Oil Pressure Sensor Gauge Fitting - NPT or M10x1?, Can we end this? Options

[FlacaProductions]

I've been doing a lot of reading/searching here concerning the installation of an oil pressure gauge. This obviously includes installation of the requisite sender.

There is a LOT of information on this - especially the need to relocate the sender.

Some say the fitting on the motor is NPT. Some say it's M10x1.

Reputable voices and vendors with lots of experience are on both sides have ample evidence supporting their position.

So which is it?

Isn't the OEM sender M10x1? Period? Full stop? How did we get to NPT for this connection - are the threads just thaaaaaat close and it ends up working most/all of the time?

The last thing I want to do is do this install and end up with A: a leak and B: ruining the threads on the case.

My plans are to use the NLA 42 Draft Designs relocation block. Their supplied hose uses an adapter that is M10x1.

Their block has two M10x1 ports and one NPT.
NPT is for the aftermarket VDO sender and the OEM sender will go in one of the M10x1.

That tells me that the threads in the case are M10x1 - right?

[jaredmcginness]

Got these questions a lot during my time at 42DD.
Our cases should all be threaded M10. As are all aircooled VW cases. However most aftermarket senders are NPT. I suppose that's where a mix up comes in.

I recall 0-80 PSi and 0-150. Options in both thread pitches from VDO.


Not sure if this answers your question. M10 should thread in fine, I doubt someone has retapped it. The threads are not compatible.

[Superhawk996]

Setting aside the Metric vs. NPT debate the key issue is whether the threads are straight or tapered.

There are metric taper fittings. Getting into the weeds of DIN standards for metric taper is something that is probably beyond the scope of this thread.

My experience with OEM pressor sensor is that it is tapered.

Out of curiosity I just measured a OEM sensor while I'm on lunch. P/n 021.919.018A. I would agree that this is a M10x1.0 taper

Diameter at the 3rd thread is 9.94mm. Diameter at top is 10.25. M1.00 thread pitch gauge fits PERFECTLY.

But . . . . .

1/8 NPT measures 9.89mm at 3rd thread and the top is 10.27mm. For all practial purposes the delta between them could just be manufacturing tolerance of the parts I measured.

The 1/8 NPT thread pitch is ever so slightly different than M1.00 pitch. I would have to do some serious study of the NPT pitch via The Machinist Handbook to figure out exactly how minor the difference is. My metric pitch gauge says 1.00 is a bit too coarse and 0.9 is too fine. Probably 0.95mm pitch but of course my gauges don't have a 0.95 pitch so I'm guessing / estimating.

For all practical purposes the difference in pitch between 1/8 NPT and M10x1.00 Taper is so minimal that is why 1/8 NPT aftermarket products screw right in.

Since either is a taper and counting on an interference fit to seal the threads, the slight mismatch in 1/8 NPT thread pitch just means you get that interferece slightly sooner or later than would with a M10 x 1.0 taper.

Conclusion: they are so close either works. Sort of like using a 19mm or 3/4 socket interchangably since the only differ by 0.002" which is smaller than the manufacturing tolerance of the socket.

There you go - my lunchtime study of the situation.

I've never owned a set of metric tapered taps -- might have to think about ordering one (IMG:style_emoticons/default/laugh.gif)

However, using a M10 straight thread in the M10 x 1.0 TAPER sender hole will still be a leak risk since you are mating straight threads with a taper bore. Do not do that!

[jaredmcginness]

^ that is an awesome, educated response. Whenever I have questions, I'll make sure to call you on your lunch. (IMG:style_emoticons/default/biggrin.gif)

[Montreal914]

M10 x 1 tapered, makes sense:

- The block doesn't have a machined sealing surface on top.
- Why would the Germans use NPT?

[GregAmy]

Conclusion: they are so close either works. Sort of like using a 19mm or 3/4 socket interchangably since the only differ by 0.002" which is smaller than the manufacturing tolerance of the socket.

(IMG:style_emoticons/default/agree.gif)

I've successfully screwed in 1/8" NPT sensors/lines into various VW engines for many years. They seal and I've not had a problem (knock on wood!)

You can find adapters. I honestly think it's not necessary.

[davesprinkle]

OK, Ima go on record and say that I'll buy Superhawk a beer whenever I see him.

Nicely done, man.

[FlacaProductions]

(IMG:style_emoticons/default/agree.gif)

Thanks @Superhawk996
I like the way you lunch.

[brant]

I had used the NPT hoses for years
had a leak start after many years, and bought the M10 adapter

it fixed my leak

and given how big of the job was to pull the motor, strip the tin, clean everything for the new configuration...

I'd recommend to people to use the adapter from the beginning.
I wish I had... just so I didn't have to go through the extra work

I don't remember where I found it.. belmetric I think
brant

[davep]

Brake master cylinder sizes are in Inch increments.