Hi all its been a while, i have just a quick question on aftermarket water temp. senders did i read (or did i dream about it ?) you could put a sender unit into the water pump drain plug hole??? if so what is the thread size i would need for the sender. many thanks Tim

You most definitely do not want to place a plug into the drain plug hole. You need a switch that threads into the second hole on the combined oil/water pump. Unscrew the Allen head plug sitting next to the oil pressure switch (12x1.5) and look at its threads (it measures 10x1.0). Another method is to look up the item in ETK because, as in this case, the dimensions are spelt out for you in the description, as is the correct part number. I use www.realoem.com. You don't mention a year or model so it's guesswork as to the exact bike you have there, tho' they're common to many models. I recommend placing this info in the signature section of your profile.

Here's a link to an 8V K100:

http://www.realoem.com/bmw/enUS/showparts?id=0503-EUR-11-1986-K589-BMW-K_100_RS_83_0502,0503,0513_&diagId=11_1818

Yes. 10x1.

The BMW OEM sender has a flush face that does not extend into the coolant passage.  When I was looking for a non-BMW replacement it seemed that nearly every other sender had an extended probe that I was afraid would block flow or possibly contact the wall of the passage.

Two Wheels Better wrote:You most definitely do not want to place a plug into the drain plug hole. You need a switch that threads into the second hole on the combined oil/water pump. Unscrew the Allen head plug sitting next to the oil pressure switch (12x1.5) and look at its threads (it measures 10x1.0). Another method is to look up the item in ETK because, as in this case, the dimensions are spelt out for you in the description, as is the correct part number. I use www.realoem.com. You don't mention a year or model so it's guesswork as to the exact bike you have there, tho' they're common to many models. I recommend placing this info in the signature section of your profile.

Here's a link to an 8V K100:

http://www.realoem.com/bmw/enUS/showparts?id=0503-EUR-11-1986-K589-BMW-K_100_RS_83_0502,0503,0513_&diagId=11_1818

Thanks for the info the age and model stated in the title

The drain plug is the one next to the oil pressure? number 16

Yes. As it happens, only last weekend I exchanged the drain plug for a temperature sensor on my K100.

Thanks for that ? what length was the sender unit ie how far in did it go? can you restrict the flow?

I used a s/h K1100 sensor from Motorworks. I didn't measure it but it looked to be about 15mm of thread. Give me a few hours and I'll pull it back out and measure it - I'm still working under the bike and there's no coolant in the ATM.

thanks

The threaded portion on the sensor is 10mm long.

And the sensor bit without the thread ?

Makes no odds as it hangs down into thin air with nothing to foul, but it's 14mm AF and about 6mm high. Too lazy to go out there again

The OEM sensor has a flush face where the coolant contacts.  It seems that every aftermarket alternative has the probe extend from the threaded portion of the body into the coolant passage, obstructing the flow.  Because the cooling is marginal at best on these bikes, I am loathe to use anything other than the OEM sensor so as not to reduce the cooling efficiency(what there is of it).

The temp sensor in the water pump has no impact on how the cooling system operates.  The thermostat and fan work the same whether you have a drain plug or a sensor in that hole.

Thanks all 
''has the probe extend from the threaded portion'' this is my only worry 
The length the probe enters the water pump, the one i have doesn't have a flush face.

timo27 wrote:Thanks all 
''has the probe extend from the threaded portion'' this is my only worry 
The length the probe enters the water pump, the one i have doesn't have a flush face.

That was my concern when I was looking for a sensor.  There are lots of low cost sensors with the right temperature curve out there, but they all have large probes that extend into the coolant passage while the flush face of the OEM sensor doesn't.  I figured that the special to BMW configuration was necessary to prevent blocking the flow.  The extra 30 bucks for the BMW sensor was cheap insurance for peace of mind when riding around the deserts of New Mexico and Arizona in the summer heat.