The engine pros

Uwe Rosenau and his son Florian are professionals in engine repair. Among other things, piston replacement is part of their everyday business. In this interview, they tell us what matters and why you can dare to fix an engine.

The engine pros Uwe Rosenau and his son Florian.

MAHLE: How did you come to specialize in engine repair?

Uwe Rosenau: That was a stroke of fate, as it were. I originally wanted to work for Deutsche Bahn. However, there was a freeze on recruitment at the time (1976), so I applied at an engine repair business in Osnabrück without really knowing what it meant or what the job entailed. I served my apprenticeship there and discovered my passion for engine repair right from day one—and I’ve never wanted to do anything else since. I was simply fascinated at how a “dead engine” could be “brought to life” again.

MAHLE: Your son has now also joined the business with his own workshop. What do you offer your customers?

Uwe Rosenau: We are specialists in the field of engine repair and also have an automotive workshop next door. We can thus offer our customers an all-round service in addition to our expertise. We repair all types of vehicles—from lawnmowers to 1,000 HP engines. This includes agricultural and construction machinery, special vehicles with special structures (e.g., road sweepers, fire engines, concrete pumps, etc.), off-highway engines (e.g., cooling units), as well as passenger cars, trucks, and vintage cars.

MAHLE: Of the orders you have received to date, which represented a highlight or particular challenge for you?

Uwe Rosenau: Although the installation steps are always the same, every engine is different, so the respective torque and dimensions need to be taken into account during assembly. For instance, we once had a Caterpillar engine model 3412 in for repair. The engine was delivered partially dismantled and didn’t have the old pistons, rod bearings, and connecting rods. After consulting with the customer, they sent us 12 connecting rods. When assembling the engine, the mechanic inquired about the piston protrusion or recess dimension. I wasn’t able to get a dimension from either the OE or the aftermarket. We decided to carry on with the assembly regardless. During the test run, we then noticed thick gray smoke coming out of the engine upon cold start and during the warm-up phase. We had to dismantle it yet again and plane both end faces of the block to reduce the recess of the piston. This process cost us a lot of time and patience.
We particularly enjoy repairing vintage cars. It’s funny that the vehicles that were new at the time of my apprenticeship are vintage cars today. That’s why I know so much about them. More often than not, these cars don’t just need to be repaired; they need the entire engine block, including paint, etc., to be aesthetically revamped so that everything looks “good.” But that’s of course another story altogether.

MAHLE: Why do you think workshops no longer dare work on engines?

Uwe Rosenau: This development is not new. It already began when I was an apprentice. Several manufacturers started to offer replacement engines at the beginning of the 70s. Many workshops took the easy route and installed a replacement engine if there was engine damage. In contrast to us, many workshops do not replace pistons as part of their everyday business. There’s naturally great respect for this work because the smallest error can cause great destruction and subsequently lead to engine damage. Many therefore prefer to leave this area well alone as it’s easier and safer. Moreover, the majority of independent workshops don’t have the necessary equipment to carry out engine repairs (e.g., boring, honing, or grinding machines, etc.). In addition, trainees attending vocational schools or industry-wide training are now rarely taught anything about engine repair. The vehicles nowadays have a lot more mechatronics and electronics. I am from the old school and was trained in this area. However, the “old hands” are gradually retiring and the next generation can no longer perform this work. That’s why we retrain the employees that didn’t do an apprenticeship with us in this field. I always tell them: Don’t be afraid to examine the engine and see where the damage is. You’ll probably be able to do the simple things yourself. If you can’t figure out how to do something by yourself, then go to an engine reconditioner “you trust.”

At the engine repair 'Rosenau' dead engines are brought to life again.

MAHLE: To what extent is it still worth it to replace engine components and at what point does the entire engine need to be replaced?

Uwe Rosenau: The engine’s mileage is a deciding factor here. If it’s very high, it’s not worth replacing individual components. It’s better to overhaul the engine completely or install a replacement.

MAHLE: Do you see a trend whereby only the pistons are replaced as opposed to the complete engine?

