Surface Roughness (Ra) and Why I Think Its All Wrong.. Learn Me Right!

Automotive
1

As I am reassembling my under-hood situation in the Mazda, my multi layer steel head gasket replacement has me worried. And I think I have just cause. Let me explain.

I used Felpro PermaTorqueMLS gasket which has steel and sealants in its construction. My heads are aluminum and I think my block is aluminum alloy as well but may be iron. I really am that dumb when it comes to metal.

But where I am less dumb is measurement and mathematics. And this is where my question comes from. You see, felpro specifies that max 30 Ra is required for most MLS applications. But that their PermaTorqueMLS line can handle up to 60. The problem is that they did not specify units. In fact, no one in automotive industry is taking much care to properly specify their units when discussing this Ra requirements for head gaskets of MLS variety. A few have mentioned microrinches (otherwise known as mils), and I am under the impression that this is the requirement felpro is suggesting.

But there is a huge problem with .03 inch or 30 mil Ra requirement. It is not smooth at all. This much average roughness is quite rough, actually. Andj to understand why, lets think about what .03 inches of average roughness means.

One way to measure surface roughness is to use a profilometer. This device uses a sharp diamond tip, and slowly drags itself along the surface while measuring and recording the height the diamond tip reads. To compute Ra, the tool must take all its sampled measurements and average them. Then it would use the average to determine the deviation of each sampled measurement (taken as it drug across the surface) from that average. Since the deviations from this average surface height are both positive and negaive, the tool must then take absolute value. From this, the tool now has a set of positive deviations from the average for every measurement point (or sample) taken. To get the overall average, the tool must then average all of these deviations together.

So a .03 mil roughness surface would actually have .06 roughness since the absolute values had to be taken. And depending on shape of these deviations, likely peak to peak is double that, so now we have .12 inches peak to peak roughness. Most handheld metal files I have used are much smoother than that. The .12 inch peak to peak surface would for sure never be described as ā€˜mirror-likeā€™ which is what I see the gurus calling a .03 Ra surface.

Where did I go wrong?

Secondly, I donā€™t think Ra can be used to completely characterize the surface requirement needed to properly seal an MLS (multilayer steel) head gasket. The situation is more complex than that. You see, a single surface discontinuity of .06 inches (like a step function) would result in .03 Ra if it were perfectly smooth otherwise. And I doubt a surface like this would do a great job of holding the coolant, or oil, or pressurized hot gasses in the cylinders. There are a lot of other surface imperfection shapes that would be massively problematic, but could still pass the .03 Ra requirement. So an additional surface parameter must be specified and required. Maybe even two or more additional surface specs are required. Has anyone seen a gasket maker or OEM equipment manufacturer nail this down properly?

To summarize my questions:

  1. What is the correct unit the automotive industry intends to specify when discussion Ra, as mils makes no sense at 30 being mirror-like?
  2. Has anyone seen surface roughness requirement called out in such a way as to appropriately restrict the sorts of surfaces that would work in an MLS or other sort of gasket application?
2

Ra is used a lot in US aerospace, automotive probably uses whatever ISO uses.

Hereā€™s what we called the ā€œFinger Testā€
Ra 125 when you run you finger over it should be able to catch some but not really take much to force to get past

Ra 64 is a pretty good finish, say half a record groove in height

Ra 30 is getting down to where if specified we had to use an instrument to gage it because ā€œfinger testā€ not reliable.

Also, note: when we did Finger test we had a calibrated panel at different Raā€™s that are produced by various processes. Youā€™d run you finger nail over panel then compare to part. You could only report ā€œBetter than Ra XXā€ not the exact amount. Most machined parts non-mating surfaces were Ra 125, mating surfaces 64, 30 and below were usually on shafts or certain holes, if on a part surface other than shaft/hole usually a flatness and parallelism requirement went along with it. Always the case at 16 or lower which are usually ground/polished surfaces. Mirror like finished.

Here are some examples, one I used was similar to this: Surface Roughness, various processes, made of metal $125

Hereā€™s one probably good enough for what you need Plastic Surface roughness $10

Hope this helps, the $10 buck one may be plenty good for you purposes.

3

microinch (Āµin) is .000000x inch (millionth of an inch), rather than .00x inch (hundredth of an inch).
I donā€™t know how much that helps bring some of this into focus, but it reduces the peak-to-valley by an order or so.
The Ra is the traditional method of measuring finish, and in true vernacular adherence, units are unnecessary. When working with anything automotive in the USA, (except metric fasteners, but especially anything relating to machining) measurements are always Standard. So the Ra as stated for anything produced for USA use by a USA company shall likely be thus.

For what itā€™s worth, ā€œthe industryā€ admits Ra is not really up to the task of modern gasketry, and has ā€œadvancedā€ to Rz, and in some cases RMS (Rq). Hereā€™s a writeup (slanted toward stainless steel, but it doesnā€™t matter).
But when youā€™re a part of an industry as ingrained as aftermarket auto parts, changes in phrasing donā€™t happen quickly.

Iā€™m not sure if I really addressed your questions, but I think I have addressed the absolute value of your concerns, on the average. :slight_smile:

EDIT to add: David (@Photomancer) makes a solid point, too, that people who work with this stuff, and teach it to others, operate on ā€œfeelā€ frequently, and this can be taught or learned using comparators not unlike those he suggests.
None of this make it especially easy for us acorn dodgers, but thatā€™s just fine with the industry, for the most part, because then, if something fails, they can blame us, and avoid spending warranty $$.

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4

You make a good point about microinches. I guess I was thinking milliinches, which should be .001 inches. One of the docs I referenced does call it microinches, which means I may be in trouble. My finishes will be nowhere near .00003 inches average roughness.

