Monday, December 4, 2006

CC'ing Your Heads

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(From the unpublished manuscript 'How to Build A Reliable Aircraft Engine')

Most folks arrive at this point not because they are building an engine but because they are repairing one. Doing a valve job on a VW engine alters the chamber volume. Or they've dropped a valve and have had to replace one of the heads. Replacement or repair of a head alters the chamber volume and chamber volume is a factor in the compression-ratio equation. Before they can re-assemble the engine they have to measure the chamber volume and re-compute the CR to ensure the repair work has not upset the compression ratio.

Measuring chamber volume is easy to do and the procedure is all the real service manuals for the VW engine. But just to keep all the information in one place, I'll give you a quick sketch of the procedure.

Using the catheter-type syringe and a sealing disk similar to those shown in the photos, you should be able to measure the volume of a Volkswagen combustion chamber to within 2cc. That is, if you measure it a multiple number of times your results should not vary from the average result by more than plus or minus 1cc. (As a general rule, always measure each chamber at least three times.)

Herez how: Install a spark plug in the head and torque to spec. If the valves are not perfectly fluid-tight, pop them out and put a light smear of Vaseline on their sealing surfaces. Level the head in both directions then smear a light coat of Vaseline around the periphery of the sealing disk and drop it into the chamber. If the sealing disk is 3/8" or thicker it will be heavy enough to stay in place by itself (the specific gravity of cast acrylic sheet is only 1.19). For thinner sealing disks you'll need to hold them in place with your thumb while filling the chamber or add a few ounces of weight to their edges.

The disk(s) doesn't have to be perfectly round. You can saw the thing out with a jig saw, smooth the edge with a file and you've kept it to within +/- sixty thou or so, it will work perfectly well. Nor do you need five holes in the disk as shown in the factory workshop manual; you can do a quick check with just one.

Using a suitable fluid, fill the chamber and the hole through the sealing disk. Subtract the volume of the hole from the total. Do both chambers. For OHV engines I think you'll find ‘wetter' water does a better job than the traditional kerosene. This is because you need to measure the chamber several times and average the result and that means you have to remove all of the fluid from the chamber. With a flat-head engine it's pretty easy to simply wipe the chamber dry but with an OHV the kerosene tends to cling to all the nooks & crannies. So use water with a drop of wetting agent as your fluid. When it comes time to dry the chamber, simply dump it out, slosh it with a bit of alcohol then blow it dry. Alcohol is hygroscopic - - it will mix with any remaining water and once mixed with alcohol it takes only a modest blast of air to dry even the deep recess around the spark plug's insulator.

A quick check using the syringe shown in the photos will allow you to determine the volume of your combustion chambers to within 2cc (i.e., plus or minus 1cc). To compute your compression ratio use the largest volume you measured as the factor in the CR equation.

That's it. If you're a shade-tree mechanic you're all done. Good luck in the contest.

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CC'ing TOOLS & EQUIPMENT


Why am I doing this?

Truth is, some of you aren't. Although this article was written to explain how to improve your engine's efficiency by adjusting it's volumetric balance, many builders are interested only in determining their compression ratio. If that's all you're here for, drop down to MEASURING CHAMBER VOLUME.

By applying modern-day standards of balancing, of both mass and volume, you can pick up as many as ten free horsepower from a bone stock VW engine. And I mean ‘free' in the sense that it will use the same amount of gas. That seeming impossibility reflects the fact the engine must first overcome any internal imbalance before it can deliver any usable power. That means an imbalance generating a loss of five horsepower costs an additional five horsepower to balance the books, resulting in a net loss of ten horsepower. This is a key factor in the importance of balancing and one most people overlook. That is, for each imbalance you eliminate you will gain approximately twice that amount of improvement.

Of even more importance to builders of flying Volkswagens is the fact that a properly balanced engine is more durable and the reduction in wear is even more pronounced than the improvement in fuel efficiency. That's because most of the internal imbalances appeared in the form of friction and heat, which could only be overcome by generating more friction and heat. The bottom line is that a 3hp imbalance could cost you up to four times that in additional wear. Eliminate the imbalances and the engine's wear-factor takes a dramatic drop.

