Engine Paint and Painting

I've read a bit here about the Commando engines being painted silver after a certain point in their production. For the record, my frame tag says 7/74 and it was painted, as was the transmission. Has anyone happened to find a close match for the factory silver? I am going to pull the top end so that I can repaint the barrels and while I'm there, I'll service the head, hone the bores and put in fresh rings, so long as nothing is out of spec. Can anyone see any compelling reason to go deeper into a running 9k mile engine or suggest anything else to address while I have the top off?

I'm not an early Commando expert at colors and such but I do know it was the earlier ones that Had the silver barrels. Your '74 would have had black painted barrels and a natural finish transmission. The original black paint is very hard so if you do go back to black a light bead blast on the silver someone has painted it may get rid of it and expose a good deal of the original hard baked finish. I would not try to remove this, but just high temp paint over it.

Black radiates about 3 times the heat as white or silver. Also, avoid tumble polishing the barrels. The surface roughness breaks up the boundary layer (the thin layer of non-moving, or slow moving air) near the fin surface, and aids heat transfer. Don't worry if you have already polished them....you will not overheat, but will run slightly, but not significantly hotter. Any racers who are pushing the limits should not polish, if you have, and want to increase cooling, consider blasting off the paint, mixing some paint with aluminum powder, and brush painting it on.

I would not recommend powder coating the barrels. Powder coating relies on electro-static attraction of the powder to part, and I do not believe the fin separation is condusive to getting uniform paint thickness.

Slick, I'm not arguing about this one way or the other, really, but can someone explain to me why color has anything to do with getting rid of internal heat? :?:

boostedsuby said:

I've read a bit here about the Commando engines being painted silver after a certain point in their production. For the record, my frame tag says 7/74 and it was painted, as was the transmission. Has anyone happened to find a close match for the factory silver? I am going to pull the top end so that I can repaint the barrels and while I'm there, I'll service the head, hone the bores and put in fresh rings, so long as nothing is out of spec. Can anyone see any compelling reason to go deeper into a running 9k mile engine or suggest anything else to address while I have the top off?

Click to expand...

You're referring to the engine and gearbox cases rather than cylinders. My '74 JPN also had painted cases and I used Hi Temp Aluminum color in a spray can. Spray it on then rub it with a cloth before it dries. It dulls the finish and then resembles the original.

Biscuit said:

Slick, I'm not arguing about this one way or the other, really, but can someone explain to me why color has anything to do with getting rid of internal heat? :?:

Click to expand...

Stefan's Law (also called Stefan-Boltzman) says for a body warmer than its surroundings, radiative heat emission Qr = s T(4th power). Stefan's constant of proportionality s = 1.0 for a perfect black body radiator. Nothing is perfect, and s = about 0.9 for a black painted body, and about 0.3 for white or silver. These are experimentally determined values. Thus, my statement that black radiates about 3 times more (0.9 / 0.3 = 3).

Stefan's Law works both ways: eg for a body cooler than the source of radiative energy impinging on it, the body will absorb heat according to the equation given above. That is why black cars are hotter in the sunshine than white ones. Have a light colored roof on your house if you want to reduce air conditioning energy cost, a dark one if you live in cold climes and want the sun to warm your house.

All the above pertains only to RADIATIVE heat loss by an engine. Convection by moving air accounts for most of an air cooled engine's cooling requirements. I knew a guy in college who painted his cylinders gold and over-heated. Why push your luck? Or as Henry Ford once said "You can have any color you like, as long as you like black."

Mine is 2\74 engine cases and gearbox painted silver from factory.
You might want to just vapor blast them.

Here's the thing Slick, I can understand the black vs. white car in the sunshine example, but I can't get my head around heat radiating more or less because of color. If I'm getting this right, a black container, filled with hot water and covered will lose it's heat faster than the same container in white?..... At night?

Biscuit said:

Here's the thing Slick, I can understand the black vs. white car in the sunshine example, but I can't get my head around heat radiating more or less because of color. If I'm getting this right, a black container, filled with hot water and covered will lose it's heat faster than the same container in white?..... At night?

Click to expand...

