This is a topic that you will get 100 variations on when asking 200 different folks. It is also a topic that ultimately does not have a precise and simple answer because of a number of variables (engine build quality, fluids used in the build and running in procedure, initial fuelling and ignition settings, climate, geographic location) and one crucial but very variable factor – the 'operator'. The person doing the running in. When done on an engine dyno, typically it is being done by an experienced pair of hands, and is in a completely controlled environment. The consequence of this is a far more rapid and generally 100% successful running in. By successful I mean in particular the cam and followers, piston rings and bores have bedded in without any galling or other stress or heat damage. On a rolling road, you may have the experienced pair of hands effect, but not quite the controlled environment. In particular the load on the engine, as some rolling roads struggle with maintaining a constant load over a period of time. Some rolling road operators have worked out a way of getting round this and achieve a decently run in engine. Mostly it is done by the enthusiast, many of which have no experience of running in engines, on their country's highways and by-ways. In an attempt to offer some path through the jungle, I have decided to take a punt at explaining how I do it. This article will be a good deal more wordy than illustrated as it is not possible to describe the process pictorially. So strap yourselves in for a good read. 
From the outset, let us get something aired out. It is not at all necessary to run an engine in for 1,000 miles. Or even 500 miles. That is all very, very old hat. These periods hark from days of yore when engineering was not privy to the precision of machining and component quality in terms of materials and manufacturing precision we have today. And then there are the lubricants available. Oil technology has moved on dramatically. And we are talking about the aforementioned distances dating back to the 1960s. More than 50 years ago. A properly engineered and built engine today needs no more than 100 to 150 miles. The aim of running an engine in is to remove high spots on components that rub together. That in itself is a misnomer as there should never actually be any direct metal to metal contact. There should always be a film of lubricant between the parts, no matter how thin that film is. And those high spots are not mountainous peaks, but near-minuscule pips. 
Discussions on running in tend to start with what type of oil to use. We'll get to that later, because way more important than that is the initially fuelling and ignition settings. This is because you need to be able to start quickly and allow the engine to run immediately at a very fast idle of around 2,500rpm to bed the cam and followers in before you drive the car anywhere. The reason for the high idle rpm level is to ensure all slack is taken out of the valve train to prevent any components clattering against each other. If the fuelling and/or ignition are willy out you can not achieve this. Even with the fuelling and ignition in the ball park, you may only get to run the engine for 10 minutes at a time at that rpm level. But better to run it, stop it when the engine temp starts getting over normal, allowing it to cool, then running it again, and repeat until you get the necessary minimum 15, but better 20 minutes on the clock. And this is all pre-supposing you have not dropped the ball with the cooling system capability. I have been witness to far too many engines trashed at the initial running in stage because the owner has spent the whole budget on achieving the most engine output possible and none on the cooling system. Understand that the standard cooling system is barely adequate to cool an engine with a standard output. A 20% increase (so a decent stage one kit) will be running it hard. Particularly on a 1275cc engine. And that's when the system is in excellent health. Not something that can typically be applied to a vast majority of Minis out there that have had no cooling system servicing for years on end. 
Back to the fuelling and ignition. It is not unreasonable to expect the vendors that sell you the go-faster parts to build your desired spec engine to be able to advise you on both of these. If they can't, I would be somewhat sceptical about handing over your hard earned in exchange for tuning parts to them. Once parts have been bought, you could always try contacting one of the well-known specialist engine builders and ask for their advice, but don't be surprised if they are reluctant to pass on any information. Their stance is likely to be that the folks that flogged you the bits should be able to help in that direction. If they can't they shouldn't be selling such parts. Understand that the standard fuelling and ignition set-up is unlikely to be suitable for engine up-grades that will produce more power. From the fuelling point of view, to get more power you have to burn more fuel. For ignition, the standard advance curve is unlikely to provide sparks in the correct places for optimum burn. For the majority, the fuelling tends to be relatively cheap to sort out initially as it usually only requires a new carb needle at around £9.00 + VAT. Many of the misinformed ask for a needle and jet to suit their new spec engine. The SU range works on a standard sized jet for the carb size. It is only the needle that needs changing. In the vast majority of cases anyway. For ignition it will cost more. As a minimum you can get your existing dizzy re-curved for around £35.00 plus postage to and from your chosen ignition tuning expert. If the dizzy needs a re-build it may cost a few quid more, depending on what needs to be done. A brand new dizzy with transistorised electronic ignition to get away from (the now very troublesome) points and condensers will from around £160 + VAT. Initial settings should be forthcoming with both products. Equipped with suitable fuelling and ignition, we can move on to the oil discussion. 