The benefits of mass-balancing became self-evident as the normal operating speed of internal combustion engines was increased but the role of volumetric balance was not fully appreciated until computers came along. Once they had a super-computer to play with automotive engineers discovered that a lot of what we thought we knew about the process of combustion was not entirely correct, that there were transient phenomenons due to volumetric imbalances that we'd attributed to other causes. This is one of those non-intuitive kinds of things, not easily explained without a bit of background but the basic reason has to do with the process of combustion and the fact those seemingly insignificant differences in volume appear on both sides of the compression ratio equation. Turns out, relatively small variations in volumetric balance can produce some relatively large losses of power. Since that time they've taken exquisite pains to reduce those imbalances. How this is accomplished in a mass-production environment is quite interesting (at least, to me :-) but the bottom line is that we can take advantage of what they've learned by simply paying more attention to the volumetric balance of the engines we build.

Thanks to computerized equipment a modern balance shop can do mass-balancing to a fraction of a gram at a very reasonable price. But matching the volume of a pair of heads (i.e., four chambers) to a tenth of a cubic centimeter remains more art than science. In the racing world it isn't uncommon to pay a thousand dollars for a pair of full-trick heads, their volumes accurate to 0.1cc across all four chambers. The cost isn't in the tooling; it doesn't take a lot of tools for head-work. The money goes to buy the head-man's time. That fact is in our favor because time is about the only resource of which lo-buck homebuilders have a surplus. By acquiring a few tools and devoting a bit of time to your heads, you can bootstrap yourself into a better engine than you can afford to buy.

At this point a majority of non-mechanic home-builders are sitting there with a large black question-mark floating in the air over their heads. If adjusting chamber volume is more art than science, how can they hope to tackle the job themselves?

The answer is so simple it will make you smile. Anything you do to reduce the magnitude of the existing imbalance will improve the efficiency and durability of your engine. The fact the pro's regularly balance to a tenth of a cubic centimeter is like running the four minute mile. But we're just a bunch of week-end joggers so let's not even think about that degree of precision. At least, for now. Instead, let's look at the basic problem.

You have four chambers. You measure them (the procedure is outlined below) and come up with four values, probably accurate to within half a cc or so. Using real numbers now... two heads measured 56.5, 58, 58, and 60cc. Rather than trying to turn you into a head-flow guru capable of achieving 0.1cc accuracy on a repeatable basis the question becomes: "Can we reduce that 3.5cc spread to something smaller?"

The answer is a decided ‘Yes!' And it really isn't all that difficult because you only have to work on three of the chambers. That is, you can't make a chamber any smaller so the largest chamber becomes your target-size. So relax. You can do this. And you'll end up with a better engine because of it.

Then comes the question, how much metal are we actually talking about? Just what the hell is a ‘cubic centimeter'? The answer to both: Not much.

If you've been following the Practice Wing project you should have some high-density urethane foam on hand. Print out the ONE_CC drawing, glue it to a piece of foam then use a razor blade to make a cube the size of the printed square. That is, a cube 0.3937" on a side. No foam? Then make the cube out of wood. Or plastic. Or tool steel (if you can). The goal is to give you a tangible reference to the volume of a cubic centimeter.

Hold a cubic centimeter in your hand and three things are immediately apparent: Firstly, it isn't very large. Just a tad over 3/8" on a side. Secondly, you can appreciate the heroics it takes to achieve repeatable tenth-cc accuracy. And finally, you can see why a lot of people consider such imbalances to be insignificant. To the uninitiated, something that small has to be unimportant; something they can ignore.

Got some sixteenth-inch ply? Okay, howz about some poster board a sixteenth of an inch thick. Or a piece of .063 aluminum. Three-eighths wide, six inches long, you're looking at about one cc. Now go back and look at what we're trying to do. To balance the 56.5cc chamber we need to remove three and a half cc's of material. Just unshrouding the valves is usually good for that volume.

So how do we do all that? I'll get into that in a minute but it all begins by measuring the volume of your combustion chambers.

MEASURING CHAMBER VOLUME

If you've never cc'd your chambers, relax; it isn't difficult nor do you need a lot of equipment.