Absolutely....if the container is hotter than its surroundings, black will lose heat faster than white. If the surroundings are hotter than the container, black will get hotter faster than white. This is Stefan's law working both ways. This is true at night as well as day...however in the daytime, the temperature of the "surroundings" is not simply the local air temperature, but the temperature of the source of impinging or absorbing radiation (the "sky" temperature). In a windowless room at night, the source of impinging or absorbing radiation would be the temperature of the walls, ceiling, etc. By impinging, I imply radiative energy is imparted to the body (surroundings hotter than body), and by absorbing I imply radiative energy is absorbed from the body (surroundings cooler than body).

Your black/white container example is a good one. Do you dabble with electronics? Substitute a heat sink device for the container....aren't all heat sinks black?

I like my coffee hot....that is why I drink it black! No! No! No! I just contradicted myself! I hope my little joke here doesn't confuse anyone.

Slick, I understand radiant heat transfer well, but one thing I've wondered: If you paint your cylinders black to increase radiant, you are also adding an insulating film to the fins which will reduce convective heat transfer. Ideally, we'd want a black finish with no thickness. Any ideas on how to do this?
Jaydee

Maybe I wasn't clear. Yes, I talking about the engines and trans cases. The barrels are black and will stay that way, though with a good blasting first. A few questions:

1) What is a vapor blasting?

2) Any input on my plans for the mechanicals?

jaydee75 said:

Slick, I understand radiant heat transfer well, but one thing I've wondered: If you paint your cylinders black to increase radiant, you are also adding an insulating film to the fins which will reduce convective heat transfer. Ideally, we'd want a black finish with no thickness. Any ideas on how to do this?
Jaydee

Click to expand...

You are partially correct....any film added will reduce CONDUCTIVE, not CONVECTIVE heat transfer. The heat first conducts its way thru the iron of the barrels, then thru the film. It is conceivable the film might even be more conductive than the iron, but the added thickness will reduce the overall heat transfer as you state.

A zero thickness solution might be the black oxide finish as applied to firearms (the classic ones anyway), or black chrome plating (not zero thickness but only a few mils and highly conductive). Chrome, or any plating in fins will have the same problem as powder coating.

Once the heat is past the film, it finds its conductive path obstructed by a thin layer of non-moving air (the boundary layer). Stagnant air is a rather good thermal insulator. Surface roughness breaks up the laminar sublayer into a thinner turbulent layer enhancing the heat transfer.

So far in this discussion, all heat has moved by conduction...thru the core of the iron, the paint film, and into the boundary layer. Now convection takes over. Hot air near the body surface mixes with the moving mass of the cooling airflow, the mixing enhanced by the turbulent action of the boundary layer. Turbulence is microscopic eddies and swirls that move hot air from the film surface into the mass of adjacent air. This is greatly enhanced heat transfer than that of laminar mixing...thus we want surface roughness to induce turbulence.

Radiation, meanwhile goes on irrespective of convection, depending only on the surface temperature of the body, and the temperature of the radiation absorptive surroundings.

Mostly this is an academic discussion.....don't worry about the paint film thichness....worry more about surface roughness...it is an order of magnitude (that is engineering speak that means a bunch more but don't ask me how much 'cause I don't want to compute it) more than the loss of conduction by an increase paint film....avoid polishing the jugs....and paint them black.

"Alrighty then" !

Biscuit said:

"Alrighty then" !

Click to expand...

There will be a pop quiz on this next time!

Prof Slick

Supporting texasSlick here. When I had to work for a living I had to specify (design) a series of cooling fins on a piece of electronic equipment. I did a whole lot of research and can remember the major findings which were....
1/ A medium grit blast of a well finished machined surface can increase the surface area by a large amount, 30% comes to mind but I couldn't justify/defend it.
2/ Painting the reflecting (grit blasted) surface with a thin coat of matt black would also help.
The end result was that we were able to get rid of the heat that the electronics generated without increasing the volume of the overall "black box"

Now before anyone jumps down my neck, I realise that the "as cast" fins on a Commando are pretty rough, certainly not a well machined surface. The point is though, that if the cylinder barrels are masked to protect the bores and machined surfaces, there should be a fair bit to be gained by a light grit blast to get rid of any rust/crud/any old paint that's flaking, then immediately spraying with some matt black heat resistant paint. In other similar situations I've used rattle can VHT paint, which I believe is from the US. A thin coat to cover, not enough to run.

cheers
best of luck
wakeup

OK....one more post to completely wrap this up! Wakeup makes two good points.....first, it does not matter how you achieve surface roughness....either by leaving the sand casting texture (a jumble of mountains and valleys ), or by blasting depressions into the surface. It does not take much roughness to induce turbulence, and like most things in life, a little may be good, but more not necessarily better. Secondly, he points out that matte black is preferable to gloss. Matte black has a higher emissivity (scientific speak for Stefan's constant) than gloss.

