Recently there has been a lot of forum discussion about the best way of running in a freshly built motor. This topic seems to pop up every now and then, and always seems to provoke more than its fair share of opinion and controversy. The information in this article is not aimed solely at those of you building full-house race motors. The information is equally valid for those looking to import a crate motor for their project, or even anyone who is about to buy a new car from a dealer.

It seems as if everyone you talk to has their own preferred running-in technique, but the problem is that they all appear to be at odds with one other. Some will recommend the engine is driven carefully for the first 5000km with only the lightest touch on the accelerator, while others advocate leaving the car dealership’s showroom on the rev limiter with smoke billowing from the tyres.
Chances are that the truth lies somewhere between these extremes, and we will explore what actually happens during the running-in period and how we can achieve the best results possible.

Running In: What Is It All About?

First up we need to have some idea of what we are actually trying to achieve in running in our fresh motor. This is the first area in which misinformation abounds.

Three of the more common myths are that the engine must be heat cycled, that it requires a certain number of kilometres for the bearings to bed in, and that the pistons need some time to ‘free up’ in the bores. Let’s consider these myths in a bit more detail.

1. Heat cycling the engine All the parts that require heat-treating have had this process applied by the manufacturer during production. The sort of heat involved with treating of steel is significantly higher than what we would ever expect to see in our engines, so little can be expected to be gained once the engine is running.

2. Running in bearings Crankshaft bearings and conrod bearings do not require running in. A plain bearing either has sufficient clearance or it doesn’t. They rely on a thin film of oil to be present between the surfaces, so there should never be any metal-to-metal contact. If there is insufficient clearance, no amount of gentle running in will ever help.

3. Piston clearance The amount of clearance between the piston and the bores is set during the machining process and, as with the bearings, the engine either has enough clearance or it doesn’t. No amount of gentle running will change things if these clearances are too tight.

The only components we are really worried about during the running-in procedure are the piston rings, and their seal against the cylinder walls. Let’s explore these in a bit more detail.

Piston Rings

Piston rings are amazing devices. They are responsible for sealing all the combustion pressure, as well as preventing oil consumption by scraping excess oil from the bores. The rings provide a slight radial tension against the cylinder walls, which is how the oil is scraped away. However, this radial tension on its own is not enough to be able to hold in the pressure of combustion. During the combustion event, the cylinder pressure travels over the top of the ring and gets behind it, helping force it out against the cylinder wall.

A brand new ring set will always have imperfections that mean it doesn’t seal the entire way around the bore. Likewise the bores are never quite perfect either. During the running-in process these imperfections get worn away to produce a perfect interface between the ring and the bore. The better this seal, the more power an engine will make and the less oil it will burn. An added bonus is that a perfect ring seal will normally result in an engine that will last longer.

Getting The Perfect Seal

During machining, the engine’s bores are honed. Honing produces a very fine and controlled crosshatch pattern on the cylinder wall. This hone pattern acts like a file on the ring surface, gently removing and smoothing the irregularities and ensuring that the ring contact is perfect.

The important point here is that the hone pattern only lasts for a short while before the peaks get worn down and it stops acting as an abrasive on the ring surface. It is this window of time that we need to get our rings to seal perfectly.

Running In Oil

During the running-in process, it is important to only ever use a high quality mineral-based oil. Under no circumstances should you ever use a synthetic oil. Synthetic oils do such a good job of lubricating that they prevent the ring wear that is necessary to achieve a good seal. While the majority of ring bedding happens in the first 20km of driving, the process will continue for some time. For this reason it is important not to switch to a full synthetic oil too early. In a street car it is advisable to stick to a mineral-based oil for at least the first 1000km.

The initial run-in period will produce a lot of fine metal particles from both the rings and the bores. Mixed in with these particles, there will also be engine assembly lubricant and sealants. With all these nasties floating around, it is imperative that the engine oil is changed after the first 100km to prevent them being recycled through the engine.

Driving Technique

The way a freshly built engine is driven immediately after it is first started will influence the performance of the engine for the rest of its life, so it is critical to get it right. If the rings don’t seal well, the only fix is to re-hone the block, fit new rings and start again. The idea is to get a moderate amount of load on the engine right away to get the rings to bed in.

The absolute worst thing that can be done to any fresh engine is to allow it to idle for extended periods of time once it is hot. At idle, there is no load to force the rings against the cylinder walls, so the rings will quickly remove the rough peaks of the hone pattern, but because they are not under load, the rings do not wear properly to conform to the cylinder walls.

What we want to do is alternate between periods of light to moderate acceleration, and then overrun deceleration using engine braking if possible. During acceleration the rings are loaded against the bore walls and begin to bed in. This produces metal debris as well as a fair amount of heat in the rings. When the engine is allowed to decelerate on overrun, the vacuum produced will draw the oil mist up the cylinder, which helps cool the rings as well as flushing away the debris.

Note that I’m not suggesting the car is simply thrashed. If you drive the car too hard, the heat produced in the rings will become excessive and this will also prevent a good seal being attained. For the first 20 to 30km it is advisable to only use 50 to 60 per cent of the engine rev limit as a maximum rpm, because high rpm will also cause excessive heat in the rings.

With modern rings and honing techniques, it is difficult to totally mess up the running-in procedure, so don’t get scared off the idea. Following the above strategy, though, may be the difference between a great performing, long-lasting engine, and one that is simply average. Our suggestion is to keep in mind the points above, consult with your engine builder or supplier, don’t baby your fresh motor too much, and above all else remember to have fun ” that’s what you built or bought your car for in the first place.

Words: Andre Simon