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PostPosted: Tue May 21, 2019 11:26 am 
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TBI Slant 6
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Location: Helsinki Finland
Car Model*: 1966 2D Dart
My new rebuild pulledout the filter seal. Probably the NOS pump overpressure valve is stuck. Will install a mechanical gauge to see real oil pressure. For this I need adapter. What is the thread size of the oil pressure light sender?

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PostPosted: Tue May 21, 2019 11:35 am 
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I am pretty sure it is 3/8 NPT (pipe thread).

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PostPosted: Tue May 21, 2019 10:26 pm 
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I think it's actually 1/8" NPT. The threads are approximately 3/8" but tapered. The 1/8" refers to the nominal pipe size.

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PostPosted: Wed May 22, 2019 3:10 am 
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Car Model*: 65 Valiant 2Dr Post
All that I have ever seen are 3/8 NPT

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PostPosted: Wed May 22, 2019 4:09 am 
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3/8 NPT then. No way it is 1/8, although you can use adapters to get to 1/8.

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PostPosted: Wed May 22, 2019 2:05 pm 
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1/4"-18 NPTF (Dryseal) is the smaller thread size. 3/8"-18 NPTF (Dryseal) is the larger thread. All Slant-6 oil pumps are tapped for 3/8"-18 NPTF. For whatever reason, early cars had a reducer bushing screwed into the pump, and a small-thread sender screwed into the bushing—I can only guess Chrysler had a warehouse full of small-thread senders (which they had started using in '54 or so) and wanted to use them up, or perhaps it was a result of Chrysler's shady supplier-kickback scheme of that time. Other than the thread size difference, the senders are the same, so the bushing setup does nothing but introduce another potential leak point.

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PostPosted: Wed May 22, 2019 2:40 pm 
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It probably was a surplus of 1/8"NPT senders...and no one was willing to change the block machining to suit.

The one 'technical' advantage to the reducer is that it *can* protect the block threads....which are generally a lot harder to remedy if someone crosses them. If the reducer gets damaged on the 1/8" side, you can replace the reducer and be on your way. But I doubt that's enough reason to do it.

But the more I think about it....the real question is why did they tap the block 1/4" NPT...that makes it the oddball of the Chrysler family. All the rest used 1/8"NPT so they probably didn't want to order special 1/4" Slant Six senders.


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PostPosted: Wed May 22, 2019 3:23 pm 
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GregCon wrote:
It probably was a surplus of 1/8"NPT senders...and no one was willing to change the block machining to suit. The one 'technical' advantage to the reducer is that it *can* protect the block threads....which are generally a lot harder to remedy if someone crosses them.


We're talking about NPTF threads, which are extremely difficult to cross-thread. We're not talking about block threads, though; we're talking about threads in the oil pump body. On some Slant-6 engines Chrysler did briefly (from sometime after late '63 through sometime before early '65 production) put the oil pressure sender at the rear of the block's oil gallery. When owners started complaining about the oil pressure light coming on under hard deceleration, they reverted to the oil pump location for the pressure sender and issued a TSB calling for removing the plug from the pump and the sender from the gallery and reinstalling them in swapped locations on those vehicles.

(We're also not talking about 1/4" or NPT—we're talking about 1/8" and 3/8" NPTF. Those were the two sizes used, no matter whether the engine had six or eight cylinders.)

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PostPosted: Wed May 22, 2019 4:38 pm 
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Oh. I thought the discussion was on the block threads. On my '68 that's where the idiot light sender was. NPT threads are (apparently) very easy to mangle. My old machine shop job included fixing them after the maintenance boys got through with them....


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PostPosted: Wed May 29, 2019 9:07 am 
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The original engine in my '64 Dart also had the sender in the block oil gallery rather than the pump. I assume that's why it used the smaller 1/8" NPT threads. Maybe when Chrysler transitioned to putting the sender on the pump, they used adapters until they ran out of the smaller-threaded senders.

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