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I have had many Alfa's and I have only seen NPT threads on the oil pressure switch. The original thread into the block may well be M10 and maybe the original switches were too, but they were all NPT by the time they got to me. Use teflon tape on the threads and don't worry about it.
 

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Push hard and live
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As noted earlier...

NPT really should not be installed into an Alfa block intended for a 10mm straight thread. The tapered NPT will stress and wear the 10mm hole. While you might get away with it, I suggest we should not knowingly damage something that may lead to scrapping the block in a future year.

Aluminum is soft, and fairly easily fatigued. Every time an NPT is screwed into that hole, it will remove some aluminum, and force the hole and it’s threads to be a little displaced, and wider.

Using the correct parts is a small cost compared to gaining long term service and retaining value.
 

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Don,
Can you suggest a source for suitable oil pressure switches with M10x1 thread?
 

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But Mad North-Northwest
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I'm going from memory but I'm pretty sure at some point OP light switch was NPT, at least on the later cars, so there may be some grounds for confusion here.

On my '91 the light sender is attached to the block via an L-shaped brass adapter. The connection at the block is an M10x1 banjo bolt (with copper washer at the mounting surface) but the sender is clearly tapered thread. This is how the S4, at least, came from the factory. Found this photo that shows it, my car looks the same.

If the sender screws directly into the block it should be M10x1 straight thread, but not sure if that part is available. It looks like Classic Alfa EN057 is M10x1? Maybe ask them, they tend to be pretty responsive.

OP685 from Centerline is for '65-'89 Spiders, so that's probably M10 as well. Maybe the cutoff for NPT was the S4 engine in 1990?

1618216
 

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But Mad North-Northwest
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Actually, a better option may be Rock Auto. If you search for the S3 or earlier, they have multiple options listed as M10x1 for about $6 each.

They don't have a part listed for the S4, which suggests that the S4 used a different sender per above. No idea why they would've switched to NPT, but they appear to have done so?
 

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1978 Spider
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Discussion Starter #67 (Edited)
I'm gonna have to side with Don...the right thread has to be M10. I just don't see any explanation why any European car would even have anything but a metric fitting. All US cars are metric since the 80s. This has to be a pre-80s Ford/Chrysler/Gm spec sender?

I've searched Rock Auto, Autozone, Oriellys, ebay…I think every brand sensor or part number I find is for M10 threads. they don't always say but if I google some of the part numbers and brands I often get M10x1.0 in the results.

How I ended up with an NPT one I don't know. It's likely the PO was sold an NPT one by mistake and 1/8 NPT is just close enough that he didn't notice.

I would even venture to say that probably any oil press light sender from any car any year, US or Euro/Asian, metric or NPT is likey functionally identical...they will all trigger the light at approximately the same psi...if it "fits the hole" it will work.

I wonder how many spiders are running around with NPT sensors? by accident or is that OEM? anyway to know for sure?
 

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1978 Spider
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Discussion Starter #68
Here is a Tee I machined from brass stock. My mechanical oil pressure gauge, electric pressure gauge, and low pressure warning Pro Lite. I think my bases are covered.
is that gonna clear the starter?
 

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I just checked my spare oil pressure switches. Two are 1/8" NPT with a 21mm hex labelled elbi po-100.
One is M10x1 with a 24mm Hex labelled Messmer W Germany. All three were removed from 105 Alfas, either S2 Spider or GTV.

AFAIK, there is no metric tapered thread so someone wishing to use one has a choice between NPT and BSP.

I agree that the blocks are most likely tapped M10x1
 

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Ed: I'd/we'd be interested in the resistance reading from the base/frame to the output connector on those . In the bench.
Even if you think they are bad. Please list the markings if any.
 

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1978 Spider
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Discussion Starter #72
ok..found this old discussion...seems the light sensor is tapered...but is it metric or NPT or a British standard taper? Is there even a "metric taper" or is it really British or NPT? And how are British and US NPT any different, or different name for the same thing?


 

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Joe, They are switches that have about 0.4 ohm contact resistance.
 

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But Mad North-Northwest
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ok..found this old discussion...seems the light sensor is tapered...but is it metric or NPT or a British standard taper? Is there even a "metric taper" or is it really British or NPT? And how are British and US NPT any different, or different name for the same thing?
Well, in thinking about it it would probably be M10 tapered vs. the NPT I said above. Alfa wouldn't use standard thread, it would be metric.
 

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Ed: Sorry. I thought you were talking about oil pressure senders.

If you have any of these would you (and any others) get a reading on an ohm meter for us?
As I said before I got a reading of 32 ohms on one that I just took a 74 2000 engine.
Didn't make sense looking at that chart posted a while ago.
I'm waiting for a replacement from Centerline and I was just
curious as to what others are seeing on an Ohm meter.
 

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With no pressure it should be about 340 ohms
 

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I know what it "should be" but curious minds want to know, as Joe Friday said "just the facts mam".
 
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