After a major engine overhaul, should cold running-in or hot running-in be performed first?

[LeAddyson]

After a major engine overhaul, cold running-in should be performed first, followed by hot running-in. Below are some introductions about cold and hot running-in: 1. Avoid high speeds: Mechanics at repair shops usually advise that overhauled engines have speed limits, try to keep it below 80 kilometers per hour, and also avoid letting the engine speed exceed 3000 RPM. 2. Avoid emergency braking: Emergency braking not only impacts the braking system during the running-in period but also increases the impact load on the chassis and engine. It is best to avoid emergency braking within the first 300 kilometers after the overhaul. 3. Pay attention to oil maintenance: Use high-quality, high-cleanliness oil for the car, which should not be lower than the manufacturer's specified grade. Avoid using oils that contain anti-wear additives or various other additives to prevent further damage to the engine.

[Neil]

After assembling a rebuilt engine, it's common practice to perform a cold run-in before a hot run-in. A cold run-in involves rotating the engine without ignition using an external machine, allowing new components like piston rings and cylinder walls to gradually wear in smoothly, avoiding scratches or damage caused by sudden friction under high temperature and pressure. This step is crucial as it helps identify assembly issues such as loose parts, poor sealing, or abnormal noises, saving time on rework. Only after this is completed does the hot run-in begin, where the engine is ignited and run to operating temperature, testing compression ratio, oil leaks, and overall performance under load. Skipping the cold run-in and jumping straight to the hot run-in risks exposing new, un-worn parts to high temperatures, potentially leading to cylinder scoring or bearing failure, rendering the repair pointless. I always follow these steps meticulously to ensure the engine's longevity and smoother operation. Remember, a cold run-in typically takes 20-30 minutes, adjusted based on the condition of the parts.

[PaisleeAnn]

I've worked on several car repairs with my friends, and every time after a major engine overhaul, I insist on doing a cold run-in first. Cold run-in means mounting the engine on a stand and rotating it with an electric motor without fuel injection or ignition, which reduces friction between parts and allows them to adapt to each other's clearances. Only then do we proceed with the hot run-in, gradually heating it up to operating temperature for testing. This sequence has helped us avoid many issues. For instance, once we skipped the cold run-in, and during the hot run-in, the spark plugs failed to fire. Upon inspection, we found the ring positions were misaligned, nearly ruining the engine. I believe skipping this step is not worth the risk. Cold run-in is like a warm-up, while hot run-in is the main course—proper warm-up prevents strains. A cold run-in typically takes about half an hour, followed by a hot run-in with load, starting from low speeds. This approach ensures better engine durability.

[Katie]

After an engine overhaul, perform cold running-in followed by hot running-in. Cold running-in involves externally rotating the engine without ignition to allow new components to smoothly break in and reduce initial wear. Hot running-in involves ignition operation to test the actual working conditions. Direct hot running-in carries high risks, easily leading to cylinder scoring or component damage. Typically, cold running-in lasts over 20 minutes, followed by checks for noise and leaks. If no issues are found, proceed with hot running-in while gradually increasing temperature.

[ChristopherLynn]

After installing a rebuilt engine, cold running-in is essential for safety. During cold running-in, the engine does not fire; instead, external force rotates it to allow parts to gently adapt. If hot running-in with ignition is performed directly, high temperatures and high speeds may instantly scratch the cylinder walls or burn out piston rings, increasing repair costs. Cold running-in helps detect assembly errors, such as oil leaks or looseness. After completion, hot running-in should start with small loads and gradually increase to full speed to ensure reliable sealing. With my extensive experience, engines treated this way are more durable.

[DelElena]

After a major engine overhaul, cold running-in is the first step, like giving it a starting buffer. Cold running-in involves externally driving the engine to rotate without ignition, allowing the piston rings and bearing shells to smoothly break in, avoiding direct hard friction and part damage after ignition. Then, hot running-in follows with ignition, testing performance and compression under high temperatures. This sequence protects new parts and reduces the risk of failure. I think it's similar to fitness—warming up before high-intensity exercise to maintain engine health. Cold running-in takes about half an hour, and monitoring oil and coolant temperatures during hot running-in is more prudent.