To Hone a rusty cylinder bore or not

[Thruster87]

Talking about engines that have been standing around for a long period of time, the discussion followed on to the need to hone or not [bore 4041 rings cast iron as per the Jabiru engines] after a top end inspection,especially on low time engines. Found the following WEB debate intersting :

 

I was once in attendance at an Auto Mechanics course at Technical College, dancing thru the hoops for a degree in Industrial Ed. One topic we kicked to death in there was the subject of honing and ridge reaming cylinders. It all came about when the instructor greeted us one morning with the news that you should *never* hone cylinders except to finish a bore job, and that it's *pointless* to ream ridges, unless you need to get the piston out. (Cars)

 

What an uproar! Most of us had had a fair amount of experience with engines, and we were pretty incensed. You *have* to "deglaze," or the rings will never seat! You *have* to deridge, or the new rings will hit the ridge and break on the first stroke! Everybody knows that!

 

Well, our instructor, who was a pretty savvy guy, knocked the wind out of the sails of the more scientifically inclined among us with one move: he handed out some reprints of actual SAE test results involving deglazing/not deglazing rering jobs, and ditto for ridge reams. It seemed that the compression test figures after 10000 miles were *better* by far for the unhoned engines. Also the increases in piston clearance were less, as was the bore wear, and - get this - they also showed *significantly* less bearing wear.

 

How come? Well, it seems that when you scuff up a perfectly good smooth cylinder with a rude sort of carborundum device, you are in fact only putting scratches on it, giving us a new, larger diameter at their base - sort of like root and pitch diameters in a nut, say. And when you put it back together, the new rings and this rough surface proceed to grind each other down until smoothness is re-attained. By this time, needless to say, you have larger ring gaps, due to more ring wear, and more piston clearance, since the cylinder has been taken down now to the base diameter of the honing scratches. And the valuable metal so removed has been converted into grinding compound circulating in your oil, so you have also reduced the size of your piston, and contributed significantly to wear of all the other parts as well, from rocker bearings to cam bearings to timing chain rollers to rods and mains.

 

Proven. Scientifically demonstrated in an actual SAE sponsored research project. But you gotta *deglaze* or the rings will never seat, correct? Well, yes and no, Chet. Although it is true that *chrome* rings are so hard and smooth that it can take from a long time to forever for the cylinder to wear them to perfect fit, this is *not* the case for Cast Iron rings. In fact, that's the secret. Chrome rings only in rough (honed) bores. And hone bores only when you have to, ie as the final "finish" of a bore job. Since the boring bar tool in effect runs a screw thread down the bore in its journey toward the land of largerness, it is necessary to do a final "polish" with a somewhat cruder tool - the hone. This is the *only* reason to put a hone in a cylinder. And when it's done, the machinist has purposely left some extra meat in there for the hone to finish to final diameter. And the final diameter is a tad small as well, in anticipation of the initially rapid wear that is inevitable as the new rings and the scratchy bore get acquainted. This is also the reason for very careful cleaning of said bores before assembly, and for very frequent oil and filter changes, until the motor has stopped manufacturing grinding compound during the

 

break-in process.

 

 

[Bruce Tuncks]

Thanks Thruster, great stuff. I reckon you would also like to read about the Waddington Effect, where it was proven that more maintenance can do more harm than good.

 

Its about time we got more scientific about what is really proven by proper evidence to be the best practice. Our intuition has been shown to not be reliable, as your story shows.

 

....Bruce

 

 

[facthunter]

Its got a lot to do with maintaining an oil film. A polished bore will not do this. A firm I worked for introduced a "mirror finish" rebore and virtually every motor scuffed the pistons during run in. This technique was abandoned very quickly.

 

Why would the cylinders wear down to the stage where the new size becomes the base of the hone scratches, which would be an extremely small percentage of the area of the metal. If you look at most cylinder surfaces after quite a while, ( even well over 100,000 kms in a clean running engine) , most of the cylinder will still show these scratches except for the high wear area near the top of the bore.

 

If the rings do not seat in and seal very quickly the blast of high pressure gas will heat up and distort the ring and it will warp and lose tension. Aircraft engines do not have the opportunity to run in gently as the first take-off will be at full power. If the correct clearances are used the motor will survive this. Most new rings have a rough surface on the part that contacts the bore. This allows an accelerated "bed in" and seal of the rings essential for them to do their job. A well machined "new" cylinder will get good compression within minutes of starting it up.

 

The hone process is only to "deglaze" the bore except where it is used to achieve the final size and acceptable smoothness, after the boring bar, for the engine to initially operate. It should not be used to true it, if it is worn, where the correct procedure is to resize it (if permitted) by boring or grinding, and fit oversize pistons with the correct clearance, or replace the assembly..

 

Hone "grit" should be removed from the parts carefully, as it will embed in the softer aluminium and lap the cylinder. This can promote glazing which is not desireable, because of the oil retention problem. Dust will do the same thing.

 

Modern oil filters filter particles to a few microns where they are not likely to cause much wear, if they are working properly.

 

Some of the assumptions ( and that is what they are) are not really plausible ( as far as I'm concerned) and I have been working on engines since I was 11 years old. Nev

 

 

[Thruster87]

Continuing with the debate [lifted from the WEB]

 

1) There IS no "glaze"! That's a polished surface. Call it

 

"final machining" done by the last set of rings. It's a

 

wonderfully smooth surface of a certain size.

