Your turbo gets fed pressurized oil to keep the bearings alive. That oil also has to get back out, and it drains by gravity, not pressure. If the drain is restricted, kinked, or too small, oil backs up in the center section and pushes past the seals. Now you are smoking oil and blaming the turbo when the real problem was the drain.
This matters even more when you swap turbos or run a second gen swap, because the factory drain rarely lines up clean with the new setup. That is where a proper AN turbo drain kit comes in.
Why AN Line for the Drain
- Smooth, full size path lets oil fall out as fast as it comes in
- No crushed or kinked hose choking the return
- Flexible routing so you can run it clean around a swapped manifold or turbo
- Real fittings that seal and do not weep oil down the block
10AN vs 12AN
For most builds a -10AN drain is plenty. If you are running a big single like an S400 family turbo or pushing serious flow, stepping up to -12AN gives you margin so the drain is never the restriction. Bigger turbo and more oil moving through it means you want the drain sized so it is never the bottleneck.
Build It Yourself
Our Universal Cummins DIY Turbo Drain Kit gives you the flange, fittings, and braided line to set up a clean drain for your exact combination. If you are running a high flow S300 or S400, we also stock the High Flow -10AN Turbo Drain Flange for S300 and S400. If you have never assembled AN line before, we walk through the whole process in our AN Fittings 101 guide.
Get the Drain Right
A turbo drain is one of those things nobody thinks about until a fresh turbo is blowing oil. Size it right, route it clean, and it is one less thing on the list.