S400 Turbo Oil Drain Guide: Sizing, Routing & Why Turbos Leak

If your turbo is pushing oil, the drain is the first place to look — not the seals. Turbocharger center sections do not pump oil out. They rely on gravity, and gravity is easy to defeat with a restrictive fitting, a bad routing angle, or a drain that is one size too small for the frame.

This guide covers how to size an S400 turbo oil drain, how to route it so it actually works, and how to read the symptoms when it does not. Black Sheep Industries machines billet drain flanges in-house in every common bolt pattern.

Why Turbo Drains Fail — The Part Most Builds Get Wrong

Oil enters a turbo center housing under pressure through the feed line. It leaves under gravity alone. That single fact explains almost every turbo oil leak.

Inside the center housing, oil is whipped into an aerated foam by a shaft spinning at over 100,000 RPM. That foam has to fall out of the housing, through the drain, and back to the pan faster than the feed line supplies it. If it cannot, the housing fills, pressure builds, and oil takes the only other exit available — past the piston-ring seals and into your turbine or compressor housing.

Then you replace the seals, and it happens again, because the seals were never the problem.

The Four Things That Cause It

  • Drain too small. A 10AN port on a large-frame turbo that needs 12AN.
  • Drain not vertical enough. Anything shallower than about 15 degrees off vertical and the oil stops falling and starts pooling.
  • Crankcase pressure. If the pan is pressurised, the drain has to fight it. Blow-by from worn rings will push oil back up the drain.
  • Drain entering the pan below the oil level. The return has to terminate above the oil in the sump. Below it, you are asking oil to drain into oil.

Sizing an S400 Drain Flange

There are two independent measurements. Get both right.

1. Bolt Circle — 35mm, 65mm, or 100mm

This is the bolt pattern on your turbo's center housing drain pad, and it is the number in the product name. It is not the port size — a common point of confusion. S400-frame turbos ship with different drain pads depending on the manufacturer and vintage, so measure yours rather than assuming.

To measure: put calipers across the centres of the two drain pad bolt holes on the turbo center housing. That distance is your bolt circle.

2. Port Size — 10AN or 12AN

This is the oil return passage itself.

  • 12AN — the correct choice for S400 and other large-frame turbos. Larger passage, keeps up with the oil volume a big frame flows, less risk of the center housing pooling.
  • 10AN — suitable for small-frame and some mid-frame turbos. Undersized for a hard-working S400.

If you are building an S400 and are unsure, run 12AN. There is no downside to a drain that flows more than you need.

S400 Drain Flange Selector

All flanges are billet-machined, available in M8-1.25 or 3/8-16 thread, and shipped from Canada in CAD.

12AN — Mid-Frame and Large-Frame (recommended for S400)

Bolt Circle Port SKU
35mm 12AN BSI12MlF35
65mm 12AN BSI12LF65
100mm 12AN BSI12MlF100

10AN — Mid-Frame

10AN — Small-Frame

Browse all turbo drains and plugs

Why Billet Instead of Cast

  • No porosity. Cast flanges can have voids that weep oil through the material itself. Billet has none.
  • True sealing face. Machined flat off the tool, so the gasket seals and stays sealed instead of you chasing a warped casting.
  • Crack resistance. Solid stock handles the vibration and thermal cycling of a hard-working turbo better than a casting.
  • Consistent bolt pattern. Machined, not shrunk from a mould. It bolts on without persuasion.

Drain Routing — The Five Rules

  1. Keep it as vertical as you can. Aim for within 15 degrees of vertical. Beyond about 35 degrees you are asking for trouble.
  2. No loops, no dips, no low spots. A sag in the drain line becomes an oil trap, and a full trap is a blocked drain. Route downhill the whole way.
  3. Terminate above the oil level in the pan. The return fitting has to enter the sump above the maximum oil level, including under hard acceleration and braking when the oil sloshes.
  4. Use the largest hose that fits. Match the hose ID to the port. A 12AN flange feeding a 10AN hose is a 10AN drain.
  5. Avoid tight bends. A sharp radius restricts flow the same way a small port does. Use a gentle sweep or an angled fitting.

Supporting Parts

Oil Feed — The Other Half of the Equation

Drain problems and feed problems produce similar symptoms, so it is worth understanding both.

  • Journal bearing turbos need a relatively high oil volume to maintain the bearing film. Most run unrestricted or lightly restricted feed.
  • Ball bearing turbos need less oil, and over-feeding one will flood the center section and push oil past the seals even with a perfect drain. These require a feed restrictor.

If you are running a ball bearing S400-class turbo and have oil in the compressor housing, check your feed restrictor before you blame the drain.

Symptom Diagnosis Table

Symptom Most Likely Cause What to Check
Blue smoke on startup only Oil pooling in housing at shutdown Drain angle, drain terminating below oil level
Blue smoke under load Drain restriction or crankcase pressure Port size, hose ID, blow-by
Oil in the compressor housing Over-feeding or blocked drain Feed restrictor on ball bearing units, drain routing
Oil in the turbine housing Drain restriction Drain size, low spots in the line
Repeated seal failure after rebuild Drain was never the right size Bolt circle and port size, upgrade to 12AN
Oil weeping at the flange face Warped or porous cast flange Replace with billet, new gasket
New turbo, immediate oil push Reused undersized drain hardware Whole drain path from flange to sump

Installation Checklist

  1. Measure the bolt circle on the turbo center housing drain pad
  2. Choose 12AN for an S400-frame turbo
  3. Order the matching billet flange in M8-1.25 or 3/8-16 to suit your housing
  4. Clean both sealing faces — no old gasket material
  5. Fit a new gasket, torque evenly, do not over-tighten into billet
  6. Route the hose downhill with no low spots, within 15 degrees of vertical where possible
  7. Terminate above the sump oil level
  8. Confirm feed restrictor is correct for your bearing type
  9. Prime the turbo with oil before first start

Related Guides

Frequently Asked Questions

What size oil drain does an S400 turbo need?

An S400 needs a drain flange matched to the bolt circle on its center housing drain pad — commonly 35mm, 65mm, or 100mm — with a 12AN port. The millimetre figure is the bolt pattern, not the port size. Measure the distance between the drain pad bolt hole centres before ordering.

Is 10AN or 12AN better for a turbo oil drain?

For S400 and other large-frame turbos, 12AN. The larger passage keeps up with the oil volume a big frame flows and reduces the risk of oil pooling in the center housing. 10AN is suited to small-frame and lighter-duty mid-frame applications.

Why is my turbo pushing oil into the exhaust?

Most often a restricted or badly routed oil drain rather than failed seals. Oil leaves the center housing by gravity only, so an undersized port, a shallow drain angle, a low spot in the hose, or a return terminating below the sump oil level will back oil up and force it past the seals.

How steep does a turbo oil drain need to be?

As close to vertical as the install allows — ideally within 15 degrees. Past roughly 35 degrees off vertical, oil stops falling freely and begins pooling in the center housing.

Does a ball bearing turbo need a different oil setup?

Yes. Ball bearing turbos need less oil than journal bearing units and require a feed restrictor. Over-feeding one will flood the center section and push oil past the seals even with a correctly sized drain.

Why use a billet drain flange instead of cast?

Billet has no porosity for oil to weep through, comes off the machine with a true flat sealing face, and resists cracking under vibration and thermal cycling better than a casting.

Can I reuse my old drain flange on a new turbo?

Only if the bolt circle and port size are correct for the new turbo and the sealing face is flat and undamaged. Reusing undersized drain hardware is one of the most common reasons a freshly installed turbo starts pushing oil immediately.