On the other hand, the only other doc I found specifying what the units were said thousandths of inches, in which case my argument applies. It must be microinches, with one careless author mistaking it for mils, and the rest unsure so just saying 30 of whatever it is that high-dollar, newfangled machine gives us.

I used 220 grit sanding screen to do my finish on the heads. The surface on the block is that of Mazda, plus whatever damage I may have done with plastic ice scraper and paper towels. Mazda knows I will be using a MLS gasket, and their surface is rougher than the heads are. That is the only reassurance I have right now as I reassemble things. And that one goes out the window if the standard procedure for replacing with new short block includes surfacing the new short block. Does it?

There is also this consideration: I tend to think that certain styles of roughness within reason would be beneficial to seal. This would mean that peaks and valleys do exist and create points of heightened pressure on the gasket. If these points pressed against the gasket with appropriate frequency and/or were clustery ridgelike in a helpful way, I would think this would aid fluid blocking capability. Troughs, like scratches, would be all sorts of bad news.

@jast I think the sky may indeed be fallingā€¦

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5

This is worth checking out.

I note that 1 micrometer = 40 microinches. So which one is the standard in automotive?

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6

There needs to be a (relatively) cheap optical instrument for automotive. Say < $1000, small, inferometer based, gives you all sorts of roughness info including waviness,Ra, peak to peak, etc. ā€¦more good business ideasā€¦

I hope my gaskets seal. Iā€™m really worried at this point. The head bolts are already in. It either seals or it doesnā€™t, and it is for sure worth a try, especially given that felpros are known to be forgiving of roughnesses exceeding requirement. Guess I gotta finish putting air intake on and crank it to find outā€¦ getting closeā€¦

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7

To shorten my response: inches (standard) is the standard.

I doubt it.[quote=ā€œsemaphore1999, post:4, topic:14206ā€]
And that one goes out the window if the standard procedure for replacing with new short block includes surfacing the new short block. Does it?
[/quote]

It will not include resurfacing, but it will include inspecting. And in all likelihood, the inspection will show that a resurface is necessitated.
In short: itā€™s VERY likely it was resurfaced to an appropriate texture.
EDIT: This should probably say: theyā€™ll have sold it with an appropriate surface finish for the recipient to have cleaned it off and put it into use without further surfacing work necessary.

Gaskets seal in both directions, so troughs=valleys in this case. They are the same. And their absolute value should be about the same, for success. Further, in my opinion, it is important where they begin and end. A ā€œmarkā€ traveling from a sealing surface to a sealing surface, doesnā€™t really matter how deep it is. A ā€œmarkā€ traveling from a water jacket hole to the outside of the block, after a certain depth, will be problematic.[quote=ā€œsemaphore1999, post:6, topic:14206ā€]
There needs to be a (relatively) cheap optical instrument for automotive. Say < $1000, small, inferometer based, gives you all sorts of roughness info including waviness,Ra, peak to peak, etc. ā€¦more good business ideasā€¦
[/quote]

if you can attach it to a smart phone, youā€™ll likely have a runaway seller. Iā€™ll settle for 10% (ROFL)

Iā€™m betting on it being OK. No sense worrying. Just get on with it, and be pleasantly surprised when you have no problems :slight_smile: (thatā€™s funnier the better you know me)

8

Your response, and the following document make me feel a whole lot better. And notice that the microinches are in range for the 220 grit sanding screen that I used.

9

Re: micro inches, in that range always with an electronic gage or maybe an optical flat, depending on feature. Anything under .00001" is going to be considered a polished/honed surface.

Re Car: Iā€™d be more concerned with flatness and squareness of mating surface than difference in surface roughness, Iā€™m sure you are okay if it is the way if came from factory and wasnā€™t damaged during cleaning. I seriously doubt it is anywhere near Ra 125, 64 or less not counting pits (which may be a real problem if near edge of cylinder wall)

But flat and square is what will ensure the best chance the gasket seats properly over the surface when properly torqued down (very very important), also aligned properly before torquing. Warpage and twist is usually what causes a gasket to blow due to the metal expanding and contracting, especially if block and head are different materials.

And yes, I/we did go through training and were tested, it is surprising how many other tests are tactile, the human hand is really sensititive and if trained for what to feel for itā€™s amazing at detecting, not so much measuring. But after the first couple thousand parts you get pretty good at between Ra 32 and 125.

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10

The block is new (<400 miles) and the heads I sanded until all evidence of rings at cylinders disappeared. Once I cleaned the heads well, and the stone as well, I was not able to put the 2 mil feeler under them anywhere. So from this I infer that the flat sanding method is a good one, just like the dude in video said. He said he does it all day for a living, and then proceeded to witness to the audience, saving souls. This told me he truly believed in the method, as doubtful a man with that sort of religious conviction would lie to the multitudes.

We will know for sure soon. I just found a vacuum issue working on the air intake, so may me a minute to get things all properly installed and all vacuum leaks sealed. Thanks for words of encouragement.

11

When I last replaced a head gasket, I was under the tutelage of an ā€œold handā€ who had me use something like this:

https://www.amazon.com/gp/aw/d/B003Y3BMAW/ref=mp_s_a_1_1?ie=UTF8&qid=1479047631&sr=8-1&pi=SL75_QL70&keywords=Bora+sharpening

Using the fine side with little 3-in-1 as lubricant, I knocked down the surface finish as evenly as possible, cleaned, and then put it back together. Ran just fine for another 50k miles until I sold it.

12

Dead link.,care to repost or edit? I would love to see what you gotā€¦ :slight_smile: