At Circle Track we’re all about tech, all the time. And at least a few times a year we’ll show you how to assemble your own engine from air filter to oil pan. But one area of tech we rarely touch on in depth is machine work of the engine block and cylinder heads. That’s because high-quality machine work requires very expensive equipment and specialized training. So honestly, even if they are going to handle engine assembly, most racers are better off handing the machining duties to their local engine builder.
But that doesn’t mean it isn’t necessary to understand what’s going on. Quality machine work that’s done with the care and precision required to hold tight to the tolerances required isn’t always a given--after all, you can wind up hiring the services of a machinist that’s poorly qualified and cuts corners just as easily as you can a plumber, house painter, or barber. The key to being confident that you are getting what you pay for is understanding what’s involved, and what separates good work from bad.
The first step is to talk to other racers. Does the engine builder or machinist have a good reputation at your racetrack? Teams that can consistently gather top finishes don’t get that way by accident. It’s a good bet that any company they are willing to associate with does quality work.
But there are often times when you can’t simply rely on a recommendation. Then you need to understand what you need, and be able to determine if you are getting what you paid for. There are a handful of critical machining processes that should be a part of every engine build. This time around, we are concentrating specifically on honing the cylinder bores.
Although they are often spoken of together, honing is not the same thing as boring the cylinders. Boring uses a carbide steel cutting tool to cut away metal from the cylinder bore. Honing, on the other hand, uses stones of varying coarseness to abrade away smaller amounts of the cylinder bore to improve concentricity (roundness, in plain English) and prep the surface of the metal.
Let’s take, for example, a new engine build. You will probably want to go ahead and bore the cylinder 0.030-inch oversize--to a final bore diameter of 4.030 inches--to take advantage of typical rules that allow a maximum engine displacement of 358 inches for both small block Fords and Chevrolets. That extra 0.030 of width doesn’t do much in terms of cubic inches, but it can really help improve compression ratio when stuck with flat-top pistons.
To get there, most machinists will bore the cylinders to 4.025 inches, leaving 0.005 of material to be honed away to get to the final 4.030 target. Honing is done in steps, beginning with a more coarse set of stones. Honing with the coarse stones will bring the bore size all the way to 4.027- to 4.0275-inches. The final 0.0025 is done in one or two more steps--with finer stones--that not only leaves a perfectly round cylinder bore, but also creates the surface finish on the cylinder walls.
When it comes to machining the cylinder bores, the surface finish of the walls is just as important of the roundness. Good honing work will create a "crosshatch" pattern that’s consistent along the length of the bore. The crosshatch is a fine texture of ridges and valleys in the metal that catches and helps hold a fine film of motor oil in the cylinder bores. This cuts down on the friction between the piston rings and the cylinder walls as the pistons move up and down the bores, and also keeps the rings from welding themselves to the block.
The end result of high-level honing work is an engine that seals up well so that an absolute minimum of combustion gasses leaks past the rings. That means more power making it to the crankshaft, and fewer contaminants in the oil for improved protection of the bearings, cam lobes, and other friction prone surfaces. Great honing work also helps the rings seal well so that you aren’t burning oil, either. So here are a few tips to make sure you are getting the best honing work possible.
Properly honing your cylinders is one of the first steps to building a solid race engine. Here’s how to make sure your machinist is doing it right.
Ken Troutman, owner of race...
Ken Troutman, owner of race engine shop KT Engine Development, has built numerous race-winning engines at all levels and says that the value of properly machining your block and heads—and especially honing the cylinder bores—before an engine build can’t be overstated. No matter how much you spend on pistons, rods, and other components, it’s impossible to overcome mistakes or sloppy work at this stage.
1 A honing machine is a pretty...
1 A honing machine is a pretty advanced piece of equipment. But even more important is the proper skills and experience to use a piece of equipment like this properly. We can’t teach you how to hone your own blocks in this space, but we can pass along some pretty important tips Troutman gave us to make sure the machinist you hire to perform the honing work on your next engine build does a good job.
2 This is a block that KT...
2 This is a block that KT Engines has in the shop for a rebuild. But before going into the honing machine, Nathan Allmond makes sure all the main caps are torqued to spec. In order to make sure the cylinder bores are as round as possible after the engine is assembled, you must replicate all the stresses possible the block will see.
3 Allmond locks the block...
3 Allmond locks the block in place in the honing machine using fixtures that ensure the cylinder bores are perpendicular to the line of the crankshaft. Then he spends a few moments prepping the deck surface for the torque plate.
