Deliberate piston chamber scoring

[Tony]

Deliberate scoring of the combustion chamber and respective cams has been said to assist the oil-ways so lubricating the power-house. Is this true?.... If yes then i am troubled by this! hence my questions..

 

1: What happens to the compression

2: Wouldn't the engine smoke like mad

3: Isn't this action accelerating wear to the pistons/rings/liners

[Sam@TDi]

Can you elaberate a little on what you mean by scoring?

 

We need to "hone" the cylinder bores so that the surface finish retains oil, but that surface finish is far far removed from a scored surface, we're talking a regular pattern only microns deep

 

We'd actually look for a very smooth surface finish in the combustion chamber it'self to promote fast flame travel and discourage carbon build up and so any resulting hot spots.

[Tony]

Can you elaberate a little on what you mean by scoring?

 

We need to "hone" the cylinder bores so that the surface finish retains oil, but that surface finish is far far removed from a scored surface, we're talking a regular pattern only microns deep

 

We'd actually look for a very smooth surface finish in the combustion chamber it'self to promote fast flame travel and discourage carbon build up and so any resulting hot spots.

Remember i am a bit of a numpty in the engine department It was on American hot rod.. A specialist dismantled a new V8 and honed all the cylinders and the liners the cam shaft spins on. After you could visibly see the score marks on the metal.

[Sam@TDi]

Well I can't really tell you exactly what that guy was doing, but it is normal engine building practice to apply different types of surface finish to different components, mostly to control or manipulate the speed or severaty of the bedding in proceedure between metal to metal surfaces.

 

Too aggresive with the surface finsh and you reduce component life, too gentle and it's possable for surfaces to "glaze" which is to form a thin ultra hard shell which ultimately won't wear on a microscopic level and therefore will never seal properly.

 

It's common practice when building short life "maximum effort" engines for motorsport to apply very aggresive surface finish's to cylinder bores and alike, this has the effect of reducing the engine running-in time to only a few laps rather than 1000 or so miles required for a normal road engine. The trade off of course is that the involved componants such as piston rings lose maybe a third of there life span during those first "running-in laps" and so will be up for replacement sooner.

[jon]

I wish I had the time and money to rebuild my 3sgte after every trackday. With the solid bottom end you could have some real fun

[TDIPLC]

When I was a boy, when we built the engines for Noahs Ark, we used to machine horizontal grooves in the piston skirt. The theory behind this was to provide an oil retention groove which could help reduce piston scuffing.

 

That sort of operation is no longer required now that the manufacturing tolerances and designs of the pistons and cylinder blocks have greatly improved

[Tango]

Quite a few years ago I was being shown around the Delapena facility in Cheltenham. They manufacture honing machines, honing 'stones', and cutting fluids. During the course of discussion mention was made of a process first introduced for Rolls Royce which used specific stroking speeds and feeds for three grades of honing stone. The finishing stones provided a very smooth bearing surface for the piston rings, but with an underlying cross hatch pattern made by the first and second roughing stones that retained oil for efficient lubrication and cooling.

 

Sounds something like what you described Tony, but we're only talking in (fractions of) microns of surface finish, although the honing finish described would look 'scored' to the naked eye. A cross section of the surface finish under a microscope would look like a series of peaks and valleys, but with the peaks flattened off (it's known as a plateaued surface). This finishing technique has the ability to control the surface peak characteristics separately for the valley characteristics, such that the peak and valley distributions can be controlled to allow substantial bearing load capabilities (broad, flat areas) with well-defined lubrication, debris collecting valleys. The technique extends predictable engine life and improves the sealing of the piston ring to the bore.

Looks like this if viewed (not cross section obviously), and second image is a trace of the surface finish ..your 'score' marks are around 1- 3 microns deep, the bearing surface variation is only around 0.3 micron or less.

 

[Tony]

Thank you for your insight Tango. I think you have hit the nail on the head with your reply. Finally i have a reason to why any engine designer would score the chambers.