My first exposure to cc'ing heads was in the mid-1950's. I bolted a piece of plex to the freshly milled head of a Ford V-8, leveled the thing up with wedges and used a turkey baster to fill each chamber with kerosene. After doing both heads I plugged the smallest volume into the compression-ratio equation and came up with a CR of about 11:1, assuming I used a sixty-thou gasket. Life was good, assuming I could swipe enough avgas for a couple of runs through the lights :-)

Determining the volume of the combustion chambers is a chore common to all engines and the basic procedure is the same for all. The procedure requires:

1. Some means of holding the head level.

2. Some means of sealing the space to be measured.

3. A method of accurately measuring the amount of fluid needed to fill the chamber.

Working with nothing more than a 1/4" drill motor and a couple of carbon steel rotary files, cleaning up the marks with a strip of sandpaper spiraled around the split end of a quarter-inch dowel, you can expect to produce a cc-job accurate to about 0.5cc across all four heads. Indeed, the usual problem when a novice does his first set of heads is going too far, opening up the smaller jugs to a volume larger than the target-jug. Which means they have to take a bit out of the big chamber, which usually leads the them chasing the volume back & forth until they get sick of it.

SAME PRINCIPLE, DIFFERENT PROCEDURE

The secret to hitting your chamber volume dead-on is exactly the same as for hand-cutting a piece of aluminum to a precise dimension in that you don't shoot for the exact dimension. You always give yourself a clean-up allowance. Cutting aluminum by hand you snip or saw the thing about forty-thou over-size then dress the edge with a file. You do much the same when cc'ing your chambers in that you use the rotary files to get close to the finished size then begin smoothing things up with abrasives, checking the volume periodically.

I mention this now because the next subject has to do with the precision of your cc'ing job and the fact balancing your chambers to a fraction of a cc might take you a couple of weeks (!) if you've never done it before, whereas balancing them to 1cc (across all four) should take only a few hours. If you don't want to spend the time then there's no need to spend the money for an accurate liquid dispenser, such as the one shown in the photos, which is graduated to 0.1cc. Turkey baster or the big syringe, you can work down to 1.0cc with fair accuracy. And there's a big difference in price between a surplus syringe and a certified-accurate, lab-grade burette.

All engines get their chambers cc'd, not just Volkswagens. The cc'ing of combustion chambers is illustrated and talked about in the various manuals on engine building. The three key elements mentioned above reflect the basic core knowledge for any cc'ing job. Then comes a host of details; the How-To stuff, most of which is engine-specific but some of the common factors address the accuracy of the finished job, the degree of precision, how long it takes and various convenience factors. As with most other aspects of proper engine fabrication, cc'ing your heads calls for a keen attention to detail. Most of the time.

The point is that you really don't need much to do a quick check.

Lots of after-market retailers will sell you a ‘cc kit.' It usually consists of a plastic syringe like the one shown in the pictures plus a plastic disk with a hole in the middle.

What's it cost? Hard to believe but some outfits want as much as thirty bucks for a cc kit.

See the syringe in the photos? It comes from a veterinary supply house. New, quantity one, you're looking at three or four dollars. But syringes have a certain shelf life and once they're past it, the supply house often flogs the stuff off as surplus, out-dated or whatever. Buy them a dozen at a time, the syringe shown cost about half a dollar.

The plastic sealing disk is any sort of flat plastic, eighth of an inch thick or more. Doesn't have to be clear. The sealing disks I use for 77mm jugs are made from a hunk of red Plexiglas I dug out of the scrap box at a plastics retailer. In a pinch I've even cast my own using polyester resin on a sheet of waxed glass. It only has to be flat on one side and just clear enough so you can chase the bubbles. Chasing the bubbles is one of those little details no one mentions :-) The bitter truth is that some bubbles don't like to be chased and you can spend the best years of your life trying to coax the thing over to the hole.

The sealing disk in the typical kit has just a single hole in the middle. Cheap, easy to make and so forth. But harder to use than one with more holes. More holes, you don't have to chase the bubbles so far but more importantly, with more holes there's a better chance you'll be working with a truly level head. And that's worth mentioning because a lot of after-market heads, especially rebuilts, the sealing surface is not parallel to the valve gallery gasket rail, which is what you use to sit the head on.

