV system gets checked too, since trapped crankcase pressure pushes oil past seals. For a broader leak search, see [engine oil leak repair](https://route66autorepairshop.com/oil-change-services/engine-oil-leak-repair) or the [oil pan replacement](https://route66autorepairshop.com/engine-services/engine-oil-pan-replacement) page. ## Our crank seal replacement process 1. **Dye trace:** Confirm which seal is leaking and photograph the evidence. 2. **Gain access:** Remove belts and balancer, or the transmission for a rear seal. 3. **Inspect the sealing surface:** Check the crank for a wear groove where the old lip rode. 4. **Install the seal:** Seat the new seal square with the correct tool, using a repair sleeve if the surface is grooved. 5. **Reassemble and recheck:** Torque to spec, refill fluids, and recheck for leaks after a road test. ## What's included in crankshaft seal replacement - UV dye trace with photos - New crankshaft seal - Sealing surface inspection - PCV check - Fluid top-off - Leak recheck after road test ## Need-now or need-later? **Need now:** - A leak that drops the oil level between changes - Oil soaking the drive belt **Need later:** - A damp rear seam with no drips, rechecked at oil changes - A rear seal best paired with an upcoming clutch job ## How pricing and timing work Front seals are usually shorter jobs. Rear main seals carry the labor of transmission removal, which is why we suggest pairing them with clutch work on manual cars, covered under [clutch repair](https://route66autorepairshop.com/drivetrain-and-additional-mechanical-services). Phone quotes are ranges until the source is confirmed. ## Aftercare Some residual oil may drip for a day as the area dries. Check your parking spot after a week. Fresh drips mean a call to us. ## Frequently asked questions ### How do I know if my rear main seal is leaking? Oil dripping from where the engine and transmission meet is the common sign, but other leaks can run to that spot. A dye trace confirms it. ### Can I drive with a rear main seal leak? A slow seep is often a need-later item if you keep the oil full. A steady drip that lowers the level should be fixed sooner. ### Should I replace the rear main seal during a clutch job? It is a common pairing, since the transmission is already out. We list it separately so you can choose. --- Call (909) 608-7699 or visit https://route66autorepairshop.com/contact for a written estimate.