Question on oil sender on BBC - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ptc · 01-21-2017, 08:50 PM

Ok Im perplexed...

On a BBC block (mines actually a Merlin BBC Clone) right above the oil filter are two threaded holes that look to be about 1/4" NPT. But they arent. In actuality a 1/2"-13 bolt threads into them both. But it seems to me that this is where the Oil Sender unit goes on these engine blocks yes?

So is 1/2"-13 thread the same as 1/4" Pipe Straight thread? or what am I missing?

Im looking to put a tee into this hole with a male thread into the block and then the two runs be 1/8" NPT - does anybody make this sort of tee?

Thank you gents!

sorry for the sideways pic

**hogfarm** · 01-22-2017, 08:02 AM

1/4"NPT is 18 threads per inch and uses a 7/16" tap drill

SideStepper · 01-22-2017, 10:10 AM

Yes, the oil sending unit location is just above the oil filter. On my factory 396 cast block there are also 2 tapped holes in this location. The hole located closer to the rear of the block is a "blind" hole (not drilled thru the block, therefore not able to detect oil pressure). The one next to it (on the same centerline as the filter) is for the oil sending unit. That hole should be a pipe tapped hole (NPT) for the sending unit fitting not a 1/2-13 standard tapped hole as you described.

A NPT tread is designed with a tapper tread to seal fitting connection and eliminate leaks. A standard tapped hole does NOT. It is only is designed to secure 2 assembled components together.

If in fact you have a 1/2-13 tapped in that location it needs to be corrected. It may be possible to re-drill & tap the hole to a larger size NPT tread & then use a reducer bushing fitting to mate with the sending unit.

dsraven · 01-22-2017, 11:21 AM

Its been awhile since i worked on a big block isnt the blind hole used for a clutch linkage ball stud on some cars? It wouldnt go into an oil gallery.

**mr48chev** · 01-22-2017, 11:33 AM

Quote:

Originally Posted by dsraven

Its been awhile since i worked on a big block isnt the blind hole used for a clutch linkage ball stud on some cars? It wouldnt go into an oil gallery.

I'd have to say your memory is correct. I think that several chassis used that spot to mount the clutch ball. Too much snow on and around my 77 dualie to go look at it to see if it does.

ptc · 01-23-2017, 01:12 PM

Thank You guys for all the replies. I think I got it figured out.

tmoble · 01-25-2017, 02:41 AM

yeah, it's a Dart block. for whatever reason known only to themselves they tap all the pipe plug holes with NPS, National Pipe Straight instead of NPT. PITA, problem saves a few pennies on taps or something.

I re-tap all the pipe plug holes on those blocks, but you really can't do that after the engine is built. Engine builder should have taken care of this.

01-21-2017, 08:50 PM	#1
ptc Registered User Join Date: Mar 2015 Location: Morada, CA --- (Near Lodi) Posts: 1,443	Question on oil sender on BBC Ok Im perplexed... On a BBC block (mines actually a Merlin BBC Clone) right above the oil filter are two threaded holes that look to be about 1/4" NPT. But they arent. In actuality a 1/2"-13 bolt threads into them both. But it seems to me that this is where the Oil Sender unit goes on these engine blocks yes? So is 1/2"-13 thread the same as 1/4" Pipe Straight thread? or what am I missing? Im looking to put a tee into this hole with a male thread into the block and then the two runs be 1/8" NPT - does anybody make this sort of tee? Thank you gents! sorry for the sideways pic Attached Images __________________ My Build: 57 Chevy Pro-Street

01-22-2017, 08:02 AM	#2
hogfarm Senior Member Join Date: Jan 2016 Location: Appleton Washington Posts: 592	Re: Question on oil sender on BBC 1/4"NPT is 18 threads per inch and uses a 7/16" tap drill

01-22-2017, 10:10 AM	#3
SideStepper Registered User Join Date: Jun 2002 Location: Southern California U.S.A. Posts: 279	Re: Question on oil sender on BBC Yes, the oil sending unit location is just above the oil filter. On my factory 396 cast block there are also 2 tapped holes in this location. The hole located closer to the rear of the block is a "blind" hole (not drilled thru the block, therefore not able to detect oil pressure). The one next to it (on the same centerline as the filter) is for the oil sending unit. That hole should be a pipe tapped hole (NPT) for the sending unit fitting not a 1/2-13 standard tapped hole as you described. A NPT tread is designed with a tapper tread to seal fitting connection and eliminate leaks. A standard tapped hole does NOT. It is only is designed to secure 2 assembled components together. If in fact you have a 1/2-13 tapped in that location it needs to be corrected. It may be possible to re-drill & tap the hole to a larger size NPT tread & then use a reducer bushing fitting to mate with the sending unit.

01-22-2017, 11:21 AM	#4
dsraven Registered User Join Date: Nov 2010 Location: calgary alberta Posts: 8,333	Re: Question on oil sender on BBC Its been awhile since i worked on a big block isnt the blind hole used for a clutch linkage ball stud on some cars? It wouldnt go into an oil gallery.

01-23-2017, 01:12 PM	#6
ptc Registered User Join Date: Mar 2015 Location: Morada, CA --- (Near Lodi) Posts: 1,443	Re: Question on oil sender on BBC Thank You guys for all the replies. I think I got it figured out. __________________ My Build: 57 Chevy Pro-Street

01-25-2017, 02:41 AM	#7
tmoble Registered User Join Date: Feb 2009 Location: Glendale, Az Posts: 1,062	Re: Question on oil sender on BBC yeah, it's a Dart block. for whatever reason known only to themselves they tap all the pipe plug holes with NPS, National Pipe Straight instead of NPT. PITA, problem saves a few pennies on taps or something. I re-tap all the pipe plug holes on those blocks, but you really can't do that after the engine is built. Engine builder should have taken care of this.



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