For those who just have to spiffy up your jugs, you have my permission to sand and polish the fin EDGES, then matte black spray the jugs, next using a roller, gloss black paint the edges. Theoretically this goes against all that has been said above, but the edges account for an insignificant percentage of the fin's heat transfer, so you do not lose much. In fact, this is exactly how I intend to treat my jugs when I swap out my high compression pistons for lower CR.

Oh....why do I have high CR pistons? Because on a transcontinental trip, I took my Atlas onto the Bonneville salt flats, not knowing the salt was hard and dry only limited times a year, and got brine over everything, which turned white when it dried. Even tho' I was a graduate engineer and should have known better, I failed to wash it off (being 3 days late for graduate classes 2000 miles away, having dallied too Long in San Francisco with a hot mamma, may have had something to do with it). At first, everything was cool (pun intended), but when I got down to lower elevation flatlands, and the air temp. heated up, I overheated the Norton, destroying one piston and bending up a valve. So lest anyone still think color does not matter......

Here's something else to think about, I have been anyway because of this thead. My MK3 has around 80% of the original black barrel finish on it and it looks to me as though all the rough casting surface has been purposely coated with a thick, very hard (as if enameled), and smooth finish. This is a finish that is not particularly conducive to loosing heat apparantly. My theory is this was done purposly to 'control' the cylinder heat rather than dump as much of it as possible all the time. Here is my reasoning based on my experience. About 10yr's ago I rebuilt this engine with +.020" pistons but gave myself a clearance of only .0035", not enough I know. This engine, however, ran great all summer with no issues. Early in the fall though, I took a run up the Kancamaugus in NH, a mountain road that climbs steeply towards it's summit where it can also get a lot colder as on this day. So here's the conditions. Spirited ride, warm at the bottom, engine running moderately fast but easy. Toward the summit, much steeper and more curves which equaled more shifting and a much harder working engine generating more internal heat, particularly piston heat. In the space of a mile the ambient temp fell from f 70ish to around f40 and my engine siezed, let go, siezed again and let go once more as I nursed the somewhat rattley engine down the other side. My theory is that the cyl block hitting the cold air cooled and contracted rapidly enough to close up the admittedly too little clearence of the now heat expanded pistons to zero and....! My point being, and question, could the hard, smooth enameled like surface of the stock 850 barrels be to dampen the effect of rapid overcooling that even with correct piston to cyl clearance but under extreme conditions could lead to seizure? Discuss.

There be pirates in them waters. They be takin' yer threads and twistin' 'em to all manner 'o meanings what have nothin' to do with yer question, Matey. I've seen it 'appen with me own two eyes. Well, back afore that dark-souled devil of a parrot prysed out me other'n for 'is dinner, that is.

They painted them black because it looked good and they had the paint on the shelf.
If it is smooth that looks good too. The factory paint is pretty durable I will admit.
That it helps dump heat may have figured into it but probably more because they wanted the
850 to look different than the 750.
Look, Norton was just a dying company trying to make a buck and stay alive another week.

RE: Biscuit theory

I think the first part of your theory has merit....the rapidly falling air temp. would cool and contract the cylinders before the pistons had time to respond, and would close up the clearance.

As for the second part, that the hard, smooth, enamel-like coating tempers the heat exchange, and moderates the contraction, preventing seize up, I would like to think over further. I usually defer to the manufacturer's engineers, thinking they know, or are aware of, something I do not. If this is a factory coating (others may wish to verify this), I would be curious to know the reason for it. Perhaps it is just a marketing strategy....looks pretty.

Maybe the former NV engineer who posts here frequently, can comment on this. You might want to PM him to alert him of this thread.

Ah!...Looky here....Onder confirms my "looks pretty" theory