S300 Turbo Oil Drain Guide: Sizing, Routing and Why Turbos Leak
More S300s are killed by their oil drain than by anything else. Not by the turbo being wrong, not by boost, not by tuning — by oil that cannot get out of the centre housing fast enough and ends up pushed past the seals instead.
A drain flange is defined by three independent specs. Getting one right does not get the others right. This guide covers all three, plus routing, and what to check when a turbo starts leaking.
The Three Specs
- Bolt centre — whether it physically bolts on
- Port size — whether it flows enough oil
- Drain length — whether it clears what is in the way
Customers routinely conflate these, usually because the number in the product name is the drain length and gets mistaken for one of the other two. Take them one at a time.
1. Bolt Centre — The S300 Is Mid Frame
Bolt centre is the distance from the centre of one bolt hole to the centre of the other on the turbo's oil drain pad. It is set by frame class and it is not negotiable — it either matches or it does not.
| Bolt Centre | US Standard | Frame Class |
|---|---|---|
| 38mm | 1.50" | Small frame |
| 50mm | 1.97" | Mid frame — this is your S300 |
| 50.8mm | 2.00" | Large frame (S400) |
Measure With Calipers
Mid frame and large frame are 0.8mm apart — about 1/32 of an inch. A tape measure held against a warm turbo in a driveway will not reliably tell you which one you have. Use calipers.
Bolt centre is also why the S300 and S400 are not interchangeable at the drain even though the turbos look related. If you are stepping between frames, the drain flange changes.
2. Port Size — 10AN or 12AN
This is where the 300 frame differs from the 400. On an S400 there is one sensible answer: 12AN. On the 300 frame you have a genuine choice, and both are stocked at the 50mm (1.97") mid-frame bolt centre.
10AN
- Adequate for the majority of street and towing S300 builds
- Smaller hose, easier to route in a tight engine bay
- The sensible default unless something below applies
12AN
- Larger return passage, more margin when oil return is working hard
- Worth it on high-boost, sustained-load, sled pulling and competition builds
- Worth it in compound setups, where the high-pressure turbo can see demanding conditions
- Worth it if you have had drain-related trouble before
Oil leaves a turbo by gravity, not by pressure. There is no such thing as a drain that flows too well — the only cost of going up a size is hose, fittings and routing space. If you are undecided and have the room, 12AN is the safer error.
3. Drain Length — The Number in the Product Name
The 35mm / 65mm / 100mm figure leading every drain flange product title is the length of the drain — how far the drain body stands off the flange face before your hose connects. It is not the bolt centre and not the port size.
| Drain Length | US Standard | Use When |
|---|---|---|
| 35mm | 1.38" | Clearance is not the problem and you want the assembly compact |
| 65mm | 2.56" | The middle option — enough standoff for most obstructions without bulk |
| 100mm | 3.94" | You need to reach past a manifold, frame rail, block accessory or tight turbo mount |
A longer drain moves the hose connection further from the centre housing, which does two things: it gets the joint out from behind whatever is in the way, and it helps establish the near-vertical drop the drain needs. Pick the shortest length that clears everything.
The Full Mid-Frame Drain Range
10AN Mid Frame — 50mm (1.97") Bolt Centre
- 35mm (1.38") Billet 10AN Mid-Frame Drain Flange
- 65mm (2.56") Billet 10AN Mid-Frame Drain Flange
- 100mm (3.94") Billet 10AN Mid-Frame Drain Flange
12AN Mid and Large Frame
- 35mm (1.38") Billet 12AN Drain Flange
- 65mm (2.56") Billet 12AN Drain Flange
- 100mm (3.94") Billet 12AN Drain Flange
Thread Options
Every drain flange is offered with M8-1.25 (metric, roughly 5/16") or 3/8-16 (US standard coarse) hardware. Match it to the threads in your turbo's drain pad.
Sealing
These flanges use a captured high-temperature o-ring rather than a paper gasket or RTV. The o-ring stays in place during installation instead of falling out while you are trying to get bolts started behind a manifold.
Routing: The Part That Actually Kills Turbos
The flange is the easy part. Most drain failures are routing failures.
The Rules
- As close to vertical as you can get. Oil leaves the centre housing by gravity. Every degree away from vertical slows it down. Beyond roughly 35 degrees off vertical, expect trouble.
- No low spots. A dip anywhere in the drain line traps oil and creates a standing column the centre housing has to push through. The line must fall continuously from turbo to sump.
