An LS1 engine rebuild can involve thousands of dollars in machine work, bearings, pistons, valvetrain components and hardware.
And then there’s a small 16mm metal expansion plug at the front of the block that absolutely needs to be there.
It’s easy to overlook because it isn’t a performance component. It doesn’t add horsepower. Nobody shows it off when the engine is finished.
But the opening behind it communicates with the engine’s oiling system.
According to the LS engine-building reference used for this guide, the front left—or driver’s side—of the LS block contains an open oil-galley passage created during factory machining. That opening is sealed during final assembly with a 16mm press-in expansion plug. Building the Chevy LS Engine – …
If you’re rebuilding an LS1 from a bare block, this is one of those small assembly details worth checking twice.
Where Is the LS1 Front Oil Galley Plug?
The plug is located at the front of the LS engine block on the driver’s side, near the oil pump.
The reference describes the opening as a factory-drilled passage associated with oil-passage machining. A 16mm cup-style expansion plug closes the opening before the engine’s front cover is installed. Building the Chevy LS Engine – …
That location is important because the plug eventually becomes much less convenient to inspect.
During bare-block assembly, it’s sitting right in front of you.
Once the engine is together, not so much.
What Size Is the LS1 Front Oil Galley Plug?
The engine-building reference specifies a:
16mm press-in metal expansion plug
It is a cup-style plug rather than a conventional threaded pipe plug. Building the Chevy LS Engine – …
The PDF also specifically identifies this opening on Gen III/IV LS blocks as an open oil-galley passage and says a new plug should be installed during final assembly. Building the Chevy LS Engine – …
Don’t confuse it with the various threaded plugs elsewhere in an LS block.
Why Is There an Opening in the Front of the LS1 Block?
Engine blocks require drilled passages during manufacturing.
The PDF identifies this particular opening as a factory-machined hole associated with the block’s oil passage. Once the machining operation has served its purpose, the opening has to be sealed.
That’s what the expansion plug does.
This is different from simply plugging an unused accessory hole.
You’re closing an opening associated with the engine’s lubrication system.
Should the LS1 Oil Galley Plug Be Removed During a Rebuild?
If you’re dealing with a used bare block that is being thoroughly cleaned, yes.
The reference specifically recommends removing the original front oil-galley expansion plug when preparing a used engine core for cleaning. After the block has received its final wash, a new plug is installed during assembly. Building the Chevy LS Engine – …
That sequence makes sense:
Remove old plug → clean block and oil passages → final wash → install new plug → continue final assembly
Leaving the original plug in place while trying to thoroughly clean a used block can prevent proper access to the passage behind it.
For a complete rebuild, you’re trying to remove old oil residue, debris and whatever archaeological discoveries the previous owner left inside the engine.
Access matters.
Should You Reuse the Original LS1 Oil Galley Plug?
The source recommends installing a new plug during final assembly rather than reinstalling the one removed from a used block. Building the Chevy LS Engine – …
That’s the sensible approach for a press-fit component.
Once you’ve gone far enough into an LS1 rebuild to have a stripped and cleaned block sitting on an engine stand, reusing a tiny removed expansion plug isn’t where the budget needs rescuing.
Use a new correctly sized component.
Which Direction Does the LS1 Front Expansion Plug Face?
This is one of the most useful details in the reference.
For the cup-style 16mm plug, the book specifies installing it with the:
Concave side facing outward
The plug is driven into the opening until it sits flush with the front block face. Building the Chevy LS Engine – …
So the basic orientation is:
Concave side → outward/front of engine
Plug depth → flush with block face
That’s considerably more useful than discovering a mystery plug on the workbench after the timing cover is already installed.
Does the LS1 Oil Galley Expansion Plug Need Sealant?
According to the engine-building reference, yes.
It calls for applying a smear of RTV to the outer wall of the expansion plug before tapping it into the block. Building the Chevy LS Engine – …
The key phrase here is outer wall.
The source does not call for filling the passage with RTV, coating the inside of the oil galley or applying half a tube because more sealant apparently means more horsepower.
Use the sealant as specified around the plug’s exterior sealing surface.
How Do You Install the LS1 Front Oil Galley Plug?
With the block clean and ready for final assembly, the procedure described by the source is straightforward.
First, verify that the oil passage and plug bore are clean. Apply a light smear of RTV around the outside wall of the new 16mm cup-style expansion plug.
Position the plug with its concave face pointing outward.
Then carefully drive the plug into the opening until it is flush with the block face. Building the Chevy LS Engine – …
Avoid driving it unnecessarily deep into the passage.
The source’s installation endpoint is flush.
When Should You Install the Plug?
The PDF makes an important point about assembly order: although many block plugs can be installed at different stages, some need to be installed before the front and rear engine covers go on. The 16mm front oil-passage plug is specifically discussed in that stage of assembly. Building the Chevy LS Engine – …
That’s a good reason to incorporate a plug inspection into your bare-block checklist.
Before installing the front cover, physically inspect the front block face.
Don’t rely on remembering that you installed it.
Look at it.
Engines are remarkably unsympathetic to sentences beginning with, “I thought I…”
Don’t Confuse Oil Plugs With Coolant Plugs
The LS block contains several plugs, and they don’t all seal the same type of passage.
The PDF distinguishes the 16mm press-in oil-passage expansion plug from the engine’s threaded water-jacket plugs.
Those threaded coolant plugs use straight metric threads and are sealed differently. Building the Chevy LS Engine – …
There’s also a large threaded water-jacket opening on the left side of the block that uses a 30mm straight-thread plug with an O-ring. Building the Chevy LS Engine – …
These components shouldn’t be lumped together simply as “freeze plugs.”
