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If is is the 3/16 NPT hole just above the filter, it is for an oil preasure line. It needs to be plugged before you try to start it. If it is the 3/8 in hole just below the head, that is for the clutch pivot ball.
 

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I believe it is a 3/16 pipe thread. I found one at Advanced Autoparts.
 

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On most 1 pcs rear main seal blocks just below the pivot ball mount there is a 1/8 pipe port for a oil sending unit.

This is the oil sending switch for the light and fuel electric fuel pump/start bypass.

It is 1/8 NPT pipe Most parts stores and hardware stores will have this plug.

Al
 

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On most 1 pcs rear main seal blocks just below the pivot ball mount there is a 1/8 pipe port for a oil sending unit.

This is the oil sending switch for the light and fuel electric fuel pump/start bypass.

It is 1/8 NPT pipe Most parts stores and hardware stores will have this plug.

Al

1/8 inch pipe is the size in back of the int. manifold where most oil sending unites go and above the oil filter on the SBC has been 1/4 pipe not 1/8 pipe
 

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CNC and Al are the experts. On my ZZ4 it was not 1/4 inch. It may have been 1/8. The 1/4 inch NPT fittings at Lowes and Home Depot didn't fit.
 

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1/8 inch pipe is the size in back of the int. manifold where most oil sending unites go and above the oil filter on the SBC has been 1/4 pipe not 1/8 pipe
CNC and Al are the experts. On my ZZ4 it was not 1/4 inch. It may have been 1/8. The 1/4 inch NPT fittings at Lowes and Home Depot didn't fit.
It is 1/8 pipe

I am no expert!

Al
 

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It is 1/8 pipe

I am no expert!

Al

Here is a pic of a ZZ4 block that is in the shop for splayed caps and if you look at the blue print for a GM block that galley hole that goes into the block is 7/16 which is .4375 and that is the same size used for 1/4 NPT

1/8 inch is a letter Q drill which is .332

I machine a couple hundred blocks a year and to this day I have never seen a block with a .332 hole above the oil filter. We buy a lot of the 10105123 blocks from GM to machine and all have had a 1/4 pipe plug in this location.

Here is a pic of a ZZ4
 

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It is 1/8 pipe

I am no expert!

Al
The only 1/8 in pipe plugs on the GM SBC blocks is where the oil pressure gauge or sending unit screw in the back of the block just behind the intake manifold and on some blocks just above the timing cover are 1/8.

It should be the same size pipe plug used in the water drain holes which is 1/4 pipe
 

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Here is a pic of a ZZ4 block that is in the shop for splayed caps and if you look at the blue print for a GM block that galley hole that goes into the block is 7/16 which is .4375 and that is the same size used for 1/4 NPT

1/8 inch is a letter Q drill which is .332

I machine a couple hundred blocks a year and to this day I have never seen a block with a .332 hole above the oil filter. We buy a lot of the 10105123 blocks from GM to machine and all have had a 1/4 pipe plug in this location.

Here is a pic of a ZZ4
This is wild. My headers make it difficult to get to, but the the 1/4 fittings I bought didn't fit (I'm working from memory). I will look at it tommorrow and see what size wrench it takes. According to reciepts, the engine was purhased in the summer of 2000. It does have ZZ4 stamp on the engine, so I believe the receipts are correct.
 

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This is wild. My headers make it difficult to get to, but the the 1/4 fittings I bought didn't fit (I'm working from memory). I will look at it tommorrow and see what size wrench it takes. According to reciepts, the engine was purhased in the summer of 2000. It does have ZZ4 stamp on the engine, so I believe the receipts are correct.
Some where 1/8 npt.

And I am very aware and know pipe thread!

Like most machine shops, they don't see every block.
Just because they see a few hundred is minuscule to the amount produced every-year.





Al
 

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I don't need to plug that hole?? Really? I stuck a screwdriver in it and it is pretty deep.
not needed, it's used for clutch leakage ball
 

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Some where 1/8 npt.

And I am very aware and know pipe thread!

Like most machine shops, they don't see every block.
Just because they see a few hundred is minuscule to the amount produced every-year.

Al


I really can't see GM restricting the main oil galley hole that feeds the whole engine to .332 as their blue print says they are 7/16

I bet I have seen pretty close to 100 if not more ZZ4 engines and none of them are restricted to .332 as the all had a 1/4 pipe plug above the oil filter.

Do you have any pics of the 1/8 plug above the oil filter??
 
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