William Polowniak wrote:
>One posting
>suggests that the NPT 1/8 fitting is so close tot he 11 mm metric that it
>does not matter which is used. Any thoghts about that?

Yeah,  Use the right threads.  You won't be sorry.  1/8" NPT and M10x1
metric are not the same.  NPT threads are tapered.  M10x1 threads are
straight.  That's why VW puts a sealing washer under their oil pressure
sensors.  However, I looked at the link below and VDO says all their
sensor threads are tapered.  I haven't checked lately to be sure.

In my one and only excursion of this type I once screwed a Vanagon oil
pressure sensor (M10x1) into a 1/8" NPT brass tee to do exactly the
thing you are trying to do.  It was so tight I thought I would twist off
the sensor before I got it tight enough in the brass TEE not to leak
(more later).  I will never mismatch threads again.  Period.  If you
mismatch threads brass into aluminum, the aluminum will be the one that
gives.

Now, let's see... A $25 oil pressure sensor vs a $2000+ engine ...
Choose wisely, my friend.

Metric and NPT sensors cost the same.  You can find the correct part
number for the VDO sensor you need here:

http://www.hotrodsusa.com/buynow/manufactures/vdo/online%20catalogs/performance%20catalog/Perf%20Cat%20HTML/perf%20cat%20senders%20frames.html

Ok.  Now the first question.
>Is there any reason to use a Tee and two senders instead of the dual sender?

Use a TEE and two senders.  It's better to leave the original sensor
attached to the dynamic oil pressure system so you know when the buzzer
goes off it means something and you won't have to wonder if maybe the
sensor is wrong.  In my '86 I used the correct M10x1 compression fitting
to attach a piece of steel brake line from the oil pressure port between
the cylinders.  This line ran toward the flywheel end, straight up, and
then out to the left cylinder head to a TEE in which I fitted a VDO
pressure sensor and the original dynamic oil pressure sensor.  Using a
tubing bender the whole thing was very neat and efficient.  I had
purchased a VDO flexible extension tubing kit but I only used the clamp
for the VDO pressure sensor to attach it to a convinent hole on the top
of the cylinder head.  This worked great for many years although I never
got rid of a very small oil leak in the TEE caused by the mismatched
threads.  If I were to do it over I would have used a metric TEE, a
metric VDO sensor, and another metric compression fitting to attach to
the brake line tubing.   Metric fittings and TEEs are available from
ParkerHannifin (or whatever they are called now).

Now if your are going to mismatch threads, don't try to screw a M10x1
sensor into a 1/8" NPT hole.  It just won't fit.  A 1/8" NPT sensor
will, however, screw into a M10x1 hole, but the threads will be altered
in the hole, not the sensor and it might leak or strip out later.

I WOULDN'T DO IT!!!

I can send a picture of my oil sender setup if you like.

Ed