S400 Turbo Flange Guide: Marmon, Half Marmon & V-Band Fitment

Most S400 builds do not stall on the turbo. They stall on the four connections around it — the turbine discharge, the compressor discharge, the clamps, and the gaskets that hold it all together. Order the wrong flange and your build sits on the bench waiting on a part that ships from three provinces away.

This guide covers every flange interface on an S400 turbo, how to measure yours, and exactly which part fits. Black Sheep Industries machines all of this hardware in-house in Canada and ships flange and clamp together as one part number.

The Four Flange Interfaces on an S400

Every S400 install has four bolted or clamped joints. Miss one and the truck does not run.

  1. Turbine inlet — where the turbo bolts to the exhaust manifold. T4 or T6. Determined by your manifold.
  2. Turbine discharge (exhaust outlet) — where the downpipe attaches. Marmon-style V-band. This is the one most people get wrong.
  3. Compressor discharge — where the charge pipe attaches. V-band.
  4. Oil drain — covered in the S400 oil drain guide.

What Is a Marmon Flange?

A marmon flange is a V-band style connection where the flange has a tapered outer profile that a matching band clamp pulls against. Squeeze the clamp and the taper draws both halves together, forming a seal without bolts through the joint.

It came out of aerospace and heavy commercial diesel, where you need a joint that seals under thermal cycling, survives vibration, and can be broken loose for service without fighting seized studs. That is exactly the load case on a turbo exhaust outlet.

Why Turbos Use Them

  • Clocking freedom. A bolted flange locks your downpipe angle. A marmon joint lets you rotate the downpipe to any position before you tighten the clamp.
  • Thermal cycling. The taper keeps clamping force as the joint heats and cools, where a bolted flange warps and starts leaking.
  • Service access. One clamp off and the downpipe drops. No penetrating oil, no snapped studs.
  • Sealing without a gasket. Machined face to machined face, the taper does the work.

Full Marmon vs Half Marmon

A full marmon has the tapered profile machined all the way around the circumference. A half marmon has the taper on part of the profile with a flat section — used where the turbine housing casting does not leave room for a full circular flange. S400 T6 turbos use a full 5.75" marmon. S400 T4 turbos use a 4.62" half marmon.

S400 Turbine Discharge — Which Flange Do You Need?

This depends entirely on your turbine housing flange, not on compressor size. An S467 and an S488 on the same T6 housing use the identical discharge flange.

Turbine Housing Discharge Flange Clamp Also Fits
S400 T6 5.75" full marmon 99502-0588 S410SX, other T6 400/410 frame
S400 T4 4.62" half marmon Included Garrett G-Series 42, G-Series 45
S200 / S300 4.25" marmon Included S200 and S300 frame
S500 / GT55 / G57 T5 discharge Included GTX55, PTE ProMod

5.75" S400 T6 Marmon Downpipe Flange

The standard discharge flange for every S400 and S410SX on a T6 turbine housing. Sold with the quick-release clamp so you are not sourcing the clamp separately.

  • 5" to 5" 304 stainless — BS0575SST6S4005T5
  • 5" to 4" 304 stainless — BS0575SST6S4005T4
  • 5" to 5" aluminum — BS0575ALT6S4005T5
  • 5" to 4" aluminum — BS0575ALT6S4005T4

Shop the 5.75" S400 T6 Marmon Downpipe Flange with Clamp

The "5 to 4" and "5 to 5" refer to the flange bore stepping down to your downpipe tube diameter. Pick based on the downpipe you are building, not the turbo.

4.62" Half Marmon T4 Downpipe Flange

For S400 turbos on a T4 turbine housing. Also the correct flange for Garrett G-Series 42 and 45, which share the same discharge profile.

  • 3.5" 304 stainless — BS0035SST4S400
  • 4.0" 304 stainless — BS0040SST4S400
  • 4.0" aluminum — BS0040ALT4S400

Shop the 4.62" Half Marmon T4 Downpipe Flange with Clamp

5.75" Marmon Quick Release Clamp (99502-0588)

The clamp for every 5.75" marmon joint. 304 stainless. Buy a spare — clamps get lost, stretched, or left under a truck more often than any other part in a turbo install.

Shop the 5.75" Marmon Stainless Steel Quick Release Clamp

S400 Compressor Discharge Flange

The compressor outlet is a V-band joint to your charge pipe. Unlike the turbine side, the flange you need depends on your intercooler piping diameter, not the turbo.

This one flange covers a wide range of large-frame turbos — Borg S400, PTE ProMod, and Garrett G-Series 42, 45, 47, 50, 55, and 57 all share the compressor discharge profile.

