First I got hold of a spare 1/8" NPT temperature sender that is compatible with the Acewell unit. The only place that this can be fitted is in the Raceline sump plug. I called Raceline and they kindly informed me that the sump plug is 1/2" BSP, whereas the front plug that is used to access the gauze filter, is slightly larger at 3/4" BSP.
So, to get the sender to fit in the sump plug I needed a 1/2" BSP male to 1/8" NPT female adapter. Thankfully, eBay came to the rescue again with a nice anodised blue item. Thought I'd get blue to replace the blue Raceline sump plug.
First job was to fit the sender in the adapter. As the NPT thread is a tapered one I used just a couple of turns of PTFE tape to lubricate the threads. Then I swapped the Acewell plug for a waterproof 2-way AMP plug. The white Acewell connector was used at the other end to connect into the short wiring harness that plugs into the back of the digital dash. Some spiral wrap was added to give some protection to the wires.
The best metric copper crush washer would have been 21mm, but the nearest commonly available is 22mm. This was slightly too large and leaked. In part because the head of the adapter is not very wide, so the crush washer needs to fit very closely to the threaded part. In the end I got hold of a 1/2" BSP washer, normally used for sealing air lines and tools I believe. This seems to be holding up well so far.
I'm conscious that the end of the sender, along with the associated wiring, is a little vulnerable hanging out the side of the sump. I'm not planning on putting any additional protection on just yet, as one of my next job will be to fit a sump guard. This can be made in such a way to provide some cover for the sender and wiring.
I've been out twice now: once at around 5oC ambient air temperature and once at around 15oC. Both times the oil temperature got to only the high 70's oC. It did fall a little when moving at speed and did rise a little when at rest, so there is certainly a degree of cooling of the sender, or even sump, due to airflow under the car. I think the oil temperature is not rising further due to the large volume of oil in the Raceline sump (5 litres) plus the large surface area, including cooling/strengthening fins.
It will be interesting to see what temperature the oil gets up to on a quick run on a hot summer's day. I think once the sump guard is fitted, this will allow the sump to run a little warmer. Also, the sender is shielded a little inside the cavity of the adapter so this might knock a few degrees Celsius of the reading.