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Discussion Starter #1 (Edited)
I want to tap the front cam and lifter oil galleries on a 1970 350 truck block. Everything I can find online and on this site claim 1/4" NPT is the correct tap, but my block won't take 1/4". The bores mic to a little over .53" back into the gallery 3/4" before tapering to approx. 7/16" bores. I tried tapping the center lifter gallery to 1/4" NPT, but the threads don't cut until hitting that tapered bore behind the cam bearing hole even with the tap buried in the bore, so the plug wouldn't block oil. I tried both a Blue Point and Matco tap, so it's not my taps.

My thought is to use 3/8" NPT since that tap calls for a 9/16" bore and these bores are just shy of that.

I'm running into the same issue trying to tap the oil filter gallery. Rather than bugger that bore, and since a 3/8" tap won't fit down in there, I'm just going to use a factory soft plug there.

Has anyone run into this issue before of not being able to use 1/4" NPT on the front of the galleries? Will 3/8" work, particularly with the lifter gallery so as to not block the cam bearing hole? Thanks.
 

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Discussion Starter #2
For posterity in the event someone down the road runs into a similar issue, I was able to drill out the first 1" of the galleries to 37/64 and tap them 3/8 NPT without issue. A steel allen plug flush with the block comes exactly to the cam oiling hole in the lifter gallery, so that worked out nicely.

I'm still incredulous that so many people claim a 1/4 NPT works on these holes. I suspect it might work on particular blocks used in a handful of magazines/books and then people who have never actually performed the modification simply repeat what they've read, incorrectly. I can't see how 1/4" could work when the factory soft plug is already 1/2". Maybe people who use 1/4 end up with two threads in the block to hold in the plug? Anyway, 3/8 works.
 

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I have been tapping the front oil passages in first gen small blocks for almost thirty years. I built my first engine when I was 15 and I'm 53 now. I've never seen a engine that you couldn't use a 1/4 inch npt tap and 1/4 inch pipe plugs. I don't know if someone had already opened yours up but every one I've came into contact with the quarter inch worked. I've never had to break a drill bit out to thread the front oil galleys. Are you sure you were using a 1/4 npt tap and not a regular 1/4 inch tap. I have seen a lot of strange things in engines that had had people inside them before.
 

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Discussion Starter #4 (Edited)
I don't know if someone had already opened yours up but every one I've came into contact with the quarter inch worked. . . . Are you sure you were using a 1/4 npt tap and not a regular 1/4 inch tap.
It's a virgin engine, standard bore, factory bearings, factory cam, factory timing chain, etc. Both the block and front pad numbers confirm 1970 truck engine. Factory soft plugs mic at approx. 1/2" and the plugs out of this engine mic at approx. 1/2". The cylinders have almost no taper, so it's a low-miles engine. I can't see how or why anyone other than the factory would have fooled with the galleries.

To confirm, I tried both a Snap-On and Matco 1/4 NPT tap, not a standard tap.

Maybe the factory changed the design on some blocks, or maybe the factory had to bore these holes larger for some reason, but with the 1/4 NPT tap buried 1-1/4" into the hole and cutting the inner tapered gallery to where the tap wouldn't go any further, the widest threads on the tap barely scraped the opening of the hole.
 

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I have been tapping the front oil passages in first gen small blocks for almost thirty years. I built my first engine when I was 15 and I'm 53 now. I've never seen a engine that you couldn't use a 1/4 inch npt tap and 1/4 inch pipe plugs. I don't know if someone had already opened yours up but every one I've came into contact with the quarter inch worked. I've never had to break a drill bit out to thread the front oil galleys. Are you sure you were using a 1/4 npt tap and not a regular 1/4 inch tap. I have seen a lot of strange things in engines that had had people inside them before.
Agreed! Although I don't have quite that many years, they have all taken a 1/4 NPT tap. :yes:
 

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or maybe the factory had to bore these holes larger for some reason, but with the 1/4 NPT tap buried 1-1/4" into the hole and cutting the inner tapered gallery to where the tap wouldn't go any further, the widest threads on the tap barely scraped the opening of the hole.
There's really no telling what may have happened when the engine was being put together. I have a 65 model car that came with some 66 parts from the factory. A lot of times I think it boils down to what's on hand when it's being built. I've probably built two dozen small blocks over the years but that's a first for me.
 
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