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Tapping block for oil galley plug

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CMCon98 Colin C
Dedham, Massachusetts, USA   USA
I'm in the process of tapping the oil galley in the rear bottom corner of my 3-main block, where the oil pan rail mates to the block. There's a small plug there and a larger one, and I'm working on the larger one. I'm using a 1/4-18 NPT tap, with a 7/16 hole, which is what the tap/drill chart calls for. I've cut the threads to a depth of about 9/16" into the block, but the plug will only thread in about 2 turns before it gets tight, and most of it is still sticking out of the hole. I don't want to crack the block or break off the tap by trying to force it further into the hole. Should I drill the hole slightly bigger and tap again? Use a tapered reamer and tap again? I just want the brass pipe plug to thread into the hole far enough so that it's at or below the oil pan gasket sealing surface. Thanks for any advice!

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fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
I drill the large hole out with 1/2" drill, then run the tap most all the way in and check for plug depth. Use a little blue Loctite on the plug threads as a sealer. No need to over tighten.


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boggsy64 Avatar
boggsy64 Ken Boggs
Brooksville, Kentucky, USA   USA
1977 MG MGB "Dad's Hand Me Down"
Have you used a bottom tap or normal?

CMCon98 Colin C
Dedham, Massachusetts, USA   USA
Just so I understand Dave's reply: so drill the hole out to 1/2" to the max depth the plug needs to thread (so depth = length of the plug), then tap with the 1/4"-18 NPT tap to full depth of the tap or close to it, until the plug will thread in far enough to clear the pan rail surface of the block.

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ghnl Eric Russell
Mebane, North Carolina, USA   USA
1961 MG MGA "Calvin"
Pipe threads are tapered. Run the tap in deeper to allow the plug to fit in further. I think a 1/2" drill bit is too large.



Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

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fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
!/2" would be too large if you needed to dry seal fluids under pressure. But it's OK if this hole seeps as it's inside the pan. Drill deep enough for the full thread length of a standard 1/4"NPT tap. On blocks I prep now, I use a Dorman cup plug in the 1/2" galley holes at the front and rear faces of the block. If desired, these can be tapped 1/4"NPT as well and they do not leak when plugged with 1/4"NPT plugs and blue Loctite.


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Dave Headley, dba FAB-TEK offers full service race car parts and preperation for MGB & MGA race cars, SCCA and Vintage. Dave is a mechanical engineer and has raced MGBs since 1963.
Paul755 Paul H
Richmond, Virginia, USA   USA
1974 MG MGB MkIII
1979 MG MGB MkIV "Nemesis"
You are using the correct drill size. The driIl size for 1/4 NPT is 7/16



Bigamy is having one wife too many.
Monogamy is pretty much the same thing.

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Speedracer Platinum Member Hap Waldrop
Greenville, SC, USA   USA
1967 MG MGB Racecar "The Biscuit"
In reply to # 3640863 by Paul755 You are using the correct drill size. The driIl size for 1/4 NPT is 7/16

Yeah but like he said, try 7/16 drill bit and it will be too tight for the tap. I do use a 7/16 drill bit, and just walk the drill bit a bit, this give me the size I need for that tapping, However that plug hole is slightly smaller than the main galley plug holes, which are normally perfect for the 1/4 NPT tap.



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CMCon98 Colin C
Dedham, Massachusetts, USA   USA
Thanks for all the good advice guys. I suppose I could use a drill bit that's larger than 7/16 but smaller than 1/2, like 29/64. maybe I'll drill and tap a test piece using the 1/2" bit and see how far down the plug will thread before it gets tight. If it's too loose, I can always buy a 29/64 bit and try that. I'll report back with my outcome.

Paul755 Paul H
Richmond, Virginia, USA   USA
1974 MG MGB MkIII
1979 MG MGB MkIV "Nemesis"
So far I have not had an issue plugging the MGB block. I use a USA made MAC Tools tap and die set with lots of cutting oil. I do bottom out the NPT tap to get the correct taper for the NPT plug. I can see it would be difficult if the tap is either worn or not bottomed out- or cheap Chinese junk from Harbor Freight, Lowes, Home Depot etc. If the bore taper does not match the plug, it isn't going in. The slightly larger hole would help if there is a mismatched taper. Liberal use of blue Loctite would keep it in place and leak free.
Most times I use the brass plugs from Moss or turn one up on the lathe from soft brass bar stock and stick it in dry ice before whacking it into the block.
FYI I did strip a block where the previous rebuilder had used JB Weld with a little aluminum disk to act as a bottom stop in place of plugs. I actually worked quite well, and according to the owner, the engine had 90K on it since the last rebuild. Needles to say it was not returned to him with any JB Weld in the block.


In reply to # 3641295 by Speedracer
In reply to # 3640863 by Paul755 You are using the correct drill size. The driIl size for 1/4 NPT is 7/16

Yeah but like he said, try 7/16 drill bit and it will be too tight for the tap. I do use a 7/16 drill bit, and just walk the drill bit a bit, this give me the size I need for that tapping, However that plug hole is slightly smaller than the main galley plug holes, which are normally perfect for the 1/4 NPT tap.



Bigamy is having one wife too many.
Monogamy is pretty much the same thing.

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