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I'm seeing something I didn't realize before. I thought you had drilled the same diameter as the dowel. You have a small hole at the bottom of a larger hole. Are you going to tap the large hole or the small hole?
The small hole looks like it may not have enough wall depth for a tapered thread. And the large hole doesn't seem to have enough depth for even a bottoming tap.
I would be tapping the smaller hole. The larger one is the dowel pin hole so it cannot be touched because I need to put my new dowel pins in there of course.

Fortunately, I have my engine out and apart right now so I could go have a look.

The bottom hole on the oil filter adapter, the one that you drilled into, is the pressure side of the oil pump before it goes into the filter. You can see in the 5th picture you posted that shows the back side of the block, that passage makes a 90 degree turn and goes down to the oil pump. The upper passage across the back of the block is the one that comes out of the filter and goes across to the lifter gallery, "output" from the filter. So, that hole you drilled is going to "see" the maximum oil pressure that the pump puts out.

True, that's a small hole and very shallow to be able to cut a tapered thread into for a small pipe plug, but I think the good news is that I don't think it HAS to be a tapered pipe plug to work. That's because the dowel pin will be sitting on top of it, and that ought to keep whatever you put in there from backing out. If it were me, I'd consider cutting normal threads into that small hole and putting a regular hex socket plug into it, with JB weld packed into the threads to seal it. Once that dowel pin is sitting 'behind' and on top of it, you shouldn't have to worry about it backing out, and if you make sure to get both the threads on the hole and the threads on the plug nice and clean, that very thin layer of JB weld that will be left after you put the plug in ought to seal it nicely. Make sure the plug doesn't protrude into the oil passage so as not to introduce a flow restriction. If you go this route, coating your tap with heavy grease before you cut the threads will help trap the chips so they don't wind up in your oiling system. Also with the pan off and the pump removed, you can blow air into that passage from the oil pump mounting face and that will clear it out. If for some reason you don't get a good seal, it should be obvious because you'll be able to see oil leaking from around that dowel pin after it's all up and running. It wouldn't hurt to use some kind of sealant around the dowel pin, perhaps something like regular old pumber's "pipe dope" for insurance.

My .02 - from someone who's also made more than his share of mistakes and created additional problems for himself therefrom.

Bear
As far as wall depth, if I understand you, I measured it at about 0.450". That's from the back of the dowel pin hole to the edge of the oil passage. To put it another way, this is the maximum length of the threads I would cut, or the maximum length of the plug. Is that enough for tapered threads?

A concern I have of cutting regular threads is, what would stop it from "backing in" to the oil passage? I know the dowel pin would stop it from backing out, but would the oil pressure (or JB Weld) -always- be sufficient to keep it from continuing into the oil passage?
 

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There are many ways to do what you need to do.

  • Aside from the tap itself, theres no difference between pipe threads (tapered) and standard threads.
  • Pipe threads and standard threads DO NOT share terminology. So don't expect a 1/8 pipe plug to look like a 1/8 bolt.
  • As has been pointed out, a 5/16 hole is what you'd need for a 1/8 pipe plug.
  • What will stop you from screwing the plug in too far? YOU. Use loctite thread sealer, screw the plug in so that it provides the clearance needed for your alignment pin. Stop. Verify that it does not protrude into the oil galley. Regardless of whether or not it's tight, the correct loctite will seal it.
  • Wall thickness; if you have .45", that';s almost 1/2 inch. A 1/8 pipe plug is usually about .25 thick, so you'd have way more than enough depth... assuming that your .45 is accurate.
  • Above all else. Tap slow, a lot of oil, back the tap out often, and don't tap deep.
If you do all of these things and you still have a leak, it would be very slight and no more than any other leak on every 50 year old Pontiac.

On your new pins, do those slots protrude outside of the block when seated? I'd be packing the new pins with Permatex Ultra Grey, slots and threads, 24 hours before installing them. It will work like an oil seal.
 

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As far as wall depth, if I understand you, I measured it at about 0.450". That's from the back of the dowel pin hole to the edge of the oil passage. To put it another way, this is the maximum length of the threads I would cut, or the maximum length of the plug. Is that enough for tapered threads?
So, the small hole alone is 0.450" deep? If so, that's plenty deep for what I'm about to suggest.

