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. All dimensions are given in both metric and US standard. Black Sheep Industries machines billet drain flanges in-house in every common bolt centre.

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.

Three Specs, Three Different Jobs

A drain flange is defined by three independent measurements. Getting one right does not get the others right, and this is where most wrong orders come from.

Spec What It Decides Options
Bolt centre What physically bolts on 38mm / 50mm / 50.8mm
Port size Whether it flows enough oil 10AN / 12AN
Drain length What it clears on the way out 35mm / 65mm / 100mm

Work through them in that order.

Step 1: Measure Your Bolt Centre

The bolt centre is the distance between the centres of the two bolt holes on your turbo's oil drain pad. It is the measurement that tells you which frame class your turbo belongs to, and therefore which drain flange will bolt on.

To measure it: put calipers across the drain pad on the turbo center housing, from the centre of one bolt hole to the centre of the other. Do not measure the port opening — measure hole centre to hole centre.

Bolt Centre (Metric) Bolt Centre (US Standard) Frame Class
38mm 1.50" Small frame
50mm 1.97" Mid frame
50.8mm 2.00" Large frame

Note for US builders: mid frame and large frame are only 0.8mm apart — about 1/32 of an inch. Large frame is exactly 2.00 inches; mid frame is just under at 1.97 inches. Use calipers, not a tape measure. If you read 2 inches on the nose, you have a large frame.

S400-frame turbos are large frame — expect a 50.8mm (2.00") bolt centre.

Step 2: Choose Your Port Size

Bolt centre tells you what will physically bolt on. Port size tells you whether it will actually flow enough oil.

  • 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 lighter-duty 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.

Step 3: Choose Your Drain Length

The number at the front of every drain flange name — 35mm, 65mm, or 100mm — is the length of the drain: how far the drain body stands off the flange face before your hose connects. Every bolt centre and port combination is offered in all three lengths.

Drain Length US Standard Use It When
35mm 1.38" Nothing is in the way. Keeps the assembly compact and tucked in tight to the centre housing.
65mm 2.56" The middle ground. Enough standoff to clear most obstructions without adding bulk.
100mm 3.94" You need reach. Gets the hose connection out past a manifold, frame rail, block accessory, or a tight turbo mount.

Length is a clearance and routing decision, not a flow decision — a 35mm and a 100mm drain of the same port size flow the same. What length buys you is the ability to start the hose run somewhere it can actually fall away cleanly. If a short drain forces your hose into a sharp bend or a shallow angle right off the flange, a longer one will often solve it.

Measure the clearance around your drain pad before ordering. Not sure? Send your setup through the Turbo Spec Form and we will confirm before you buy.

Step 4: Pick Your Flange

All flanges are billet-machined and available in M8-1.25 (metric, roughly 5/16") or 3/8-16 (US standard coarse) thread, shipped from Canada in CAD.

Large Frame — 50.8mm (2.00") Bolt Centre — 12AN (recommended for S400)

Drain Length US Standard Port SKU
35mm 1.38" 12AN BSI12MlF35
65mm 2.56" 12AN BSI12LF65
100mm 3.94" 12AN BSI12MlF100

Mid Frame — 50mm (1.97") Bolt Centre — 10AN

Small Frame — 38mm (1.50") Bolt Centre — 10AN

Browse all turbo drains and plugs

Metric to US Standard Reference

Everything on this page in one table, for builders working in inches.

Metric US Standard What It Is
38mm 1.50" Small frame bolt centre
50mm 1.97" Mid frame bolt centre
50.8mm 2.00" Large frame bolt centre (S400)
35mm 1.38" Short drain length
65mm 2.56" Medium drain length
100mm 3.94" Long drain length
M8-1.25 approx. 5/16" Metric thread option
approx. 9.5mm 3/8-16 US standard coarse thread option

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 centre. 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, an angled fitting, or a longer drain to move the connection somewhere the hose can fall away cleanly.

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 Port size, low spots in the line
Repeated seal failure after rebuild Drain was never the right size Bolt centre 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
Flange will not sit flat on the pad Wrong frame class Re-measure bolt centre — 1.97" and 2.00" are easy to confuse
Hose kinks or sags right off the flange Drain too short for the install Step up to a 65mm or 100mm drain length

Installation Checklist

  1. Measure the bolt centre on the turbo drain pad, hole centre to hole centre
  2. Match it to your frame class — 38mm (1.50") small, 50mm (1.97") mid, 50.8mm (2.00") large
  3. Choose 12AN for an S400-frame turbo
  4. Check clearance around the drain pad and pick your drain length — 35mm (1.38"), 65mm (2.56"), or 100mm (3.94")
  5. Order the matching billet flange in M8-1.25 or 3/8-16 to suit your housing
  6. Clean both sealing faces — no old gasket material
  7. Fit a new gasket, torque evenly, do not over-tighten into billet
  8. Route the hose downhill with no low spots, within 15 degrees of vertical where possible
  9. Terminate above the sump oil level
  10. Confirm feed restrictor is correct for your bearing type
  11. Prime the turbo with oil before first start

Related Guides

Frequently Asked Questions

What size oil drain does an S400 turbo need?

An S400 is a large-frame turbo, so it uses a 50.8mm (2.00") bolt centre drain flange with a 12AN port. Drain length — 35mm, 65mm, or 100mm — is a separate clearance choice. Measure the distance between the centres of the two bolt holes on the drain pad to confirm before ordering.

What is bolt centre on a turbo drain flange?

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 identifies the frame class: 38mm (1.50") is small frame, 50mm (1.97") is mid frame, and 50.8mm (2.00") is large frame. It is not the port size and not the drain length.

What does the 35mm, 65mm or 100mm in the product name mean?

That figure 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 a flow or fitment one. Every bolt centre and port size is offered in all three lengths.

Which drain length should I choose?

Pick the shortest length that clears everything around your drain pad. Use 35mm (1.38") when nothing is in the way, 65mm (2.56") as a middle ground, and 100mm (3.94") when you need to reach past a manifold, frame rail, or block accessory. A longer drain also helps if a short one forces your hose into a sharp bend right off the flange.

Does drain length affect oil flow?

No. Two flanges with the same port size flow the same regardless of length. What length changes is where the hose connection sits, which affects how cleanly the line can fall away toward the sump.

What is the bolt centre in inches?

Small frame is 1.50 inches, mid frame is 1.97 inches, and large frame is exactly 2.00 inches. Mid and large frame are only about 1/32 of an inch apart, so use calipers rather than a tape measure.

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 centre 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.