Charge Pipe Material SKU
3.0" Aluminum BSGT55S400CDFLAL3
3.5" Aluminum BSGT55S400CDFLAL35
4.0" Aluminum BSGT55S400CDFLAL4
3.0" 304 Stainless BSGT55S400CDFLSS30
3.5" 304 Stainless BSGT55S400CDFLSS35

Shop the S400 Compressor Outlet Discharge Flange with Clamp

Sizing rule: match the flange to your charge pipe, not to the compressor housing outlet. Most S400 street builds run 3.0" or 3.5"; high-power builds move to 4.0".

Stainless or Aluminum — Which Should You Buy?

Choose 304 Stainless When

  • The flange is on the exhaust side and sees sustained high EGTs
  • You are building a truck that tows heavy or races regularly
  • You want the joint to outlive the turbo
  • Corrosion matters — road salt, winter driving, marine

Choose Aluminum When

  • The flange is on the compressor (cold) side — charge pipe temps do not justify stainless
  • You are welding to aluminum charge piping and want matched materials
  • Weight matters more than absolute durability

Short version: stainless on the hot side, aluminum on the cold side. Both are available for most flanges because builders have legitimate reasons to go either way.

How to Measure Your Flange Before You Order

Get this right and you order once.

  1. Identify the turbine housing flange. Look at where the turbo bolts to the manifold. Four bolts in a rectangular pattern with a smaller footprint is T4. A visibly larger square-ish flange is T6. If in doubt, count the bolt spacing.
  2. Measure the discharge outer diameter. Calipers across the widest point of the tapered flange on the turbine housing outlet. Roughly 5.75" means T6 full marmon. Roughly 4.62" means T4 half marmon. Roughly 4.25" means you have an S300, not an S400.
  3. Check for the flat. A half marmon has a visible flat section on the profile. A full marmon is tapered the whole way around.
  4. Measure your downpipe tube. Outside diameter of the tube you are welding to. This sets the 4" vs 5" flange bore choice.
  5. Measure your charge pipe. Same — outside diameter, sets the compressor discharge flange size.

Still unsure? Send photos and measurements through the Turbo Spec Form and we will confirm the part before you buy.

Common Fitment Mistakes

  • Ordering by turbo size instead of housing flange. "I have an S480" does not tell anyone which flange you need. T4 or T6 does.
  • Confusing S300 and S400 discharge. The S300 marmon is 4.25", the S400 T4 half marmon is 4.62". Close enough to look right in a photo, far enough apart that it will not clamp.
  • Buying the flange without the clamp. A marmon flange with no clamp is a paperweight. Every BSI flange ships with its clamp for this reason.
  • Matching the compressor flange to the housing outlet instead of the charge pipe. The flange has to weld to your tube. Size it to the tube.
  • Reusing a stretched clamp. Marmon clamps yield over repeated heat cycles. If it does not draw down with normal torque, replace it.
  • Aluminum on the hot side. It will work, briefly. Use stainless on the turbine discharge if the truck works hard.

Complete S400 Flange Shopping List

S400 T6 Build

S400 T4 Build

Browse Everything

Related Guides

Frequently Asked Questions

What size flange does an S400 turbo use?

It depends on the turbine housing. S400 turbos on a T6 housing use a 5.75" full marmon discharge flange. S400 turbos on a T4 housing use a 4.62" half marmon discharge flange. The compressor discharge is a V-band sized to your charge pipe, typically 3.0", 3.5", or 4.0".

What is a half marmon flange?

A half marmon flange has the V-band taper machined on part of its profile with a flat section, used where the turbine housing casting does not allow a full circular flange. S400 T4 turbos use a 4.62" half marmon. A full marmon is tapered all the way around.

What clamp fits a 5.75 inch marmon flange?

The 99502-0588 quick release clamp. Black Sheep Industries makes it in 304 stainless and includes one with every 5.75" marmon flange, with spares sold separately.

Will an S300 flange fit an S400 turbo?

No. The S200/S300 turbine discharge marmon is 4.25" and the S400 T4 half marmon is 4.62". They look similar but will not clamp together. Measure before ordering.

Does the S400 downpipe flange fit a Garrett G-Series turbo?

The 4.62" half marmon T4 flange also fits Garrett G-Series 42 and 45. The compressor discharge flange fits Garrett G-Series 42 through 57 as well as PTE ProMod.

Should I use stainless or aluminum flanges?

Use 304 stainless on the turbine discharge where the flange sees high exhaust gas temperatures, and aluminum on the compressor discharge where temperatures are far lower. Both are available for most flanges.

Do BSI flanges come with a clamp?

Yes. Every marmon and V-band flange assembly ships with its matching quick-release clamp as one part number, so fitment is right the first time.