A concern I have of cutting regular threads is, what would stop it from "backing in" to the oil passage? I know the dowel pin would stop it from backing out, but would the oil pressure (or JB Weld) -always- be sufficient to keep it from continuing into the oil passage?
Here's how you make sure: when you tap the straight threads, don't cut them all the way through the hole. Then use a hex socket plug that's less than 0.450" deep so that when you tighten it down into the hole, it "bottoms" out on the area where you didn't thread all the way through. In fact, if you used a regular tapered tap, you will have created some "imperfect threads" near the 'end' of the hole what will sort of act like a tapered pipe tap when you tighten the plug. As you're cutting the threads, cut them just deep enough so that you can get your plug in far enough so that the top of it is flush with the bottom of the dowel pin hole. Easy peasy.

Bear
 

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Here's a twist on Bear's tapping advise...use a tapered straight thread tap and tap into the hole, but stop well before the full taper pops out the other side. I would do this by turning the tap a turn or two at a time and then check the depth with a longish bolt and stop tapping when the depth reaches the point that the bolt stops shy of coming out the other side (about 1/8" shy)...Then, use a bottom tap, or grind the tapered portion off the first tap, and then tap the hole again to square up the threads just shy of coming out the other side (1/8" again). This would create a more firm stopping point for the treaded plug. You could cut the threaded potion off a bolt (0.45 minus the 1/8"), cut a slot in it on the pin-side to use a screwdriver to drive it in along with your choice of JB weld/thread locker sealer. I doubt this is really necessary, but it's an option. I do like the idea of leaving the tapered section to create a wedge at the end for better sealing. The bolt plug does not have to be tightened in with much force, just snug. The sealant would be doing all the work.
 

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Here's a twist on Bear's tapping advise...use a tapered straight thread tap and tap into the hole, but stop well before the full taper pops out the other side. I would do this by turning the tap a turn or two at a time and then check the depth with a longish bolt and stop tapping when the depth reaches the point that the bolt stops shy of coming out the other side (about 1/8" shy)...Then, use a bottom tap, or grind the tapered portion off the first tap, and then tap the hole again to square up the threads just shy of coming out the other side (1/8" again). This would create a more firm stopping point for the treaded plug. You could cut the threaded potion off a bolt (0.45 minus the 1/8"), cut a slot in it on the pin-side to use a screwdriver to drive it in along with your choice of JB weld/thread locker sealer. I doubt this is really necessary, but it's an option. I do like the idea of leaving the tapered section to create a wedge at the end for better sealing. The bolt plug does not have to be tightened in with much force, just snug. The sealant would be doing all the work.
That too would work.

As an aside, because I'm running a solid roller valve train in my engine I put restrictors in all the lifter bore feed holes. Their purpose is to force good oil supply to the mains and rods while still allowing "enough" oil through the lifters, pushrods, rockers, and springs. They are made from small hex socket head plugs, about 1/8" in diameter and about 3/16" or so long, with 0.060" holes drilled through them.

I used the the exact same process I described to cut the threads into the lifter bore feed holes.

They tighten down nice and snug when installed, and I used blue thread-locker on them during installation just for a little added insurance. Needless to say, if one of them were to ever "come out" in either direction, bad things would happen. I've run for years with them in place.

Bear
 

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Had a friend do exactly the same thing. we fixed his by using an Allen screw that wasn't tapered and simply coated it with red Loctite before screwing it in place. This way you can tap the existing hole without going larger trying to fit a larger tapered plug. This was over ten years ago and still running fine. Pontiac perfectly lined up the dowel hole with the oil passage.
 

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Good evening, I think I can add some positive comments about how to fix this. I've had to do this sort of repair before and understand the groups prior suggestions and issues causing some hesitation in implementing them.
Any repair with JB Weld or similar stuff depends on some thickness of material to work. If you smear it on then press or screw a tight fitting together there isn't any JB Weld left to do anything. I'd offer up my experience in using that product for a half century that you need about an 1/8 of thickness before you have much strength. Sections over a 1/4 inch can fail in sheer or tension at less force than you'd like to believe. JB Weld works really well when you can back it up with a patch plate or fill a cavity that has an irregular surface or embed screen into it.

Story: I built claimer class IMCA engines for a long time. Fords with big strokes. I used Ford 300 cid rods from Industrial or UPS engines and messed with the crank diameters to get things fitting. The 300 inch rod has an oil squirt hole in it as does the bearing. I'd fill the hole with JB weld and also smear some into the bearing hole then fit the bearings etc to allow assembly. This boosted the oil pressure at the journal and gained us RPM. It also cut down windage so we gained torque. However after about 20 races the JB Weld would erode out of the hole and that advantage would be lost.