If there is one very basic piece of knowledge needed as a Mini owner, enthusiast, and user it is that the engine shares it's lubrication oil with the gearbox. As such it should never be considered the same as any other engine. So for all those that get suckered in to buying 'running in oil' because it seems to do what it says on the tin, bare in mind it was developed for engine only applications. Typically running in oil is a much thinner grade oil with fewer anti-wear (anti-friction) additives in it. There are the odd exceptions, and recent testing has shown that Joe Gibbs Driven Break in oil is excellent. Use a decent quality mineral oil that has all the good stuff in it essential for older style push rod engines. That means decent levels of ZDDP or MPZ. I have written about oils and what needs to be in them in the past, so just trust me on this. Modern motor oils generally do not contain any where near sufficient of these additives for our A-series engines. And gearboxes. BMC originally requiring oil that had 1300ppm of this stuff to ensure longevity of performance and minimised wear on the valve train. So either buy an oil that is known to have the required additives in it, such as Joe Gibbs Driven 15W50 Break in mineral oil, or Mini Spares Centres own Mini specific mineral oil, or a decent make mineral oil with a bottle of additive concentrate (such as Torco ZEP or ZDDPlus for non-cat equipped engines, Torco SEP for cat-equipped engines) added to it. There will be those throwing their hands in the air, a-chuffin' and a-puffin' about the mineral oil thing. They will say a good quality semi-synthetic will do the job handsomely. And they would not be incorrect. I have recently started using Millers semi-synthetic to run my engines in. But on the dyno... where I have full control over the rpm and load applied over what time period and level of load. These are simply not achievable running in on the road. 
Having made sure the fuelling and ignition are something like close, the oil is to required specification and poured in to the engine to the relevant level, the cooling system is full and no leaks are apparent, time to go for oil pressure. Remove the spark plugs, make sure the gearbox is in neutral and spin the engine over on the ignition key. Do this in bursts of around 5 seconds at a time so as not to over-heat the starter motor. With oil pressure showing, refit the spark plugs, and start the engine. Get it up to around 2,500 rpm. Using the idle screw on the carb is usually the easiest way of doing this. Break the cam and followers in as detailed earlier. That done, it's time to head out on to the road to run the engine in. This is the hardest part to describe. 
Bearing in mind what I said earlier about the precision and component quality used in building engines now, what we are looking to do is rub in any high spots on adjacent surfaces, in particular piston rings and bore walls. To get the rings to bed in properly all that is really needed is several brief applications of full throttle with the gearbox in a high gear to generate high cylinder pressures. These force the piston rings out against the bore walls. The next part is presupposing the gearbox and final drive ratios have been matched to the camshaft/cylinder head specification. With the car travelling in 3rd gear at around 2,000 to 2,200 rpm, apply full throttle for around 8 to 10 seconds, then ease off smoothly to a light throttle coasting for around 30 seconds or so, then repeat. The light throttle coasting is to dissipate the heat that builds up under the full throttle periods. Ten to fifteen applications of full throttle should get the job done. Of course you need to be aware of any traffic around you so the full throttle applications may be further apart at odd times. After this build up throttle and rpm usage over the next 100 miles. At this point you should be closing in on using full throttle and high rpm for brief periods. At this point, change the oil and filter. During this running in period you need to be paying attention to how the engine feels and sounds. Rattling under load will usually indicate either too lean a fuel mixture or too much ignition advance, if not both. And keep an eye on the coolant temperature gauge! Stop the running in and sort any issues out before continuing to avoid engine component damage. After the first 100 miles the engine should feel smooth and free revving. I would very strongly advise getting the engine set up on a rolling road at this point. Put another 500 miles on the engine then change the oil and filter again.  
Job done. 
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