Glazed bores (and piston rings not bedding)
Discussion
When new rings are fitted into new bores, they have a rough surface at the microscopic scale. The purpose of bedding them in is to burnish both surfaces by rubbing them against each other so both surfaces become polished and fit each other closely. The wear rate of each surface depends on the roughness of the surface it is rubbing against, and the speed and pressure applied to it. So for this to be successful, both surfaces have to wear at similar rates. Given that the piston rings are much smaller than the bores, and the bores are being rubbed by the pistons as well as the rings, and the pressure applied between the piston, bore and rings depends on gas pressure, revs and all sorts, it's amazing that it ever works.
If you fit new rings in glazed bores the rings won't bed in because the bores are too smooth to wear the rings. This is why it's normal to run a mop or glaze buster down the bore to roughen the surface even if the bores aren't being honed.
Similarly, the rings need a certain amount of gas pressure to push them against the bores, so if the engine is only driven gently during the bedding-in process, the bores will be polished smooth by the pistons before the rings have bedded in. And if the engine is allowed to labour you get excessive wear rates on the rings and nothing else beds in.
Edited to add:
When everything has been bedded in, all the components will be glazed. This is normal and desirable. The glazing is only a problem when some of the components are replaced and need to be bedded in - the glazing will prevent that.
If you fit new rings in glazed bores the rings won't bed in because the bores are too smooth to wear the rings. This is why it's normal to run a mop or glaze buster down the bore to roughen the surface even if the bores aren't being honed.
Similarly, the rings need a certain amount of gas pressure to push them against the bores, so if the engine is only driven gently during the bedding-in process, the bores will be polished smooth by the pistons before the rings have bedded in. And if the engine is allowed to labour you get excessive wear rates on the rings and nothing else beds in.
Edited to add:
When everything has been bedded in, all the components will be glazed. This is normal and desirable. The glazing is only a problem when some of the components are replaced and need to be bedded in - the glazing will prevent that.
Edited by GreenV8S on Wednesday 14th June 13:39
Righto thanks Pete, so bedding the rings essentially its wear at a controlled level.
So if the muts nuts of oils (ie a GCSE type oil that perfectly lubricates, never allowing wear to happen, and never breaks down) is used in a fresh engine that has had a glaze buster down it and new rings, the rings will not bed in ever.
In the real world, if a very good oil is used from the outset then the rings wont bed in very quickly, likewise if chip fat is used things will bed in quickly but potentially at the risk of damaging other components.
How do things such as running rich or lean affect things? And is there anyway to try and start from scratch again? Does the legend of Bon Ami have any bearing in reality?
Regards
Iain
So if the muts nuts of oils (ie a GCSE type oil that perfectly lubricates, never allowing wear to happen, and never breaks down) is used in a fresh engine that has had a glaze buster down it and new rings, the rings will not bed in ever.
In the real world, if a very good oil is used from the outset then the rings wont bed in very quickly, likewise if chip fat is used things will bed in quickly but potentially at the risk of damaging other components.
How do things such as running rich or lean affect things? And is there anyway to try and start from scratch again? Does the legend of Bon Ami have any bearing in reality?
Regards
Iain
An engine that runs rich will over fuel and this will wash the oil from the bores and cause premature wear. In some cases this is deliberate as it helps prevent detonation. Mineral oils are often better at coping with the fuel dilation than modern synthetics which I suspect is why TVR specced mineral oil during engine run ins before switching to a good synthetic. It is also why I use a mineral (with regular oil changes) on my engine and judging from its state after the last preventitive rebuild, it is working a treat.
John Eales runs my engine in on the dyno to get everything right before I get it and put it back in the car.
John Eales runs my engine in on the dyno to get everything right before I get it and put it back in the car.
guys - honing has moved on in the recent years, read this link below before you get them re-honed.....the latest buzz word is "plateau honing" whereby the new bore surface is similar to that of a broken-in surface.....therefore no slow driving or eraly oil changes are required.....this process is now common place in OE manufacture and pro and club motorsport
www.babcox.com/editorial/ar/eb40250.htm
You could buy a glaze-breaker for use with a pistol drill, with different stone grades, like 280grit (rough) for the first treatment, then 400 grit (smooth) for the second treatment, they are quite cheap from machine mart, but try to apply some common sense before using it, you want a rough hone, then a smooth hone, then thoroughly clean it all out - I'm told automatic transmission fluid on a cloth is great........ the thinking in the above link could be applied in your garage quite easily
www.babcox.com/editorial/ar/eb40250.htm
You could buy a glaze-breaker for use with a pistol drill, with different stone grades, like 280grit (rough) for the first treatment, then 400 grit (smooth) for the second treatment, they are quite cheap from machine mart, but try to apply some common sense before using it, you want a rough hone, then a smooth hone, then thoroughly clean it all out - I'm told automatic transmission fluid on a cloth is great........ the thinking in the above link could be applied in your garage quite easily
what engine are you talking about in particular?
on freshly rebuilt engines the biggest problems that accelerate bore glazing are letting the car idle and not varying the revs under load enough, and using synthetic oil. in my opinion of course.
every engine i've built has been run in hard, with mineral oil that is changed frequently initially.
typically i'd run it in for the first 50 miles keeping the revs below 5000, but constantly changing ( on a japanese 4 pot ).
then drop the oil, new mineral oil for a further 100 to 200 miles driving fairly hard again keeping the revs constantly varied, with lots of low rev WOT runs progressing to higher rev runs. then comes another oil change and as the majority of the bedding in should have taken place a switch to fully synthetic of an appropriate weight is done. then it gets worked properly and forgotten about.
that method has never failed me yet, and all the engines i've built have lasted and been great i'm pleased to say.
i'm a firm believer in running in engines hard, it's never failed in any engine i've seen built professionally.
this was hammered home when a friend of mine with a tuning company built his engine, gave it hell for 50 miles, then got it on a dyno
on freshly rebuilt engines the biggest problems that accelerate bore glazing are letting the car idle and not varying the revs under load enough, and using synthetic oil. in my opinion of course.
every engine i've built has been run in hard, with mineral oil that is changed frequently initially.
typically i'd run it in for the first 50 miles keeping the revs below 5000, but constantly changing ( on a japanese 4 pot ).
then drop the oil, new mineral oil for a further 100 to 200 miles driving fairly hard again keeping the revs constantly varied, with lots of low rev WOT runs progressing to higher rev runs. then comes another oil change and as the majority of the bedding in should have taken place a switch to fully synthetic of an appropriate weight is done. then it gets worked properly and forgotten about.
that method has never failed me yet, and all the engines i've built have lasted and been great i'm pleased to say.
i'm a firm believer in running in engines hard, it's never failed in any engine i've seen built professionally.
this was hammered home when a friend of mine with a tuning company built his engine, gave it hell for 50 miles, then got it on a dyno
Edited by MeLLoN Stu on Friday 16th June 02:04
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