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What follows here is a brief explanation of what I ended up doing to put oil pressure and temperature gauges in my R3. It’s not the only solution and I hope what I write here will give an idea of how it can be done.
Mazda didn’t bother putting proper oil pressure and temperature sensors in the RX8, not even for Series 2 cars, and you can’t get oil pressures and temperatures from an OBDII socket as you can for water temperature. The easiest route to taking Series 1 oil temperatures is using sensors in a sandwich plate underneath the oil filter; some people take the oil temperature from the oil pan and a few purists have tapped into oil lines elsewhere. However, the Series 2 engine has a revised oil system that includes a repositioned oil filter with insufficient room around it for a sandwich plate, see Photo 1.
In addition to general monitoring I want to investigate blanking off part of the oil cooler and radiator inlets during cold weather. See the thread on the UK owners club, RX8OC, here: RX-8 UK Owners Club ? View topic - Fitting "The Mousemat Mod"
I spent countless hours investigating alternative solutions and trying various methods of getting access for my R3, taking a good 4 times as long thinking and going down dead ends as I did doing the actual work.
Gauges
I used separate pressure and temperature gauges in a twin 52 mm vent pod (see other threads), but there are lots of alternatives, for example mounting in the ash tray cover, A Pillar pods or simple brackets on the dash. You can now get combined pressure and temperature gauges that save space.
I took the gauge feeds (12v Ignition, Earth and Lighting) from supply points I’d set up for other aftermarket electrics. The Lighting point comes from the ash tray light feed, the 12v points come from supply cables at the interior fuse box and the earth goes to a dash earth point.
The sensor cable feeds go through a hole poked in the rubber grommet taking the boot release cable through the fire wall just above the driver’s pedals (my car is RHD).
I took the pressure feed from the 1/8” BSPT blind plug used for workshop pressure testing that is located on the LHS upper rear of the, see Photo 2. The sensor that came with my gauges was too large to fit even without an adapter to take the sensor’s 1/8” NPT male thread into the engine’s 1/8” BSPT female thread. I therefore used an extension hose and block with a twin 1/8” BSPT male adapter, both freely available on eBay, as shown in Photo 3.
Access is a bit of a pain. I removed the front strut bar (8 x 14 mm nuts, 3 on each suspension tower and 2 on the bar rear), unclipped a water hose and loosened the cooling unit hose bracket (1 x 12 mm nut). To get access I put my hand in diagonally from the corner of the windscreen fluid bottle as shown by the extension bar in Photo 4. I soaked the plug in penetrating oil for a while then removed the blind plug with a 5 mm Hex/Allen bit in a 1/4” socket, ¼” articulated joint, ¼” to 3/8” adapter, 3/8” extension bars and a 3/8” ratchet as shown at the top of Photo 4; be warned though, I first tried without penetrating oil and snapped the bit.
After wrapping the 1/8” BSPT adapter’s thread with PTFE tape (it lubricates the threads to get a good mating surface seal rather than seals the threads) I inserted it fairly easily into the blind plug hole. However, turning was a PITA with a normal spanner (mine was 13 mm) as I couldn’t adequately swing it between the various obstructions; fortunately I had a short-handled (4”) spanner that made tightening a breeze. I then fixed the sensor body to a lip on the LHS tower; see Photo 5.
I ran the cable between the fire wall and its insulating blanket.
Oil Temperature Sensor
Hot oil is taken from the sump by the oil pump and sent, in order to the LHS oil cooler (inlet is the outermost of the 2 connections), the RHS oil cooler (the inlet is the outermost of the 2 connections), oil filter, passes to the Oil Control Valve and thence to the engine components and the 2 Oil Metering Pumps for the rotor tips.
I decided the best place to take oil temperature was at the outlet of the RHS oil cooler using a modified banjo bolt.
I adapted an oil cooler banjo bolt to take the oil temperature sensor that came with the gauge. I did this myself as I couldn’t get one off the shelf in the UK (there are at least 2 places in the US that do them) but I strongly recommend that you get one off-the-shelf.
After first drilling and tapping a 1/8” NPT thread in the head of the bolt I decided that my sensor protruded too far into the bolt and risked compromising oil flow. Backing out the sensor wasn’t feasible either as it left too few engaged threads for comfort. I therefore drilled and tapped a nut, lined it up on the banjo bolt head using the sensor, spot-welded it then welded it to death. I had to do a bit of tidying up of the thread as I didn’t get a perfect alignment. Next time I’ll weld on the nut then drill and tap a whole through the nut and banjo bolt.
I sealed the sensor threads with sealant and a copper washer under the sensor head. See Photos 6 and 7.
Fitting the banjo bolt and sensor was fairly straightforward:
• I jacked up the front of my car to reduce the amount of oil that could leak out and to make access easier, but this isn’t strictly necessary.
• I ran the cable under the loom on the RHS of the engine bay then down into the wheel well.
