New oil pressure switch doesn’t fit...318 LA

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Righty Tighty

Righty Tighty

Blame it on the dog
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I’m rebuilding my 318 LA, ‘73 block. The system at the parts store pulled up part number PS10 as a replacement oil pressure switch. The indicator I have is the light, not a gauge.

I brought it home and attempted to thread it in, felt a little loose but it still followed the threads in the block. I started to prime the engine and oil leaked from the base of the switch. I swapped that switch out for a new one, thinking maybe the brass threads were a bit off. Same thing with the new one, I could actually “walk” it into the block without threading it in.

Of course I can’t find the old one that I pulled out of the engine when I disassembled it.

The bore in the block for the oil switch measures .395. Has anyone had this problem, or know if a part number for a pressure switch with a larger diameter male end?
 
The threads in the block are 1/8" NPT. Doesn't matter if it is a oil light or gauge, thread size is the same.
 
Okay, here’s what I’m thinking happened: when I was prepping the block for assembly, I chased all the threads. If the oil pressure switch hole is pipe threads, if I went down all the way (which I’m sure I did), then I would’ve removed the taper.
 
Yes I'm sure that is what happened. Now I would think your only fix without drilling and taping it to 1/4" NPT, is to try and find a metric tapered fitting slightly larger than 1/8" NPT.
 
You could use a step up brass NPT bushing (1/8-1/4). They usually have a flat so if it bottoms out then use an impregnated washer ( or a copper washer for that matter) to form a seal. Then use a step down bushing to 1/8 again to mount you're switch. It will raise it up a bit but you're light switch is shorter than the gauge switch so I don't think you'll have a clearance issue.
 
Is that .395" thread diameter the inside diameter of the threaded hole? Trying to figure out what you tapped it out to....almost sounds like the minor diameter of a 1/2" coarse thread. But that thread pitch is waaaaay off of 1/8" NPT.

If you did tap it out with a straight thread, you ought to be able to find a bolt to go in there so you can figure the threading.
 
3/8” fine is very close to 1/8” likely thats what went in there. unless a coarse thread went in. Really need to know what it was rethreaded to.

Going up to the next pipe thread and a bushing seems to be the only logical path. Clearing out the chips is my concern.
 
No way I would re-tap this...... that drops directly down into the main oil passage coming up from the filter and feeding to the passenger side oil gallery.... which feeds EVERYTHING.

Figure out what the threading is and buy an adapter. If one cannot be bought, then have one made from a bolt, drilled through with a hole for oil, and the threaded on the outside for another adapter to go to the proper NPT thread.
 
SGBARRACUDA said:
I am going to guess 5/16"-24?
Click to expand...
5/16" is .312" OD.... so that can't be it. And .395" is logical for the largest diameter of a 1/8" NPT. OP, just pout various bolts in 'til yo find what fits.

Also look inside and see if the new threading pattern is different from the old one. The 1/8" NPT thread pitch is quite a bit finer than any standard size bolt thread up near the size we are talking about.
 
I found a 1/8” female 1/8” male NPT adapter, and it threads into the block *pretty* good. Still not perfect, so I used some Permatex Seal + Lock thread compound. Let’s see how it does. The only bummer is that it has a 72 hour cure time. Installed it late yesterday afternoon and it doesn’t seem to have set up at all yet.
5B798362-39FC-4472-B6BB-FB1BC54B08A0.jpeg


There does seem to be a tiny pinhole where the adapter seats, I’ll close that up just to be safe.
E23DC55A-C0AD-45EF-8470-F94CE54EBBB8.jpeg
 
That pin hole will not hurt anything, the seal is the sealant on the threads.
Actually in this condition, I think a couple extra wraps of teflon tape on the male threads that go into the block, would tighten the threads enough.
 
You’re right. If not bottomed out, it’s pretty darn close. How would it cause an issue being bottomed out?
 
