What This Seal Does
This is the front main seal at the crankshaft snout. It rides on the crank hub sealing surface and presses into the front cover bore, holding crankcase oil in behind the damper and pulley. Specs are 5.500 in inner diameter, 6.63 in outer diameter, PTFE sealing element in a press-in lip seal design sized to the ISX front cover bore.
PTFE lip seals do not behave like rubber seals. They run dry against the sealing surface and rely on a machined lip and correct installed depth rather than spring tension alone. That means no pre-lube on the lip face, no grease packed under it, and no hand-pressing it in crooked. Use the correct driver and set it square.
The elastomer and PTFE combination is rated for ISX-series crankcase heat and oil pressure. Construction covers single- and dual-cam ISX/QSX engines, which is why the same part number keeps showing up across a wide spread of Class-8 chassis.
Why 3685173 And 136139 Cross
3685173 is the headline number most shops find stamped in the service literature or written on the old seal carrier box. 136139 is the PAI number covering the same press-in front crankshaft seal for the ISX/QSX platform. Both describe one physical part: same 5.500 in bore, same 6.63 in outer diameter, same press-in front cover fitment.
Seals cross cleanly when three things match — inner diameter at the crank sealing surface, outer diameter at the cover bore, and installed depth. Miss any one and the seal either walks out, runs on a worn groove, or never seats. These two numbers share all three, so a supplier listing one will list the other.
Listing 136139 here is for identification only. It lets you confirm you are looking at the right seal when the number you have in hand came off a distributor box rather than the engine. PartStop is not affiliated with, endorsed by, or authorized by any OEM manufacturer.
Check Before You Order
Verify the engine serial number first. ISX, ISX12, ISX15 and QSX15 engines went into Freightliner Cascadia, Columbia and Coronado, Kenworth, Peterbilt, International ProStar and LT, Western Star and Volvo chassis, and the chassis badge alone does not tell you which front cover you have. Read the number off the old seal or confirm by VIN and ESN.
Inspect the crank hub sealing surface before the new seal goes in. A grooved or scored hub will leak through any new seal within weeks. If there is a visible wear track, the hub or a repair sleeve has to be addressed at the same time. Also check the front cover bore for scoring or a distorted lip from a previous prying job.
Cross-reference is a starting point, not a fitment promise. Confirm against the number on the part you pulled or against your VIN before you order.
3685173 Cross-Reference & Interchange Numbers
These manufacturer and aftermarket numbers commonly cross-reference to 3685173. Use them to identify the part — then confirm the exact fit by VIN or the number stamped on your old unit before you order. An interchange is a starting point, not a fitment guarantee.
| Brand / Line | Cross-Reference Numbers |
|---|---|
| PAI | 136139 |
Frequently asked questions
Is 136139 the same seal as 3685173?
They cover the same press-in front crankshaft seal for the Cummins ISX/QSX platform, with matching 5.500 in inner diameter and 6.63 in outer diameter. 136139 is the PAI number, listed for identification only. Confirm against the number on your old seal or by VIN before ordering.
Do I lubricate a PTFE front crank seal before installing it?
No. PTFE lip seals are designed to run against a dry sealing surface and pre-lubing the lip can cause it to leak or roll during installation. Keep the lip and the crank hub clean and dry. Use the correct installation driver so the seal presses in square and to the right depth.
My new front seal leaked again right after install — why?
The most common cause is a worn groove in the crank hub sealing surface that the new lip cannot seal against. Second most common is an off-square or over-deep install. Third is a plugged crankcase breather building case pressure and pushing oil past the lip. Check all three before replacing the seal a second time.
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Run your VIN and we’ll match parts to your exact truck, or call the counter — a person who knows trucks verifies fitment by OEM number before the order ships. No wrong-box returns.
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