SKU: FRDM6701-B351
UPC: 756122670149
MPN: M-6701-B351
The One-Piece Rear Main Seal 351W is FORD's direct replacement crankshaft rear main seal for small-block Windsor engines, carrying part number M-6701-B351. It fits 351W blocks produced from 7/11/83 through 1997, along with Ford Performance 351W blocks manufactured after 6/1/93. This is the part you reach for when oil starts weeping from the back of the crankshaft, the spot where age, heat shrinkage, and seal pitting eventually take their toll. Anyone rebuilding a 351 Windsor or chasing down a stubborn rear leak will find this seal handles the job as factory-intended hardware. Browse our full gaskets and seals selection for related engine sealing components.
FORD builds this seal from high-temperature-resistant Viton, a fluoroelastomer that holds up to the heat and oil exposure found at the rear of a running engine far better than older nitrile compounds. The one-piece design wraps the crankshaft as a single ring, which cuts down on the alignment headaches and parting-line leaks that come with split two-piece arrangements. It measures roughly 5 by 5 by half an inch in the package and weighs about 0.1 lb, so it ships easily and stores without fuss. Compatibility extends beyond standard production blocks to Ford Performance castings including M-6010-BOSS35192, M-6010-B35192BB, M-6010-BOSS35195, and M-6010-BOSS351BB. Made in the United States, it matches the original equipment fit for these applications. Pair it with a fresh oil pan gasket during a reseal.
Most builders install this seal during a rear main reseal or a full engine rebuild, when the crankshaft is already accessible and the old seal has started leaking. The Viton construction earns its place in higher-output and frequently driven engines where underhood temperatures run hot and oil chemistry is aggressive. If you are working through a complete bottom-end refresh, it makes sense to address the front of the engine too with a matching timing cover seal while you have the parts apart. We stock a wide range of Windsor and small-block sealing hardware to round out the job.
It fits 351 Windsor blocks produced from 7/11/83 through 1997, plus Ford Performance 351W blocks manufactured after 6/1/93. It also covers several Ford Performance castings, including M-6010-BOSS35192, M-6010-B35192BB, M-6010-BOSS35195, and M-6010-BOSS351BB. Confirm your block date and casting before ordering.
This is a one-piece rear main seal that wraps the crankshaft as a single ring. The one-piece design reduces the parting-line leak points and alignment issues associated with older split two-piece seals, which makes it a cleaner install during a reseal or rebuild.
FORD builds this seal from high-temperature-resistant Viton, a fluoroelastomer. Viton tolerates the heat and oil exposure at the rear of a running engine better than older nitrile compounds, which helps it hold up in hotter-running and high-mileage applications.
Replace it when oil starts leaking from the back of the engine where the crankshaft exits the block. Seals fail over time from heat shrinkage and pitting. Since access requires removing the transmission or oil pan, most builders swap it during a rebuild or driveline service.
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