4 Even more important than...
4 Even more important than making sure the main caps are torqued like they will be on final assembly is simulating the warping forces placed on the top of the block by the cylinder heads. It’s impossible to hone the block with the heads in place, so a torque plate like this is used instead. Between the plate and the block is also a gasket of the same thickness that will be used in the final build. While it isn’t absolutely necessary to use the exact same type of head gasket that you will on your final build, make sure it is at least the same size. For example, if your final bore size is 0.030 over, make sure your machinist doesn’t use a 0.060 over gasket simply because it’s big enough. The fire ring is a different size, and that’s what transfers the pressure from the torque plate to the block. It’s not much, but it’s enough to affect the final bore shape.
5 Allmond torques the plate...
5 Allmond torques the plate in place in the same manner you would a cylinder head. Make sure to use a lubricant on the threads of the bolts to make sure your torque readings are as accurate as possible.
6 The honing work must be...
6 The honing work must be accurate down to 0.0005-inch. To maintain maximum accuracy, Troutman recalibrates his dial bore-gauge before every job.
7 This block is part of a...
7 This block is part of a standard rebuild, so there’s no damage to the cylinder bores beyond normal wear. But notice the change in color around 2/3 of the way down the cylinder bores. This is the farthest the rings travel down the cylinder bore at piston BDC. Typically, you will see greater wear at the top of the cylinder bore because the strong pressure from the combustion charge pushes the rings out the hardest.
8 Troutman measures the top...
8 Troutman measures the top and the bottom of every bore before determining the maximum bore size.
9 The more coarse the honing...
9 The more coarse the honing stone, the more material it will remove. The stone on the bottom is a #523 coarse stone that’s used for the rough cuts. The stone on top is an 818, which is a smooth stone used for finishing work. There is no one single way to get the best surface finish. For example, many engine builders always finish with an 818 stone, but with new plasma-moly rings, which are popular in racing right now, Troutman prefers to finish with a 623 stone. The stone is rougher, but by making his final passes with lighter pressure between the stone and the cylinder wall, the result is a crosshatch that works well with the rings.
10 The honing machine works...
10 The honing machine works by spinning the stones inside the cylinder bore while moving the arm up and down the length of the bore. This is what produces the crosshatch pattern. It’s also important to use plenty of coolant to minimize heat buildup and keep the stones from galling against the iron cylinders.
11 Here’s the cylinder wall...
11 Here’s the cylinder wall after a quick first pass. Notice the shadowing in a line straight down the bore directly in front of the head bolt hole. This is warping that developed over time. It was revealed when the honing stones removed the metal on either side of this shallow hole or depression. Even small variances like this can cause sealing problems. When this happens, exhaust gasses are allowed to pass by the rings—instead of pressing down on the piston to produce power.
12 Troutman uses the gauge...
12 Troutman uses the gauge (on the white box on the right) to gauge pressure on the honing fixture. If the gauge goes up, that means the honing stones have hit a tight spot. When the gauge holds steady, he knows the cylinder is nice and round.
13 The holes in the torque...
13 The holes in the torque plate are slightly larger than the cylinder bores so that they won’t contact the honing stones and affect the hone. But honing will leave a sharp edge between the bore and the deck of the block, which should be chamfered before beginning assembly.
14 Few—if any—racers will...
14 Few—if any—racers will have access to a profilometer to check the surface finish of a freshly honed block, but Troutman says that after being properly honed the cylinder bore should be medium gray, and slightly dull looking. The crosshatch pattern should be nice and even, and you should not be able to see any honing marks underneath the crosshatch (that will appear as a rougher spiral moving down the bore).
15 This shot is of cylinder...
15 This shot is of cylinder bores that have yet to be honed, but notice the color. Troutman says that if a cylinder has been honed too smooth (usually by using a stone that’s too fine) the cylinder bore will be dark gray or black, similar to this.
16 Quality modern rings are...
16 Quality modern rings are lapped, which ensures its roundness. And super-hard chrome faced rings are also largely a thing of the past. Because of this, all those old-school practices of pouring Bon Ami or some other abrasive into the carburetor to help the rings seat are no longer necessary. A proper break-in period is all that should be needed to get a quality set of rings seated.
17 The honing work is complete,...
17 The honing work is complete, and the engine is on its way to be decked. But Troutman offers one last tip: Notice the vertical line down the cylinder wall? That’s a mark left by the dial bore gauge. There’s no damage to the bore, but if you don’t see this line it usually means the machinist didn’t do a final check after his last honing pass. Also, the machinist should check the bottom and top of every cylinder, so that line should extend all the way to the bottom of the cylinder bore. That witness line can sometimes be wiped off if you put some elbow grease to it, so it isn’t a foolproof method of checking behind your machinist—but if you don’t see them, it’s worth asking.