So you sit the head on the gasket rail (after taken the studs & valve stems into account) and use your machinist's level or framing level or whatever on the chamber-side of the head to use wedges and so forth to get everything perfectly level, end-to-end and fore & aft.

That's no guarantee the sealing surface of the chamber is level. But if your sealing disk has a pattern of holes as shown in the drawing (and in the VW factory workshop manual), as the chamber fills the level of the fluid in the various holes tells you if the chamber is level.

You subtract the volume of the holes, by the way. Figure out their volume ahead of time, scribe it right onto the sealing disk. You can use the same equation as for V1, just substitute hole diameter for bore and the thickness of the plastic for stroke. Then multiply that by however many holes you have and be sure to always fill the hole to precisely the same level.

Buy a cc kit, it usually comes with only one disk for each size of cylinder. Indeed, some outfits only sell you one disk, period, screwing you to the cash register if you want two disks or a set of a different size. Which is another example of how VW after-market retailers screw the kiddies, because to do the job properly you really should do both chambers at the same time... and that means two disks of each size.

Making disks is pretty simple, assuming you understand plastic. Drills for use with plastic looks like a needle :-) We usta call them ‘canopy bits' or ‘Plex bits.' Included angle of something like 30 degrees. If you don't have a set of bits for plastic and don't know how to sharpen a drill bit, you'll have to use a hand-drill. Even then, you're liable to crack it when the bit breaks through the other side. That's when it grabs the flutes. Plastic bit, you needle a pilot hole then flip it over, finish it from the other side. Slow speed, always. Kerosene makes a good lubricant for thicker stock.

Although the sealing disk drawing I've posted shows dimensions to thousandths of an inch that's just the math. You can cut one out with a jig saw and it'll work just fine. That's because the sealing lip inside the combustion chamber is at least a tenth of an inch wide. So long as the disk is close to a true circle and makes good contact with that lip, it will work okay. (Too big? Then sand the edge.)

Once you understand the purpose of the sealing disk you'll see them all around you, just waiting to be cut out. See that translucent plastic over the waterproof light in the bathroom? That'll work. You can still see the bubble. Got some curvy scraps from an a blown canopy? That'll work. Just put it on a heavy plate of polished aluminum, pop it in the oven set at about 300 and let it flow out flat.

Want to keep your sealing disks nice & pretty? Got some old 5-1/4" floppy disks? Cut one open, throw away the disk, store your sealing disks in the sleeve.

To make your chamber water-tight you smear a little Vaseline on the valve seats and a light wipe around the outer-most edge of the plastic disk. Then you fill the syringe to a precise level and use it to fill the chamber. Subtract what's left in the syringe from whatever you started with, then subtract the volume of the hole(s) in the plate and that's the volume of your combustion chamber, accurate to however accurate you are and as precise as the divisions on your syringe.

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On the other hand... there's lots of lab equipment scaled to 0.1cc increments but most of it lacks the volume to fill a chamber. VW combustion chamber volume may be as small as 40cc or as large as 80cc, depending the size of your valves and the displacement of the engine. A burette large enough to hold one chamber's-worth of juice and accurately marked to 0.1cc precision, you're talking about an expensive piece of goods. The high price of the Good Stuff comes as a shock to guys building just a single engine, often causing them to settle for half a loaf, using a turkey baster of whatever.

So use a marble or two. And a smaller but more precisely marked burette.

Got a marble? There's a free one in every can of rattle-can paint. (That's the rattle :-) Or you can buy a bag of them at the dime store. Or use steel ball bearings (but remember they rust). I like ball bearings because of their uniformity. Marbles vary by quite a bit.

So what's the volume of your marble? Half fill your burette, bring the meniscus to the line, make a note, then drop your marble into the burette. Make another note then do the math. (You may shout ‘Eureka!' if you wish :-)

By adding objects of known volume to your combustion chamber you can use a smaller dispenser marked to 0.1cc graduations. Just be sure to keep the numbers straight. (KEEP GOOD NOTES!).

When trying to achieve the smallest possible difference across all four jugs, statistics are your friend. Make all of your measurements a number of times and then average them. This helps to reduce the human error, a factor that can be deadly for the first-timer. Without experience you can't appreciate the significance of seemingly minor differences. Do everything several times, throw out the high and the low then averaging the remainder tends to reduce any errors of observation.

