[Tigers] oil gauge block adaptor

[Terry Lawell]

Ron,

  The fitting I pulled out has an NPT 1/4-18 on the block side and what 
appears to be a BSPP 1/4" on the line side. I bought a 1/4-18 tap and 
die from NAPA today and did some checking. The die fit on the fitting 
very easily and was still easy all the way to the base. The tap actually 
went into the block quite easily and then tightened towards the end of 
the tap. So the conclusion was there is nothing wrong with the fitting 
or the threads in the block. However, when I inserted the fitting into 
the block, it would progress about 4 threads before it stopped. I took 
it out and inserted a 1/4" MIP brass fitting into the block and it 
tightened down after about 7 threads. I pulled it out and measured the 
thread diameter at the point where it stopped. They were at about the 
same diameter reading but were at different depths on the fittings. It 
turns out my original fittings has a different taper than the brass 
fitting as well as being much thicker at the base end. I am thinking 
someone may have used an NPTF thread which gets crushed at insertion, is 
very hard to extract, and cannot be used again. This would address all 
of the symptoms I experienced while removing the fitting and trying to 
reinsert it back into the block. I am going to order a new fitting and 
see what happens.
  Thanks for doing additional research on it for me.

Cheers, Terry


On 7/4/21, 11:02 AM, Ron Fraser wrote:
>
> Terry
>
> I did some more digging and I find there is a difference in the number 
> of threads per inch.
>
> NPTF
>
> 1/8” 27 threads/ inch
>
> ¼” 18
>
> BSPT
>
> 1/8” 28 threads/ inch
>
> ¼” 19
>
> So these 2 systems have threads that are not totally compatible or 
> interchangeable.
>
> I believe the tubing end of the adaptor is parallel thread.
>
> The threads per inch is the same for either system tapered or parallel.
>
> I would suggest measuring the part accurately is the only way to know 
> for sure.
>
> Ron Fraser
>

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