- No kinks, no tight bends. A collapsed section of hose is a restriction that will not announce itself until the seals do.
- Enter the sump above the oil level. If the drain terminates below the oil in the pan, oil cannot leave — you have plumbed the drain into a standpipe.
- Keep it away from heat. A drain hose resting on a manifold or downpipe cooks the oil in it and shortens the hose's life.
- Do not undersize the hose. A 12AN flange feeding a 10AN hose is a 10AN drain with extra fittings.
Mounting the Turbo
All of the above is much easier if the turbo is mounted with the drain pad pointing roughly downward. If the turbo's clocking makes a vertical drain impossible, reclock the centre housing before you start fighting the plumbing.
Leak Diagnosis
Oil showing up where it should not be is usually a symptom rather than a cause. Work through this before condemning the turbo.
| Symptom | Likely Cause | Check |
|---|---|---|
| Oil in the compressor housing or charge pipe | Drain restriction backing oil up past the compressor-side seal | Drain angle, low spots, hose size, sump entry point above oil level |
| Oil in the turbine housing, blue smoke | Drain restriction, or excessive crankcase pressure resisting the drain | Drain routing first, then crankcase ventilation |
| Oil weeping at the drain flange itself | O-ring damaged on install, or flange face not seated flat | O-ring condition, flange face cleanliness, bolt torque |
| Leak appears only under sustained load | Drain adequate at idle but not at working oil flow | Consider stepping 10AN up to 12AN, and re-check for low spots |
| Leak appeared after a new turbo went on | Reused drain hardware, or oil feed pressure too high for the bearing | Drain flange and hose condition, feed restrictor sizing for your bearing type |
| Repeat failures after replacement | The original cause was never addressed | Full routing review before fitting another turbo |
If a turbo has failed and you have not identified why, fitting another one usually buys you a second failure. Send us the details through the Turbo Spec Form and we will work through it with you.
Quick Reference
- Bolt centre: 50mm (1.97") — mid frame. Measure with calipers; large frame is only 0.8mm larger.
- Port size: 10AN for most street and tow builds, 12AN for hard-worked, high-boost or compound applications.
- Drain length: shortest of 35mm (1.38"), 65mm (2.56") or 100mm (3.94") that clears your install.
- Threads: M8-1.25 or 3/8-16.
- Routing: near vertical, continuously falling, no low spots, into the sump above oil level.
Related Guides
- S300 Turbo Guide: Sizes, Specs, Flanges & Fitment
- S300 Turbo Flange Guide
- S300 Replacement Turbo Cross-Reference
- S400 Turbo Oil Drain Guide
Frequently Asked Questions
What size oil drain does an S300 turbo need?
S300 turbos are mid frame, which means a 50mm (1.97") bolt centre drain flange. Port size is a separate choice of 10AN or 12AN, and drain length — the 35mm/65mm/100mm figure in the product name — is a separate clearance choice again.
What is bolt centre?
Bolt centre is the distance from the centre of one bolt hole to the centre of the other on your turbo's oil drain pad. It identifies frame class: 38mm (1.50") small frame, 50mm (1.97") mid frame, 50.8mm (2.00") large frame.
Should I run a 10AN or 12AN drain on an S300?
10AN is adequate for most street and towing builds. 12AN gives more margin on high-boost, sustained-load, competition or compound applications, or if you have had drain-related trouble before. Oil drains by gravity, so a larger drain is never a disadvantage other than routing space.
Can I use an S400 drain flange on an S300?
Only if it is a mid-frame flange or one machined for both. The S400 is large frame at 50.8mm (2.00") against 50mm (1.97") on the 300 frame. The two are 0.8mm apart, so measure with calipers rather than assuming.
Why is my new turbo leaking oil into the intercooler piping?
Most often the drain is restricted rather than the turbo being faulty. Check the drain angle, look for low spots in the line, confirm the hose is not undersized relative to the flange, and confirm the drain enters the sump above the oil level.
How vertical does the drain need to be?
As close to vertical as the install allows, and continuously falling from turbo to sump. Beyond roughly 35 degrees off vertical, drainage slows enough to cause problems.
What is the 35mm, 65mm or 100mm in the product name?
It is the length of the drain — how far the drain body stands off the flange face before your hose connects. It is a clearance and routing choice, not the bolt centre and not the port size.