Uwe Rosenau: There are currently some engine types that are prone to high oil consumption after just a short running time. It makes economic sense here to only replace the pistons with new pistons that have a modified ring set, or to only change the ring set on the old pistons.

MAHLE: How do vehicles differ when it comes to replacing pistons?

Uwe Rosenau: The work process for replacing pistons is actually always the same. You have to be careful with some engines because different pistons can be installed in cylinders 1 and 2 compared with cylinders 3 and 4 (valve pocket arrangement in the piston crown). With some older engines, such as the DB OM 636, the pistons are installed from below (the crankshaft has to be removed).

MAHLE: What else should always be replaced at the same time as the pistons?

Uwe Rosenau: The complete piston with piston rings, piston pin, and safety clips by all means, not to mention the rod bearings, connecting rod bolts (if these are antifatigue bolts), all gaskets exposed during the repair, and the cylinder head bolts. The easiest way to identify antifatigue bolts is from the manufacturer’s tightening values: torque plus angle of rotation. If you suspect that the connecting rods are bent, then they should be tested by an engine reconditioner. And in the case of air-cooled engines, I also recommend checking and replacing the rod bearings, although they don’t necessarily need to be loosened.

MAHLE: What tips do you have for replacing pistons?

Uwe Rosenau:

  • First and foremost: “The repair already starts with the dismantling.” This means you need to make a precise note of everything (position, dimensions, etc.) when dismantling.
  • What’s more, both pistons (old & new) should be compared extremely carefully (identical design, size, etc.) before the installation. It generally goes without saying that you need to work carefully and accurately, for instance, when taking measurements. This is because when assembling a piston, it must fit exactly within a hundredth of a millimeter. Measuring the piston protrusion is also of elementary importance, as it will determine whether a thicker or thinner cylinder head gasket needs to be installed.
  • It is absolutely essential that the installation direction, which is usually marked by an arrow on the piston crown, is observed.
  • Always use plenty of oil (= lubricant) during the assembly to prevent dry running when starting the engine for the first time.
  • Another installation tip is to make sure you use a clamping band for the piston rings. The gaps in the piston rings should be offset (120° angle).
  • HERE you can watch our video series with Uwe

MAHLE: Is drilling and honing always required when replacing the pistons?

Uwe Rosenau: If the dimensional accuracy (wear rate) of a cylinder (measurement!) is still OK, the cylinder (block or liner) could in principle be left as it is. Posthoning is then at least advisable. However, as soon as the wear rate has been reached or a damage pattern (damage to the surface) becomes visible, “drilling” with subsequent honing (cross-hatch finish) is unavoidable. For this purpose, oversize pistons are available for many engines. Should an oversize piston happen to be unavailable (often the case with vintage cars), you can also subsequently equip most engine blocks with cylinder liners in order to install the standard piston variant.

MAHLE: What assistance does MAHLE offer (you) for piston assembly?

Uwe Rosenau: The first few pages in the piston catalog are extremely helpful. They contain lots of information und explanations. For example, piston designations, installation instructions. The “Engine parts and filters: damage scenarios, causes, and avoidance” brochure is also very good.

The engine repair workshop Rosenau in Bohmte near Osnabrück.

MAHLE: Which MAHLE products do you rely on?

Uwe Rosenau: When it comes to engine components, we install MAHLE pistons including rings, pins, as well as valves, bearings, and liners. The high quality of the MAHLE pistons can be seen in our ultrasonic bath: even after 20 minutes of cleaning, the piston working surface coating is still perfectly preserved, as opposed to pistons from other manufact.

MAHLE: One last question: How do you see the future of the engine reconditioner?

Uwe Rosenau: There will be a decline in orders for passenger cars. This is partly due to the newer downsizing engines, and partly due to the fact that there will be more hybrid and electric vehicles. We already have relatively few trucks because the components have a good service life. The future definitely lies in
industrial and agricultural machinery, vehicles with special structures, combined
cycle power plants, and biogas plants. The same goes for vintage cars. In the
meantime, about 50 percent of the vehicles we get are vintage cars.
This market is booming.

Thank you for the very interesting interview, Uwe Rosenau.


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


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