Work with good light. Don't be shy about using a reading glass to inspect the meniscus in the sealing plate holes and your burette. When taking a series of measures on the same chamber(s) always be sure to start with it perfectly dry. That means you'll need an air blast to blow out the spark plug (which should be torqued to spec on a new washer). Before filling the chambers I swab them with lacquer thinner or MEK to remove any oily residue that might prevent the fluid from fully wetting the surface. When applying the Vaseline, don't get sloppy -- a little dab will do ya.

Once the sealing disk is in place, it don't wanna leave :-) Use a dental tool or bend a tiny hook on the end of a piece of wire to lift it out using one of the holes. Pour the water out of the chambers, wipe them down with a clean towel, give it a slosh of alcohol and blow the plug dry.

Traditionally, the liquid used for cc'ing heads was kerosene. (I donno... it just was.) Working with VW's I've heard guys advocate the use of everything from anti-freeze to ATF and big dummy that I am, I tried them all.

Hell of a mess.

Light oil of any kind is a contaminant in the shop and can be a fire hazard. And glycol, Prestone or what-have-you is poison. In the mid-70's I started using ‘wetter' water for cc'ing and found it gave more consistent results and a lot less mess. How do you make water wetter? Originally, I added a couple drops of ‘Photo Flow,' darkroom stuff that prevents water spots on your negatives. But for cc'ing heads, a couple of drops of liquid detergent per gallon of water has about the same effect. A drop or two of food coloring will make it easier to see in the burette. Try to work in a room-temperature environment. The temperature of your measuring fluid and the heads should always be the same. (Keep the liquid in the shop with the heads. You'll need to do rough spot-checks as the works progresses. I'll have more to say about that in a minute.)

I normally use a burette similar to the one in the photos. You'll notice that it's attached to a standard that bolts to the base, which is a plywood box filled with concrete. To attach the burette to the standard, fancy name for a scrap of plywood, I twisted some welding rod around a bolt a little smaller in diameter than the burette so the resulting hair-pin-looking piece would grip the burette. To fasten it to the standard I drilled a pair of holes, poked the legs of the hair pins through the holes, bent them over and covered the bent ends with a slather of Bondo. (You can see all this in the pictures.)

The base is heavy to prevent it from tipping over and breaking the burette. It is leveled with shims or screws then the burette is over-filled using a pitcher and funnel. To dispense the liquid I use a length of surgical rubber tubing (drug stores carry it) fitted with the glass tip from an eye-dropper (American Science & Surplus (www.sciplus.com) sells all this stuff, including a pretty good Chinese burette). Flow is controlled by a clamp on the rubber hose. Once the burette is over-filled I bring the fluid down to the line by releasing the clamp and returning the fluid to the pitcher or bucket.

To fill the chamber I hold the eye-dropper/nozzle over one of the holes then play with the flow-control clamp until I bring the fluid level to the base of the sealing disk then chase out any bubbles by tapping on the disk with my fingernail or the handle of a dental tool. Once all the bubbles are accounted for I fill the central hole a drop at a time until all of the holes are at precisely the same. Then I read the burette, hunkering up or down to eliminate any parallax.

Measuring a chamber takes me less time than it took you to read the paragraph above. Using two disks allows me to do both chambers with one set-up.

Then I clean everything up and do it all over again.

To determine the finished volume, as when I'm down to the point of making fractional cc adjustments, I measure each chamber as many times as I think I have to. Sometimes I'll get three identical measurements in a row on both chambers. And sometimes I'll do it half a dozen times and get a different measurement each time, usually when I'm tired or whatever. (People wonder why I don't answer the phone or throw rocks at them when they arrive unannounced. Now you know :-)

That's how I do it when I'm balancing the volume of the heads, trying to achieve 1cc or better across all four. When I'm in the shop working on the heads, I generally use a syringe. It's only accurate to 2cc but it's a lot faster and when you are first opening up the heads, 2cc accuracy is more than enough.