• I removed the RHS front wing/undertray lining (just take out the plastic clips and screws that hold it on and slide it off), put an oil tray under the cooler, took off the inner banjo bolt, put on the modified bolt with 2 new crush washers, torqued the bolt (53.9-68.6 Nm, 5.50-6.99 Kgfcm, 39.8-50.5 ftlbf), connected the sensor, refitted the panel, dropped my car back down then checked and topped up the oil level.
The fitted sensor is shown in Photo 8
Reference Values
Standard oil pressure is 500 kPa {5 bar, 5.10 kgf/cm2, 72.5 psi} at 3,000 rpm and an oil temperature of 100 °C {212 °F}. Once I’ve checked my engine is OK at this I’ll take reference pressures to compare with later readings.
Based on first principles, I’m initially looking for a temperature of 75-90 °C (170-215 °F) in normal road use, 100 °C (230 °F) in traffic and a maximum of 110°C (250 °F) on track. I’ll modify these with experience.
Of course, all those values are subject to the accuracy of the gauges and I’m looking more for changes with time rather than absolute values.
I couldn't go out and play today because it was raining so I decided to try out my new drill press and drilled and tapped an extra oil cooler line bolt I had laying around. I don't even have stock coolers and lines anymore but figured someone will be able to use it at some point.
Guys, if you think my welding is bad you should see my driving.
Yes, the welding isn't pretty and your comments reinforce what I wrote about getting a professionally-modified bolt. However, it's what I was able to achieve with a gasless welder in the time that I had (no hours of practise on bolts), my poor choice of a flanged nut (welding too near the edge) and the genes my parents gave me. Most importantly, it does the job and hasn't leaked yet over 300+ miles use.
I did have a back-up plan of just spot-welding the nut in place then smothering it all with JB Weld. As a precaution I did a small test of JB Weld before doing the banjo bolt and, as its spec suggests, it was perfectly happy at the oil's temperatures.
You can all probably do a better job than I did, perhaps by starting with a temperature sensor that doesn't have such a long probe to interfere with the oil flow. If you want to send me your efforts to judge best in show then that's fine with me (I shall have finished nursing my hurt feelings in my cave by the time they arrive) as I want to do a similar arrangement on my project car . . . . . . .
I couldn't go out and play today because it was raining so I decided to try out my new drill press and drilled and tapped an extra oil cooler line bolt I had laying around. I don't even have stock coolers and lines anymore but figured someone will be able to use it at some point.
Hey 9k, it would be great if you would include the part numbers for each bolt for people who will need to source them.
Clearly it can be done, but I decided that the probe on the one that I had was too thick to fit in the bores of the 1/8" NPT spacers that I could find here without taking too much out of the banjo bolt with the tools that I had.
Your sensor body (if it is a sensor on the LHS of the pic) seems a bit wide to fit the adapter in the bolt shown though.
I used the blind plug it works awesome, and extremely easy to take out, I sprayed with pb blaster for a day and it came rite out, and easy to install also.... also didn't use and kind of sealant and have no leaked highly recommend using it.... thanks for all the useful info on here
I've got my gauges and sensors - can somebody point me at a ready-to-purchase source of modified / tapped Banjo bolts to accept the sensor oil temp sensor?
I've got my gauges and sensors - can somebody point me at a ready-to-purchase source of modified / tapped Banjo bolts to accept the sensor oil temp sensor?
Hi Sean! Here you go, these are in stock and ready to ship!
will a banjo bolt work for the oil pressure as well? Noob question, but is the entire oil system pressurized? or is it just in the engine where it's pressurized?
Also - IanD, you mentioned that you put it in the outlet of the RHD oil cooler, which is the oil before it goes back into the engine... don't we want the hot oil temp and not the cool oil temp?
Just seen the recent posts, thread subscription hasn't worked.
You need the temperature of the oil going into the engine in order to ensure it is cool enough for the engine to use. The temperature of the oil in the pan is a less useful indicator of cooling performance and engine health, albeit there is some debate where the latter is easier to measure.
The oil is pressurized all the way from the oil pump sucking up oil from the oil pan to the engine via the oil coolers and oil filter (in that order).
^ given that, I may just banjo bolt the temp and pressure senders with the oil temp before the first oil cooler so that I have a hot temp reading. Anything flawed with my thinking...?
IMO yes. Oil temperature at engine entry is more useful: you want to know that it's in the right range ( too cold then you have power loss and reduced cleaning; too hot you have leak down and reduced lubrication). Oil temp should be secondary to coolant temp as the indicator of engine overheating, which will be seen as rising engine entry oil temps anyway. Furthermore, that position falls between the 2 stools of entry temp and inlet temp, hence being less useful.
IMO as you're mounting in the banjo bolts then there's no reason to go for anything other than cooler exit.
Thanks Ian for putting this up, its very helpful,
One thing I was surprised by is you used imperial threaded parts to go into what I would assume would be a metric threaded Mazda plug hole? (maybe they are so close it doesn't matter,
or maybe I don't know what I'm talking about)
02-04-2015, 09:35 AM
in my cave by the time they arrive) as I want to do a similar arrangement on my project car . . . . . . . 
