OM606 Engine Building Guide – Balancing, Weighing Pistons and Rods Explained

Engine balancing is one of the most important and least understood steps in building an OM604, OM605 or OM606 engine. Fred explains the difference between balanced and unbalanced engine designs, how to correctly weigh pistons and rods and what your engine builder needs to know.

OM606 engine building starts with one critical step most builders skip – proper weighing and balancing. Get this wrong and everything else suffers. Get it right and you have an engine that runs smoother, lasts longer and makes more power consistently.

Why We Build Engines

We build engines to regain control. These engines are 25 to 30 years old. The OM604 launched in 1993. The OM605 and OM606 followed shortly after. They were excellent when new. Today they may still run well but we have lost control over the internal condition and tolerances.

Building gives us options. Replace rings, bearings, rods – anything we want. More importantly it gives us the opportunity to exceed Mercedes factory specifications using better components and tighter tolerances than the original build.

Two Types of Engine Designs – Balanced and Unbalanced

This is the most important concept to understand before building any engine.

Some cranks are balanced by design. The forces generated by the rotating and reciprocating assembly cancel each other out mathematically. Four cylinder engines, six cylinder engines and flat plane V8 cranks – like Ferrari and Ford GT350R – fall into this category.

Other cranks are not balanced by design. Five cylinder engines and cross plane V8 cranks – like a standard small block Chevrolet or LS engine – fall into this category. The firing sequence creates inherent imbalance that cannot be resolved through crank design alone.

Bob Weight Balancing

Unbalanced design engines require bob weight balancing. Bob weights represent the combined mass of the connecting rods and pistons. They are fitted to the crank during balancing so the machine simulates the full rotating and reciprocating assembly. The crank is then balanced with this simulated load in place.

Balanced design engines – including all OM604, OM605 and OM606 engines – do not require bob weights. Only the rotating components are balanced: the crankshaft, damper, crank hub sprocket, and flywheel or flex plate. No bob weights needed because the design is inherently balanced.

This is a critical distinction. If you are having your OM606 crank balanced by an engine builder who is not familiar with these engines make sure they know it is a balanced design. No bob weights.

Weighing Pistons

Place all pistons on a scale. The lightest and heaviest must be within one gram of each other. One gram. That is an extremely tight tolerance but it is correct and necessary.

Stock Mercedes pistons from 30 year old engines are often eight to nine grams apart. The factory did a good job for their era but time and wear change things. You need to correct this.

To remove weight from pistons use a Dremel carefully. On OM604, OM605 and OM606 pistons the skirt area is limited so there is not much room to work with. The area around the oil squirter opening is a good location – additional clearance there neither helps nor hurts engine operation so removing small amounts of material from that area is safe and effective.

Weighing Connecting Rods

Rods cannot simply be placed on a scale as a complete unit. You must weigh the small end and big end separately.

All small ends must weigh the same as each other. All big ends must weigh the same as each other. This is how rod balance is correctly measured and corrected.

A lathe is the ideal tool for removing material from rods because you can work evenly around the big end or small end and remove material in a controlled, balanced way. Most engine builders have this equipment.

The Balancing Process

Once pistons and rods are weighed and matched assemble your complete rotating package: crankshaft, damper, crank hub sprocket and flywheel. Everything must be in its final finished state. If you need to polish the crank that must be done before balancing.

Take this assembly to your local engine builder who has a crank balancing machine. They spin it and correct any imbalance until it reads zero. For unbalanced design engines you would also provide the rod and piston weights so they can set up the bob weights correctly – but for OM606 engine building this is not required.

When the crank reads zero you are ready to assemble.

Summary

• OM606 engine building requires careful weighing and balancing before assembly
• OM604 OM605 OM606 are balanced design engines – no bob weights required
• Pistons must be within one gram of each other lightest to heaviest
• Rods are weighed at the small end and big end separately – all must match
• Use a Dremel carefully on pistons around the oil squirter area to remove weight
• A lathe is ideal for rod weight correction
• Crank balancing is done with the complete rotating assembly in final finished state

Three more days for the next video. Love you all!