 

Microscopically, it looks like this:

 

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2) If you run a hone through it, lightly or heavily, you rough

 

it up, and create low spots. It then looks like this:

 

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Until it "runs in" to a new bigger size that looks like

 

this:

 

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3) Your new rings, and your piston, must get the surface

 

smooth again, by wearing down the high spots (what is

 

left of the old polished surface - NOT a "glaze" -to the

 

level of the low spots. This results in

 

a) accelerated wear of all engine parts, due to the

 

"grinding compound" effect of the metal being worn

 

off as your cyinder is ground to the oversize of the

 

bottom of the wear spots - as well as the GCE of the

 

abrasive residue which will inevitably be left in the

 

low spots - at the bottom of the grooves created by

 

the unnecessary hone.

 

b) A final (after break-in) size of the cylinder being

 

larger, the piston being smaller, and the ring gaps

 

being larger once this unnecessary abuse of your

 

surfaces has been completed.

 

I've told you that the reason I stopped honing polished bores, and

 

using cast-iron rings on rering jobs was because the SAE research

 

report found that identical engines treated both ways, and then

 

run for a fairly long interval after reassembly showed that:

 

a) The "deglazed engines" showed lower compression and

 

significantly greater wear upon being dissassembled and

 

examined than those which were not "deglazed."

 

b) The "deglazed" engines oil consumption was higher during

 

the so-called "breakin" period than that of the others, and

 

continued to be higher throughout the virtual service life.

 

(These were test bed engines.)

 

c) Periodic compression checks revealed that at all stages, the

 

compression on the deglazes engines was lower.

 

Furthermore, I've also said that I've built many engines since that

 

day in 1969, and I've never had a problem with bad ring sealing, poor

 

compression, or oil burning on any engine I've done since that time -

 

although I did have some troubles of this sort with engines I DID

 

"deglaze" before that.

 

And finally: there is no such thing as "deglazing." Honing is a

 

final process used to take the boring bar marks down to a smoother

 

(smoother, not smooth) surface after machining. The machinist

 

purposely bores slightly oversize, to allow for the metal the hone is

 

going to take out, and hones to a slight oversize, to allow for the

 

metal that is inevitably going to be lost as the piston and rings

 

take out the high spots left by the hone.

 

 

[Tomo]

Cross hatch hone is what you want... just 'honing' a bore any old style isn't the idea.

 

 

[brilin_air]

Cross hatch hone is what you want... just 'honing' a bore any old style isn't the idea.

Also with the correct crosshatch it holds a very fine film of oil to keep the piston to bore lubricated to prevent excessive heat that will destroy pistons etc

Brian

 

 

[Thirsty]

So who can explain how an SAE research team discovered that deglazing or honing is not a good idea? Maybe the principles are different for aircraft engines? I've always thought honing was a good idea when putting new rings into an old cylinder and I've always believed the cross hatching produced when honing properly would hold an oil film to help with lubricating the rings as they travelled up and down the bore.

 

BUT

 

we can't just refute the SAE report I reckon. Maybe we've all been doing it wrong all these years.

 

 

[facthunter]

You can't just take some research and apply it across everything either...You're not addressing the whole picture. An engine that uses no or very little oil is running the cylinder dry. ( At least above the rings). The rings are scraping all of it off, otherwise you would be using oil. (obviously, ). You should not regard some oil consumption as being bad. ( environmentalists do, but they are looking at it from a different angle).

 

Cast iron bores tend to be porous and are not so much of a problem. Most aero engines are STEEL which is not so porous and unless some light hone scratches are present to hold oil (in minute quantities) scuffing is more likely to occurr . Once the contact surface of a ring is scuffed blowby increases and the ring should be replaced. It can overheat and lose it's tension.

 

Automotive usage is not much like aero engine usage. Car engines are literally loafing most of the time, while aero engines are only running below about 75% power on descents, so they are really on fairly high load most of the time. Their specific horsepower, BHP per CU in, or Kw/ litre is lower than what you would expect in auto engines. This is done for reliability Every real aero engine I have been flown and worked with, uses oil, and in some cases quite a lot. Attempts to "modernise" aero engines by putting modern style rings and reduce oil consumption right back, hasn't resulted in an increase in reliability, either. Nev

 

 

[turboplanner]

I like to hone the rust out of the bore, the same as Tomo, leaving a cross hatch pattern. The cross hatch is accidental, being the deepest scratches from dust particles etc.

 

On the other hand I'm happy to attack the piston with a hack saw and rasp, as long at the final weight and measurements balance.

 

The end result is that the rings do the work on smoothing the bore and soon maximise gas retention, and the pistons are just held in place by resting against the bore bore what's lefty of their surfaces, reducing friction, so diverting that power to the wheels.

 

The grooves in the pistons carry oil up and down the bores, and since the piston is not the wearing surface, but theoretically just hangs there away from the bore most of the time, there are no rapid wear issues.

 

 

[Tomo]

Just for interest sake, and or those wondering what we're on about... this is the cross hatch in a bore of a 436 International engine I rebuilt today.

 

 

 

Believe it or not, this bore has done 11,000 hours, and look how good it is. Pity about water corrosion on the outside of the liner otherwise it would have lasted a lot longer.

 

 

 

 

 

[Tomo]

Not particularly relevant either, but since I was working on this today I took some quick shots with my phone for other to see what can happen... Hino truck engine.

 

This is a pretty badly glazed bore

 

 

Not real pretty in this one!

 

 

And the consequential damage...