Following on from Chips very good engine building discussion, here is the start of the “running in” technical discussion. Thankyou for volunteering me chip. I’ve done my best. I hope it makes sense.


With a modern engine the running in process is all about piston ring sealing. Not running in bearings or bottom ends. In theory big end and main bearings should always be running on high-pressure oil (apart from maybe start-up) not surfaces running together so shouldn’t need running in.

It is however critical for engine performance that the piston rings are bedded to the cylinder walls.

A common mis-conception is that the sprung rings push themselves out onto the bore to create a seal. This is wrong. Combustion pressures would easily overcome the springyness (word?) of the rings!

The only ring that uses its own pressure is the oil control ring that exerts light pressure on the cylinder
walls to scrape excess oil away.

It is the cylinder pressure that gets behind the ring and pushes it out onto the cylinder wall to create a seal.

When you run a fresh engine the bores have a slightly abrasive hone pattern that is used to help the rings bed in.

If you drive around for 1000 miles at 1500 rpm with no load you will use all the roughness of the hone with out the rings really being able to use it to bed in.

What you need is some decent cylinder pressures in the first few miles to push the piston rings out onto the freshly honed bore and enable them to run in together. This means putting the engine under load.

Basically if the rings are not forced against the walls soon enough, they will use up all the roughness before they seat. This can is sometimes referred to as glazing the bores.

This is why if you are replacing the rings on a used engine you should always get the cylinders re honed. Some people refer to it as breaking the glaze.

Years ago at a bike mag I used to read (Performance Bikes) the late great John Robinson did a test with two early SRAD GSXR750s. He thrashed one from the crate and ran one in for 1000 miles as with no load. They then put them on the dyno. The thrashed bike made 124bhp, several more than the run in one!
They then compression checked, stripped them down, examined and concluded that the extra power must have come from the better cylinder sealing.

Another mis-conception is that an engine has to go through a certain amount of heat cycles before it is run in. I think people believe that parts get heat treated as they are run??? Heat treating of engine parts is done at a temperature far in excess of that which will be reached by a running engine. Well at least one that has oil in it and is not welding itself together!

What are the advantages of better sealing rings I hear you ask.
1. No wasted cylinder pressure
2. Which equals more power
3. Less blow by of rings
4. Which equals less contamination of oil with combustion gases
5. Which equals better performing oil and an engine that lasts longer.

So we have established that what you need to do to run your rings in is put the engine under load and create greater cylinder pressures.

Ok, bearing that in mind, this is how I run my engines in, whether it be a CR125 or a 2ltr zetec.
Insert “at your own risk blah blah blah” clause of your choice!

1) Warm engine up (v.important)
2) Do a couple or three mile lap of my local lanes at 1/3 throttle with plenty of accel / decel
3) Another lap of lanes with ½ throttle, plenty of accel / decel
4) As above ¾ throttle
5) A couple of full throttle blasts.
6) Change oil and filter
7) Enjoy!

I always pull in and quickly check for leaks etc. in between laps.
Always keep an eye on the temp gauge and oil pressure if you can.
Change your oil and filter as frequently as you can afford. You can’t hurt an engine by changing the oil too much!

This how I run in my engines. If you are having an engine built by a tuner, follow their directions for run in or you may invalidate any warranty / guarantees they may give.

I’m sure there will be plenty of discussion on this topic, however it would be nice as a follow up to have some info / input from the oilman as to the technical reasons you should use different oils during break in