Sorry to necro an old thread yet again, but this really has been the best source of Clio specific info when fitting Spa gauges, and in the interest of keeping it all together it seems best to add to this one!
My 2p:
You DO need a 3/8 BSPP to 1/8 BSPP adaptor. That's parallel threads on both sides of the adaptor. I bought them from RS Online (
http://uk.rs-online.com/web/) at £8 for a bag of 5 adaptors. It was the cheapest place I could find them in the correct spec. I've still got four spare, so PM me if you want/need just one or two, save you buying a whole bag! It's not good practice to mix and match threads. Will it work? Possibly, but why bother risking it for the sake of a few quid and having a poorly functioning gauge? Threads have tolerances; one persons may go in without causing damage and create a seal, someone else's may not due to the machining tolerances involved, so it's poor advice to follow, as everyone may have different luck.
The bulkhead hole where the bonnet release goes through will fit the pressure connector through it. You don't need to drill anything if you don't want to. I modified the rubber grommet to allow my (3) cables to pass through the top of it and then refitted it. It's a bit fiddly, but it costs £0 and it works. It also keeps most of the cabling out of the way of other things.
Take the power source from the ciggy 12v live, it's yellow and orange (2003 172 is my model). If you install in the dash clock area, you can use the earthing point where the main loom screws into the dashboard support bars very easily. A 5.3mm ring connector will do the job, but try to get a flat one without the plastic connector over the crimp etc as all the others are big/flat and it won't tighten down nicely on the screw if you don't. It's really easy to get to and very easy to run a wire from, especially if you're stripped out. It's also fused separately given its for the accessories etc so you don't need to worry about it if the gauge causes an issue the car will still drive fine.
I used a Mocal sender adaptor plate, it had 2 x 1/8 NPT and 1x 3/8 BSPP (Parallel) ports as mentioned somewhere else in the thread. Good bit of kit! Bought from Merlin Motorsport.
If you're fitting a coolant temp sensor, by far and away the easiest option is to buy another thermostat housing and get a 1/8 NPT weld on ally boss from Merlin Motorsport. Take it to a shop or fit it yourself if you've got the skills. If you look at the housing, there's a fairly self evident area suitable for drilling a hole very near to the OEM sender.