Paccar MX-13 vs. Cummins ISX

Paccar MX-13 vs. Cummins ISX

Posted by Kustom Truck Parts on 10th Dec 2025

Which Engine Is Easier To Swap?

At Kustom Truck Parts, we support a lot of engine conversions in Peterbilt and Kenworth trucks. Since 2010 Paccar has offered essentially two different brands of engine in their trucks: a Cummins or a Paccar. Depending on which of the two different engines your truck started with, the conversion process is starkly different.

Even though the end goal is usually the same — install a fresh CAT, Cummins, or Detroit engine — the starting point makes a huge difference in time, cost, and headaches.

Our earlier article, Engine Conversion Basics, laid out the four pillars that apply to every swap: electronic interface, brackets, piping kits, and engine up-fit. Those four pillars still stand, but when the truck left the factory with a Paccar MX-13 instead of a Cummins, several extra steps pop up that you simply don’t have when pulling a Cummins. Below are the real-world differences we see every week, and exactly how we solve them.

1. Engine Positioning & Mounts – This is Where Paccar Trucks Fight You the Hardest

The Paccar MX-13 is a true mid-block engine. It does NOT sit in the same location in the rails of the truck as a Cummins ISX, CAT C15/3406E, or Detroit 60 Series.

Front Trunnion (Same for Peterbilt & Kenworth)

  • The Paccar sits ~2" too high and ~1/2" too far rearward.
  • You must replace the front trunnion brackets with our Front Trunnion Support Bracket Kit (PN: 19917007) or something similar.
  • Critical tip: The saddle must be reinstalled on the same side (cooling-package side vs. firewall side) as it came off the factory Paccar brackets.

If you’re pulling a Cummins ISX? This is a non-issue — reuse the factory front trunnion brackets.

Rear Frame Mounts – Throw the Paccar Mounts in the Scrap Bin

The factory Paccar rear frame brackets are completely incompatible with any engine other than the Paccar. Plus, the Paccar flywheel housing is deeper and the rear mounts bolt off the back of the housing instead of the sides — so the geometry is all wrong.

Peterbilt and Kenworth each have their own signature rear mounts:

Peterbilt options (most popular first):

Kenworth options (most popular first):

If you’re pulling a Cummins? It’s a non-issue — you can reuse the rear engine frame mounts.

Proven installation sequence (Paccar trucks only):

  1. Install the replacement front trunnion brackets first.
  2. Set the bare engine in the frame on the front mount.
  3. Use a transmission jack to lift the rear of the engine until the flywheel housing and frame brackets line up with one another.
  4. 95% of the time the original forward holes become the new rearward holes — mark and drill two new holes per side. These will become the forward holes on the replacement frame brackets.

This method keeps the transmission in its factory location — no driveline modifications, no shift-lever adjustments, no clutch linkage headaches.

2. Electrical – Alternator on the Wrong Side, ECM Power Routing & Vehicle Speed Input

Alternator Location

  • Paccar MX-13: Alternator on driver’s side
  • CAT / Cummins / Detroit: Alternator on passenger side

→ All charge, power, and ground cables are now too short or routed wrong.

You have two choices:

  • Splice and extend (cheap, but every splice is a future failure).
  • Replace with correct-length harnesses (what we recommend).

Recommended harnesses:

Pulling a Cummins? You reuse the factory charge/start and power harness.

Fused ECM Power

Paccar pulls fused ECM power through the emissions harness. Remove the DEF system and you just lost ECM power.

Solutions we supply:

  • Peterbilt NAMUX3 → Fuse Harness (PN: 19906013 or 19906014)
  • Peterbilt NAMUX4 & Kenworth NAMUX3/4 → Our engine harness plugs straight into the firewall fuse panel.

Cummins-based trucks are a mixed bag depending on year — you may have to add a new fused power source like you do with Paccar.

Vehicle Speed Signal (Kenworth NAMUX3 & Peterbilt NAMUX3 with Paccar)

One more electrical gotcha that only shows up when removing a Paccar MX-13:

Kenworth and Peterbilt NAMUX3 trucks with a factory Paccar engine pull vehicle speed through the ABS system. Older CAT, Cummins, and Detroit engines cannot read that datalink signal — they require a traditional analog signal for vehicle speed at the transmission output.

Kenworth NAMUX3 solution:

  • Add our Eaton Manual Transmission Harness (PN: 29905013) – this gives you Vehicle Speed, Transmission Temp, and Reverse Switch inputs in one plug-and-play harness.
  • Install a new speed sensor at the transmission tailshaft:

Peterbilt NAMUX3 solution:

No extra transmission harness is needed (those signals already run through our engine harness), but you still must replace the tailshaft speed sensor with one of the two styles above.

If the truck started with a Cummins, the factory speed sensor and wiring are already correct – nothing to change here.

3. Air Conditioning – Compressor & Lines

Paccar compressor is on the driver’s side and uses a completely different bolt pattern. It will not bolt to CAT, Cummins, or Detroit factory or aftermarket brackets.

You need:

  1. New compressor that matches your new engine and belt routing (example: CAT top-mount 6-groove PN: 99913004).
  2. New or modified A/C lines:

Cummins start? Suction and discharge lines are almost always reusable.

4. Fan Blade – The One Everyone Forgets

Paccar fan blade = 5" pilot, 6" bolt circle.
CAT / Cummins / Detroit fan hubs = 2.56" pilot, 3.5" bolt circle.

The old blade will not fit and will not center in the shroud. Replace it:

Bottom Line – Side-by-Side Comparison

Starting Engine Reuse Front Trunnion? Reuse Rear Mounts? Reuse Charge/Power Wires? Reuse A/C Lines? Reuse Fan Blade?
Cummins ISX Yes Yes (95%+) Yes Yes Yes (95%+)
Paccar MX-13 No No No No No

The four pillars from our original guide still apply, but starting with a Paccar MX-13 adds real work in every single category. Plan for the extra parts and labor up front and the job stays smooth.

Need help figuring out exactly what your truck requires? Shoot us the Year/Make/Model of your truck, the original engine, and the replacement engine you’re installing — we’ll send you the complete shopping list.

Happy wrenching!
— The Kustom Truck Parts Team

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