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Our '85 spider I installed a mechanical gauge, tapped into the system with a "T" fitting just above the starter. I fabricated a small gauge housing where the ashtray once was. Also included a mechanical water temp gauge, tapped into the system under the exhaust manifold near the back of the block(facing rearward). Removed the plug and drilled/tapped for 1/2" NPT.
Cheap insurance and the mechanical gauges rarely fail
 

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I have extra instrumentation in my Spider - Wideband AFR and oil temperature in the cab and fuel pressure under the hood, but I see no need for a mechanical oil pressure gauge. The normal behaviour is for pressure to increase with rpm until the relief valve lifts open and there is no further increase. This pressure depends upon the the oil you use and your bearing wear and on a warm engine is usually around 70 psi. As the oil temperature increases this peak pressure falls a little and is usually over 50 psi. If you see this on your electrical gauge and your oil pressure light does not come on at hot idle then everything is fine. The time to be concerned is when the regular pattern changes and temporary fitment of a mechanical gauge may be appropriate.
 

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I have extra instrumentation in my Spider - Wideband AFR and oil temperature in the cab and fuel pressure under the hood, but I see no need for a mechanical oil pressure gauge. The normal behaviour is for pressure to increase with rpm until the relief valve lifts open and there is no further increase. This pressure depends upon the the oil you use and your bearing wear and on a warm engine is usually around 70 psi. As the oil temperature increases this peak pressure falls a little and is usually over 50 psi. If you see this on your electrical gauge and your oil pressure light does not come on at hot idle then everything is fine. The time to be concerned is when the regular pattern changes and temporary fitment of a mechanical gauge may be appropriate.
Accurate oil pressure readings are a valuable resource not much for a spot reading but for longer term driving.
The driver becomes familiar what the gauge reads in various temps and speeds.
Any variations should set concern to driver.
Mechanical gauges are the most simple and dependable. Reading the oil pressure gauge with regularity saved two of my own engines; once when with and internal defect in an oil cooler and once when an oil plug failed in a crankshaft. Oil pressure didn’t go to zero, but I realized the pressure drop indicated immediate halt and examination. That was the difference between a massive engine rebuild and minor repair.
 

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Discussion Starter #24
Hi P-3.
1) Yes I have a separate oil pressure warning light via a different sender.
2) Sending unit is next to the starter low down on right of engine (looking forward)
3) Yes a 1973 Series 2 Europe spec with twin carbs.
4) Although there are 2 connectors on the sender only 1 wire goes to the gauge. The other connector is not connected to anything. Yes grounding the wire makes the gauge go to max with ignition on but engine not started. I believe this is a standard test to check gauge works OK. I think the gauge is fine as it shows drop in pressure on gauge from normal driving to tickover. The problem is the gauge is reading too high so I am assuming there is an incompatability between the gauge and the non-standard sender.,
 

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just get a new sender.
It's kind of obvious that a 2 wire sender is not correct, and then one that makes your gauge go right off the scale is even more dubious;)
any and all of the alfa shops will have the sender....they are all 1 wire, you just need the one that fits the gauge
for instance
Alfa Romeo Search results for: 'oil pressure sender'
 

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Discussion Starter #26
Hi spiderserie4,
Yes that is my intention. Only problem is I am in lock-down in Portugal so need to wait until it clears. No rush as can't use the car anyway. I only bought the car in February and engine has been rebuilt by PO so I guess they just put in a generic sender and not one matched to the gauge. I get quite a bit of stuff from Alfaholics and they have them in stock so just waiting for things to improve this end and new sender it is!
 

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I had the identical problem on my GTV. I sent the binnicle out to Palo Alto Speedometer in CA. They could not find anything wrong - a new sender from Centerline seemed to fix the issue. I had tried a spare sender at home before I sent the binnicle to CA.
 

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It is a fallacy that mechanical instruments are more accurate than electrical instruments. Mechanical ones are more easily understood but can be made inaccurate by vibration, shock, overpressure, friction in the moving parts, etc. Grab a few tire pressure gauges and compare them. You will be lucky to find two that read the same unless they are all brand new.
I spent 45 years as an instrumentation and controls engineer. The best electrical instruments are at least an order of magnitude more accurate than the best mechanical instruments.
 

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I seem to remember that Alfa used both fine thread, AND course 1/8npt thread on the low pressure light sensor. Always had to keep both in stock. The sender to the gauge was always straight machine threads. 99/100 times a low reading on the gauge was a faulty sender (unless you could hear the rods knocking). The one thing about the mechanical oil pressure gauges is, if-when the capillary tube breaks, it makes one heck of a mess! Also, @930cabman , what is a mechanical coolant temperature gauge? How do you plumb that?
 

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Discussion Starter #30
I have a mechanical coolent temperature gauge on my Mini Moke. It has a sealed tube running from the bulb (attached to the engine) all the way to the gauge. Not very accurate and a pain as the capillary tube cannot be opened so a bit of a pain to install. The tube and bulb contains (I think) ether which expands when hot and drives the gauge. I would go for electrical gauge if refitting..
 

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I seem to remember that Alfa used both fine thread, AND course 1/8npt thread on the low pressure light sensor.
Are you sure? What year? Mine is a 78 and I have both type of adapter fittings amongst my assorted tools...My Alfa is M10x1.0 for both sensors without a doubt. It's possible at some point in production timeline it was 1/8...either before or after 1978.
 

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It is a fallacy that mechanical instruments are more accurate than electrical instruments. Mechanical ones are more easily understood but can be made inaccurate by vibration, shock, overpressure, friction in the moving parts, etc. Grab a few tire pressure gauges and compare them. You will be lucky to find two that read the same unless they are all brand new.
I spent 45 years as an instrumentation and controls engineer. The best electrical instruments are at least an order of magnitude more accurate than the best mechanical instruments.
Ed, Impressive career and hats off

I have believed a fallacy fro many years, mostly because I am a hack wrench on most days and I prefer the simplicity of mechanical gauges.

Andylarry, the mechanical coolant temperature gauge works fine and uses a copper wire wrapped in a coiled steel assembly. Works fine. There is a threaded plug on the back of the block that I removed and drilled/tapped for the connection. I purchased a three gauge (1 1/2" diameter) set from Amazon for $33.
 
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