[w481ggc]

in the old days we used to hand scrap and that would leave a pattern . it wasnt cylinders , it was lathe beds and sliding keyways and the like , but still to aid sliding surfaces .

[Tony]

Confused again I think this example is more than 1- 3 microns deep?

 

[Sam@TDi]

That looks like an aggressive hone, the only reason you would do that deliberately would be if you wanted reduce running in time i.e a racing engine that might only have the matter of a few laps in order for it to bed in.

[Bazza]

That looks like an aggressive hone, the only reason you would do that deliberately would be if you wanted reduce running in time i.e a racing engine that might only have the matter of a few laps in order for it to bed in.

 

how would that affect the longitivity of the engine ?

[Sagitar]

Confused again I think this example is more than 1- 3 microns deep?

 

 

I certainly would not advise anyone to score bores unless they know precisely what they are doing. We are talking tribology and it's a complex subject. As engine speeds have risen there has been increasing incentive to try to reduce friction levels inside the cylinder. There are significant efficiency gains to be had so it is very worthwhile. A lot of empirical work has been done, sometimes with inconclusive (and even contradictory) results, but there does seem to be benefit to be gained by the modification of bore/liner surfaces.

 

I am a bit concerned about the use of the word "score". Looking at the readout from a specialist mensuration device such as a Talysurf may give the impression of a coarse surface, but the "micron" that has been mentioned is only one thousandth of a millimetre so it is hardly a trench. The layman may see a groove as letting lubricating fluid in, but of course it can equally let the fluid out. When two lubricated surfaces move over each other, they remain separated only for as long as the unit pressure in the lubricating fluid is higher than the unit pressure generated by the load on the surfaces. Some plain bearings may be pressure lubricated, others may generate fluid film pressure as a result of the pumping action caused by the movement of the surfaces past each other. This effect will depend upon the relative speed of the surfaces past each other, their attitude relative to each other and their nature. But it will also depend upon the nature and viscosity of the lubricating fluid.

 

There is likely to be an ideal surface finish that will retain sufficient lubricating fluid to get the pumping process started but will not have such large voids that the lubricating fluid will leak out through them when the pumping pressure rises. Finding that ideal, especially in an engine subject to major changes in operating load and speed may be difficult.

 

I'm probably boring (pun intended) everyone to death by now but I hope I have said enough to support my first comment and put off anyone who thinks he may improve his engine by putting some wet and dry down the bores.

[jon]

I'm probably boring (pun intended) everyone to death by now but I hope I have said enough to support my first comment and put off anyone who thinks he may improve his engine by putting some wet and dry down the bores.

 

On the contrary, I always take the time to double read your comments and I thank you for taking the time out to write such detailed replies. You're always very insightful.

[Tony]

I'm probably boring (pun intended) everyone to death by now but I hope I have said enough to support my first comment and put off anyone who thinks he may improve his engine by putting some wet and dry down the bores.

 

On the contrary, I always take the time to double read your comments and I thank you for taking the time out to write such detailed replies. You're always very insightful.

 

I have to thank everyone for taking time to reply on this topic... I think we can understand why a engine numpty like me was so confused with the scoring.... It's an odd practice.....very odd indeed!

[hms]

Tony,

this may be of interest.

 

http://www.autospeed.com/cms/A_109105/article.html

 

h

[Tony]

Tony,

this may be of interest.

 

http://www.autospeed.com/cms/A_109105/article.html

 

h

 

That was interesting....... Cheers hms

[TDIPLC]

I'm probably boring (pun intended) everyone to death by now but I hope I have said enough to support my first comment and put off anyone who thinks he may improve his engine by putting some wet and dry down the bores.

 

On the contrary, I always take the time to double read your comments and I thank you for taking the time out to write such detailed replies. You're always very insightful.

 

Agreed. Sagitar is a very wise an valuable member of WIM