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When building a big-bore engine, cc'ing your heads begins long before you get around to calculating your compression ratio. In fact, it starts when you first acquire the heads. If they're new, with valves already installed, just install a spark plug, grease them up and do a rough measure of volume. With bare heads or heads that have just been overhauled (ie, welded-up with new seats, etc.), rough in the valves then install a plug. Level them up, select the appropriate sealing disk (if you haven't opened them up they'll be some other size) and use a baster or syringe to get some idea of their as-cast volume. It's handy to write that volume directly onto the head using a crayon or wax pencil. The heads should already have been documented and assigned a work number and a documentation package (sounds complicated; actually, each head is just a page in a notebook.) A lot of folks think that's overkill for just one engine but during the course of the work you must have some means of identifying the chambers and even one engine has four of those suckers in two identical heads. If you're building a really good engine, you'll make up a set of spare heads, identical to the first set. Having a spare set of heads on-hand allows you to swap heads when necessary and do the valve work at your convenience.

When you open up your heads for bigger jugs you are trying compress the swept volume of the larger jug into the original compression space. There's no doubt the compression ratio will go up. The big question, is how much?

If the only thing that changed was the swept volume and the deck clearance was kept the same, going from 1600cc to 1834 is going to raise your CR by about one point. But that's the myth that leads to blown engines because nothing remains unchanged. When you open up the heads even the slightest skim cut will reduce the chamber volume. Another part of the myth is that most guys don't know what their original compression ratio was to begin with. Later model engines ran 7.5 or 7.7 (the Export crate engine from the Puebla plant is 7.7) depending on the heads but if it's been overhauled a few times you could be using heads that have been flycut. Bottom line: It isn't uncommon to see 1834's with a CR of 9:1 or more.

Opening up your heads for bigger jugs offers the opportunity to unshround the valves. Unshrouding the valves results in a profound increase in flow-rate for the same amount of lift. The procedure is covered in Bill Fisher's "How to Hot-rod Volkswagen Engines" but I'll try to insert a couple of photos to show you what I'm talking about.

To unshroud your valves you have to remove metal from the chamber. Any time you remove metal from the chamber you need to cc the chambers to make sure they all end up the same size. This is normally done several times as the work progresses.

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Okay, so you're using a syringe and a sealing plate with one hole and you've never done it before and you think my talk about attention to detail is all bullshit so you squirt them just once and come up four measurements ranging from 55cc to 60cc. Now what do you do?

What you do is go back and do it again :-)

When you're satisfied that your measurements are as accurate as they're going to get, the next step is to decide on a strategy for increasing the volume of the small chambers to bring them closer to the volume of the largest chamber. By this time you should have some idea as to the physical size of one cubic centimeter and your measurements have told you how much each of the three smallest chambers must be opened up to match the volume of the largest. The tricky bit is where to remove the metal from.

Basic starting point is unshroud the valves using rotary files. Then do a quick syringe-check. You want to STOP with the filing when you get to within 1 cc of your goal.

If unshrouding the valves does not give you enough volume then you begin ‘laying-down' the edge of the combustion chamber. Here again, start with your rotary files. Try to visualize the amount of metal you want to remove. Remove nearly that amount then check the volume. When you get to within 1cc, stop using rotary files and shift to using abrasive rolls.

The principle behind smoothing things with abrasives is to simply remove the tool marks from previous steps. That means using a tool (or abrasive) that will leave smaller marks. So you start with coarse sandpaper and when you've removed all of the marks of the rotary files you'll be left with the marks made by the sandpaper. So you shift to a finer grade of sandpaper and do it all over again. I generally use 80, 120 & 220. After 220 I shift to felt hobs and polishing compound.

A key point here is that I never try to hit the target volume dead-on, I only try to get close to it and always on the high side. A second point is the need to periodically check the chamber volume as the work progresses... and to check it with an increasing degree of accuracy.

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Running the 4-Minute Mile

The only practical way to achieve 0.1cc accuracy across eight chambers (ie, two pair of heads) is to get close to the target figure, say within 0.5cc, and to then drop one of the valves. That is, to lower one of the valves by grinding it's seat. The area of the valve is fixed; you can measure it. You then determine how many thousandths of an inch it needs to be lowered to arrive at your target volume.

Since most of you don't have the stones, tools and fixtures for re-grinding valve seats, I don't expect you to be using this procedure but I think the how-to is worth mentioning.

-Bob Hoover
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