Righty Tighty said:
You’re right. If not bottomed out, it’s pretty darn close. How would it cause an issue being bottomed out?
Click to expand...
Threads will not seal, possible leak.
Let what you have done set up and try it, you may be ok.
The teflon tape would be your plan B.
 
Gotcha, thanks. Boy I’ll tell ya, one of the things I’ve learned building this engine is patience is necessary. Nothing can be rushed, that’s when mistakes are made or something isn’t done correctly. It’s so hard staring at the assembled engine knowing I can’t go on until that seal sets up.
 
RT. don't want to be discouraging, but 'sorta sealing' is asking for trouble. If it starts to leak on a long trip, you may not catch it in time; this point is right at the heart of the oiling system. Please back up, find out the actual threading that is in the block now, and let's figure out how to properly seal this.
 
nm9stheham said:
RT. don't want to be discouraging, but 'sorta sealing' is asking for trouble. If it starts to leak on a long trip, you may not catch it in time; this point is right at the heart of the oiling system. Please back up, find out the actual threading that is in the block now, and let's figure out how to properly seal this.
Click to expand...

I’m pickin up what you’re puttin down. The adapter I found threaded in pretty good, and I think the insurance of the thread sealant is just that - insurance.

After reading your reply just now, I tried getting it out by hand, and couldn’t. Now I didn’t bear down or strain or anything, but it’s seated very well. The switch threaded very snugly into the adapter, so I’m not worried about that, and with the adapter being as firm as it is in the block, I feel pretty confident with its soundness once the sealant cures.

I’ve never gotten bad advice from you, so what do you have in mind? I could find the tap I used to chase the threads and find an adapter that’s the same? I honestly don’t think anything I found would be much different than what I have currently though.
 
Righty no too tighty
do you not have two ports at the block
is so you could plug one with a bolt and nylon or brass washer
and then use the other
 
Wyrmrider said:
Righty no too tighty
do you not have two ports at the block
is so you could plug one with a bolt and nylon or brass washer
and then use the other
Click to expand...
Haha! Don’t worry, I didn’t wrench it down very tight when installing it, just snug. And it’s snug enough to where finger strength won’t budge it.

There’s only one port on my block.
 
I must have been thinking BBM
have my fingers crossed for u
all good questions quite righty
keep them coming
 
One of those lessons learned in engine work that you will probably (hopefully lol) never do again. Pipe threads (npt) are great at sealing, straight bolt threads are not. My thoughts are if it’s going to leak it won’t be a big leak. An old pipe fitter taught me a trick that I sometimes use on projects like this. I’ve used pipe dope and I’ve used pipe Teflon tape but he recommended using both. He would wrap the tape on (of coarse wrapping it on the right way so it didn’t want to come off) but then he would put some pipe dope on it so if you did have to back it off a little the tape wouldn’t unwrap. Good luck and I hope you have her sealed up.
 
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Righty Tighty said:
I’ve never gotten bad advice from you, so what do you have in mind? I could find the tap I used to chase the threads and find an adapter that’s the same? I honestly don’t think anything I found would be much different than what I have currently though.
Click to expand...
Yes, that is what I am thinking of. What concerns me is the 1/8" NPT thread pitch is 27 tpi.....The only thing close to the thread ID that you said you measured is 1/2" coarse thread.. which is 13 TPI. But I am just guessing on that size based on your number' finding the tap would be best for sure. Do you remember if you were cleaning some other holes with that same tap? We could go from that

So if you tapped it for that, then you may have 2 thread pitches crossing each other. The new 1/8" NPT adapter may just be hanging the top 1-2 threads left in the original tapped hole if the straight tap was run down through there. Boy, this is a challenge....

Thread repair compounds would be more suitable in some way, but IDK how well that would work with the top thread or 2 still apparently begin so intact. With that adapter, maybe JB Metal Weld would have been the way to go... take off the paint around there, and use JB to make that new adapter essentially part of the block. That is the best/easiest I can think of....
 
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