You have the same situation here. Main line hot oil at full flow across a repair eating it a bit at a time. When I realized that I made the rod squirt hole bigger ( 3/16th ) so the JB Weld plug would be bigger. Then; when the plug eroded away it would cause a big enough leak that the journal would starve for lubrication blowing the rod up and wrecking the engine. What our team kept secret was how long the plug would hold. So if our engine was claimed any team protesting our car winning using a claim / swap rule they got a hand grenade with the pin pulled. I never had to compete against my own engines and we had great success - as long as we kept track of which engine in our inventory had run how many races. Take away - I'd recommend never using JB Weld against hot main line oil pressure unless you can inspect it regularly. Against coolant sure. Against fuel - Hummmmm - maybe. I've had modern gas disolve it in 3 weeks time. It didn't use to do that.

Any of the teflon tape / gasket sealer sorts of goo it in there repairs won't last very long against main line oil pressure despite having some "stick to it ness" unless backed up with a metal plate or gasket sandwitch. In this case that can't be done.

You need a metal to metal repair. This leads to two options, both with drawbacks but both with certainty of success.

Typically when you break into a coolant or oil passageway the repair method is to sleeve the hole that the break in broke into. In this case you'd make a metal sleeve for the oil filter adapter boss having an ID to match the OEM oil hole size and an OD thick enough to stand pressing it in. You'd remove the engine from the car and strip the block enough to handle getting it into a machine for making an accurately sized hole to accept a sleeve. Some engines are prone to having this happen. Caterpillar, Ford, and some ISUZU engines come to mind. A company called "Lock n Stitch" in Lodi CA makes repair sleeve kits to save these blocks and heads. Give them a call and maybe you'll get lucky an existing kit will do what you need to have happen. Mostly the break through kits are for issues in head bolt bosses and injector holes. They also make repair pins which have special threads to seal against enormous pressures which I've used for decades to crack repair all sorts of castings. But in this case I don't recommend trying to pin this hole.

Story: A drill generally cuts only on the sharp end and approximately follows an existing hole. If anything is off axis the hole will egg shape and become off center. Some drills will cut on the flute edges. They generally go wilder quicker. Machine tool end mills cut on all surfaces so can't typically be successfully used in a hand drill - but don't count out trying......
Reamers follow the existing hole no matter if it is straight or egged, or belled or whatever. Reaming a hole to size is imperative to do if you want a good fit but depends entirely on the hole starting out on location and on size and on axis. This is nearly always screwed up laying on your back under a car with a hand drill. To be successful with a reamer and pin repair you need to stay within .003 on size which is all the reamer can handle (finish or correct).

So that gets you to an old school solution. Lead. Get a couple of old lead hunks - fishing weights, bullet noses, musket balls, drapery weights from an antique house curtain, whatever. Make a glob of lead big enough to fill the bottom of your break out hole plus the empty space behind your new adjustable dowel pin. Pound it in there tight with a punch until you can see it forming into the main line oil passageway. Cross drill the passage so it's full size. Remove the distributor and spin the oil pump to flush chips out. That will happen quick - half a quart or less of oil lost. Job done. The lead plug won't leak, won't move, and will last forever. It's held in by the new adjustable dowel pin so can't move.

Best of luck to you. Hope this helps you move forward. Ladd
 

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Better off with a pencil magnet fished into the oil gallery since the breach is only a few inches from the oil filter adapter mount and should be able to grab all the chips. Oil flow from the breach is after the oil filter and direction of oil flow is into the engine heading towards bearings and lifters.
 

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...Oil flow from the breach is after the oil filter and direction of oil flow is into the engine heading towards bearings and lifters.
Checked a block today and I had the wrong gallery, and need to make the correction that the dowel hole aligns with the oil pump to oil filter gallery and if chips were left behind they should be caught up in the oil filter.
 

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So that gets you to an old school solution. Lead. Get a couple of old lead hunks - fishing weights, bullet noses, musket balls, drapery weights from an antique house curtain, whatever. Make a glob of lead big enough to fill the bottom of your break out hole plus the empty space behind your new adjustable dowel pin. Pound it in there tight with a punch until you can see it forming into the main line oil passageway. Cross drill the passage so it's full size. Remove the distributor and spin the oil pump to flush chips out. That will happen quick - half a quart or less of oil lost. Job done. The lead plug won't leak, won't move, and will last forever. It's held in by the new adjustable dowel pin so can't move.
Love the lead plug idea! Wish I'd thought of it.

Bear
 

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Thanks Bear, lol, Lets hope we never have a need to remember it....but it reminds me of a story fixing a Mini Cooper block when I moved the push rod holes over a quarter inch and found water at the deck extending 2 inches down into the block just 3 days before a national event.......that got creative.
 
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