OM601 OM602 OM603 US Swap Guide – Replacing the OM617 with Modern Mercedes Diesel

OM601 OM602 OM603 US swap builds are becoming increasingly popular as American diesel builders move away from the OM617 to the more modern, lighter and more powerful three engine family. Today we cover everything you need to know to make this swap correctly.

Why Move Away From the OM617

The OM617 has been the go-to diesel swap engine in the United States for years. It is proven, available and has a strong community around it. But it has limitations that the newer engine family does not.

The OM601, OM602 and OM603 are more modern engines that are lighter, more powerful and significantly easier to tune. They accept the 722.6 automatic transmission directly – no adapter plates, no fabrication, flexplate to transmission is a direct bolt-on. This alone simplifies builds considerably compared to the adapter plate solutions required for the OM617.

For most American builders wanting 300-400 horsepower daily drivers the OM601 2.2 litre, OM602 2.5 litre and OM603 3.0 litre are completely capable engines that are easier to work with than the platform they replace.

Which Engines to Choose

All OM601, OM602 and OM603 engines available in the US use 87mm bore. This is critical because it means standard OM605 and OM606 MLS head gaskets can be used directly – cut off the excess length for four cylinder application with an angle grinder.

Avoid the 3.5 litre OM603 with 89mm bore found in some US markets. These engines have a reputation for bending rods even at stock power levels. The standard 3.0 litre OM603 is the correct choice.

Recommended starting points:

• OM601 2.2 litre – four cylinder, 87mm bore, 92.4mm stroke, available in many chassis
• OM602 2.5 litre – five cylinder, 87mm bore, 84mm stroke, excellent mid-range option
• OM603 3.0 litre – six cylinder, 87mm bore, 84mm stroke,  maximum displacement and power potential

Always start with a turbo specification engine. Turbo engines have oil squirters and 28mm wrist pins. Naturally aspirated versions lack oil squirters and use 26mm wrist pins which limits rod upgrade options.

Head Gasket Options

Stock organic head gaskets on these engines cool better than MLS gaskets due to better water flow characteristics. However they cannot handle boost reliably. Two correct options exist:

MLS head gasket from OM605 or OM606 – fits 87mm bore directly on 601, 602 and 603 engines. Cut the excess cylinder section off the back with an angle grinder. Torque down and done.

O-ring block – machine a groove in the block deck and install a stainless wire ring around each cylinder. This is actually more common in America than Europe and is a legitimate alternative to MLS gaskets. Allows the original cooling system benefits of the organic gasket while significantly improving head sealing under boost.

Both options work. Choose based on what your machine shop can do and what your build requires.

The 2.2 Litre OM601 Bore and Stroke Curiosity

The US specification OM601 2.2 litre has 87mm bore but uses the longer 92.4mm stroke – the same stroke as the 2.9 litre OM602 and 3.5 litre OM603. This is why a 2.2 litre four cylinder has the same stroke as a 3.5 litre six cylinder. Fewer cylinders of the same size equals less total displacement.

The OM604 is also 2.2 litres but uses 89mm bore with a medium stroke. Two engines with identical displacement achieved through completely different bore and stroke combinations.

Rod Upgrades – What Fits What

This is critical to get right as the engine families do not share rod dimensions uniformly.

OM602 and OM603 turbo engines – 28mm wrist pins, 149mm rods. OM605 and OM606 turbo rods drop directly in. F-tune Performance forged IA-beam rods fit directly. Excellent and straightforward upgrade.

OM601 2.2 litre US spec – longer stroke means different rod length requirements. Standard OM605/OM606 turbo rods do not fit. This engine requires sourcing specific rods. Contact specialists for viable solutions.

OM601, OM602 and OM603 naturally aspirated versions – 26mm wrist pins. OM605/606 turbo rods do not fit due to wrist pin size difference. However, F-tune Performance do have forged rods even for the NA-engine.

A Note for US Builders Swapping Rods

If you are upgrading to forged rods in your OM606 or OM605 the original turbo connecting rods you remove are valuable. Do not export them. Sell them within the US to OM602 and OM603 builders – they drop directly into those engines and represent a significant strength upgrade at used rod prices. The community benefits more from keeping these parts circulating domestically.

722.6 Automatic Transmission

The 722.6 five speed automatic bolts directly to OM601, OM602 and OM603 engines. No adapter plates. No custom fabrication. This is a significant practical advantage over the OM617 which requires adapter solutions for most modern transmission options. The 722.6 is strong enough for any realistic power level on these engines and is widely available cheaply.

Summary

• OM601 OM602 OM603 US swap is a genuine upgrade over the OM617 in power, weight and tuneability
• All US spec OM601 602 603 use 87mm bore – OM605/606 MLS gaskets fit directly
• Avoid 3.5 litre OM603 with 89mm bore – rod bending issues at stock power
• Always choose turbo specification engines for oil squirters and 28mm wrist pins
• O-ring block is a valid alternative to MLS gaskets and more common in the US
• 722.6 automatic bolts on directly – no adapter plates needed
• OM602 and OM603 turbo engines accept OM605/606 turbo rods directly
• OM601 2.2 litre US spec requires specific rod sourcing due to longer stroke
• Keep used OM606 turbo rods in the US for OM602/603 builders – they are a direct fit upgrade

Love that you're swapping diesels into trucks. Keep it going. See you in a couple of days!