For an engine builder, the more useful distinction is what passage the plug actually seals.
There Is Also an Oil Relief Plunger at the Rear
While inspecting the block plugs, there’s another easily overlooked LS assembly component worth knowing about.
The engine-building reference describes an oil relief plunger at the left rear of the block. It is a white plastic component equipped with an O-ring.
For the documented installation, the O-ring end faces toward the rear. The bare end enters the block first, and the plunger is seated until the O-ring end is flush with the rear block face. Building the Chevy LS Engine – …
That deserves to remain a separate FAQ rather than turning this page into a generic LS oiling-system guide.
The takeaway here is simpler:
When you’re assembling an LS1 from a bare block, don’t focus exclusively on the expensive rotating assembly while overlooking the plugs and small oil-system components surrounding it.
Why Block Cleaning Matters Before Installing the Plug
The whole reason for removing this plug during a thorough rebuild is access.
The reference specifically recommends removing the plug from a used engine core before washing the block and replacing it only after the final cleaning operation. Building the Chevy LS Engine – …
That allows the builder to clean the passage instead of sealing decades of unknown material inside it.
After machining operations, cleanliness becomes even more important.
Metal particles, abrasive material and machining debris have no business remaining inside an engine’s oil passages.
Once the block has been properly cleaned and inspected, the new plug closes the passage for final assembly.
Inspect the Oil Galley Before Installing the New Plug
The PDF also recommends closely inspecting the oil-galley ports above the camshaft tunnel on a used LS block and making sure they are clean and free from burrs. Building the Chevy LS Engine – …
That isn’t the same location as the 16mm front plug, but it reinforces an important rebuilding principle:
Don’t treat oil passages as invisible plumbing.
Inspect them.
Clean them.
Then seal the block only after you’re satisfied they’re ready for service.
For builders working through an entire LS1 project, Pro Street’s existing LS1 engine parts and performance components can be incorporated as the build progresses without turning this FAQ into another general LS1 parts guide.
Check the Plug Before Installing the LS1 Oil Pump and Front Cover
Assembly order can vary somewhat depending on the components being used, but this plug needs to be accounted for before the front of the engine is permanently closed.
The PDF places block-plug installation immediately around the oil-pump stage and specifically notes that certain plugs must be installed before the engine covers. Building the Chevy LS Engine – …
This creates a useful final inspection sequence:
- Verify the block and oil passages are clean.
- Confirm the correct new 16mm expansion plug is available.
- Apply the specified light RTV coating to its outer wall.
- Install it concave-side outward.
- Seat it flush with the block face.
- Visually confirm installation before closing the front of the engine.
It’s a thirty-second inspection that can prevent an extremely stupid reason to disassemble an engine again.
What About the LS1 Oil Pump?
Keep that as a separate subject.
The LS oil pump has its own installation procedure. The PDF describes the OE procedure as initially tightening the four pump bolts lightly, centering the pump around the crankshaft drive gear with even clearance, and then tightening the pump fasteners to their final specification. Building the Chevy LS Engine – …
That’s enough material for its own focused article.
More importantly, separating the subjects prevents this page from competing for broad searches such as LS1 oil pump installation.
This page should answer one question exceptionally well:
What is the 16mm plug at the front of an LS1 block, and how should it be installed?
That’s the search intent we want to own.
What Happens If You Find the Plug After the Engine Is Together?
Stop and determine exactly what component you’re looking at before proceeding.
Do not assume an unused plug came from somewhere harmless.
If a bare LS block was assembled without the 16mm front oil-passage expansion plug described here, the correct response isn’t to shrug and see what the oil-pressure gauge says.
Verify the assembly.
The PDF explicitly states that this front opening is an oil passage and must be plugged. Building the Chevy LS Engine – …
That’s considerably cheaper than conducting an experimental investigation after startup.
LS1 Front Oil Galley Plug FAQ
What size is the front LS1 oil galley plug?
The source identifies it as a 16mm press-in expansion plug located on the front driver’s side of the LS block. Building the Chevy LS Engine – …
What does the LS1 front expansion plug seal?
It closes a factory-machined opening that communicates with an oil passage in the block. Building the Chevy LS Engine – …
Which direction does the LS1 oil galley plug face?
The 16mm cup-style plug installs with its concave side facing outward. Building the Chevy LS Engine – …
How deep should the LS1 front oil galley plug be installed?
The reference specifies tapping it into position until it is flush with the front face of the block. Building the Chevy LS Engine – …
Should I use RTV on the LS1 oil galley plug?
The documented procedure calls for a smear of RTV around the outer wall of the plug before installation. Building the Chevy LS Engine – …
Should the plug be removed when cleaning an LS1 block?
For a used engine core undergoing a rebuild, the reference recommends removing it for block cleaning and installing a new plug after the final wash. Building the Chevy LS Engine – …
Can I reuse the original plug?
The source recommends replacing the removed plug with a new one during final assembly. Building the Chevy LS Engine – …
Final Word
The 16mm LS1 front oil-galley plug is a perfect example of why engine building is about more than measuring the glamorous parts.
Crankshaft clearance matters.
Piston orientation matters.
Camshaft setup matters.
So does the tiny plug closing a machining passage at the front of the block.
If you’re rebuilding an LS1 from a bare block, remove the original plug before thorough cleaning, inspect the oil passages, install a new 16mm expansion plug with the concave side outward and seat it flush with the block face using the sealing procedure described by the engine-building reference. Building the Chevy LS Engine – …
Then physically check it before the engine is closed.
Because “LS1 rebuild failed because I forgot a $5 plug” is a search term we’d rather not have to optimize for.
