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Old 07-31-2010, 10:38 PM
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bluebee bluebee is offline
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Location: San Jose, California
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Next person to replace coolant temperature sensor: Please measure the O-ring size!

Can someone tell us the specification for the coolant temperature sensor o-ring? (I think it's BMW part number
UPDATE: I measured a new FAE temperature sensor o-ring at 0.330" ID by 0.115" thick (i.e., 8mmIDx3mmThick).

The reason I ask is that most say the sensor (which controls the electrical fan in front of the radiator) rarely fails, so, in theory, can be re-used; but the o-ring often fails after removal ... (it hardens with age and also fails before removal).

Normally you'd just buy a new o-ring and swap the old temperature sensor out of the old lower radiator hose into the new radiator hose ... but I asked today at the dealership and they said BMW doesn't SELL the o-ring separately, even though we think we have the right part number

But it's just an o-ring (right?). The closest I can find that E46'ers have reported success with is the 79 "BrassCraft 0567" from Lowes; but it's not a perfect fit (it's a tad fatter than the original).

If we had the specification for the E39 o-ring, we could try to find a more suitable metric replacement.

A typical O-ring specification is something like:
- OFH O-ring spec = 7.0x2.5 (i.e., 7mm ID & 2.5mm thick)
- Oil Filter O-ring spec = 91x4 (i.e., 91 mm ID & 4mm thick)

I see another report of a working Buna N size from Lowes being 5/16" ID 9/16" OD and 1/8" thick, but, that was for an E46 and maybe the E39 is different. Another E46 thread lists the 8mm ID x 3mm cross section $1 Viton (high temp / coolant resistant) O-rings from Macro Rubber as being more stable than Buna N .

But maybe the E39 coolant temperature sensor o ring is a different size.

It would be best to make original measurements on a known good E39 temperature sensor; if you have a new (green) BMW coolant temperature sensor O-ring handy, would you please measure it for the team?

What is the ID and thickness of a new E39 coolant temp sensor O-ring?

- Coolant Temp Sensor O-Ring is a BMW Part Number
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- E46 Cooling system temperature sensor part number
- Replacing the coolant temperature sensor DIY
- Recommended parts list for a cooling system overhaul
- E46 lower radiator hose coolant temperature sensor
- BrassCraft 0567 replacement for coolant temperature sensor o ring
- BrassCraft and Viton replacements for coolant temperature sensor o-ring
- Coolant temperature sensor o-ring dimensions
- Replace coolant temperature sensor o-ring with viton from macro rubber
- History of the o-Ring
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Last edited by bluebee; 09-10-2010 at 08:53 PM.
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