Читать книгу Modern Engine Blueprinting Techniques - Mike Mavrigian - Страница 9
ОглавлениеTo blueprint an engine block, you must accurize it. You must achieve the proper bore dimensions, but also must consider the total geometric state of the block and correct any deviations from ideal geometry. This means that you inspect and machine in order to optimize the block geometry. This primarily focuses on the main bore, cylinder bores, lifter bores, and decks.
An OEM (original equipment manufacturer) mass-produced, cast block typically requires more corrective measures in a blueprinting approach, simply due to the nature of wide-tolerance mass-production practices. By contrast, today’s high-quality aftermarket performance blocks, such as those produced by Dart, World Products, RHS, Brodix, and others have CNC-machined bore locations and angles from the very start. In essence, they are already “trued,” but still need to be checked, typically requiring only deck height and bore diameter machining to the required sizes. OEM blocks, on the other hand, generally require more extensive machining in order to achieve the design geometry.
Aftermarket blocks from Dart, World Products, Brodix, and others are CNC machined to much tighter tolerances and quality standards than cast OEM blocks. (Photo Courtesy Dart Machinery)
Generally speaking, leading-brand aftermarket high-performance blocks are made with superior materials and are made in lower numbers than OEM blocks, and therefore benefit from a higher level of attention to detail. An aftermarket block costs more, but you’ll likely be faced with less corrective machining labor.
The manufacturing process for OEM blocks have time and technology constraints. As a result, factory blocks have a range of tolerances that can and usually do deviate from the initial design. In general, the older the block, the wider the OEM tolerance ranges are. Late-model blocks generally have tighter machining tolerances thanks to advanced CNC technology. However, OEM blocks are produced in mass quantities so, considering machine calibration and tooling wear factors, you’d still be hard-pressed to find an OEM block that’s precisely accurate to the design specifications. That’s not to say that they’re unusable, but they can always be corrected according to the design specifications and performance and durability can be improved.
This drawing of an LS block clearly shows how the main bore centerline is used as the primary reference point. (Illustration Courtesy General Motors)
To accurize the block, make sure that the main bore is not only straight and round, but also exactly in plane front to rear, and corrects (where needed) the main bore height location. When accurizing the block, you are correcting all critical areas to eliminate the tolerance range that is otherwise deemed acceptable in mass-production runs.
When the average enthusiast claims that his or her engine has been “balanced and blueprinted,” in reality it usually means that the crank has been balanced. True blueprinting involves much more attention to detail (and subsequently higher machining/prep costs) than merely honing the cylinders, performing a fresh cut of the decks, and balancing the rotating assembly. When someone claims that his engine was blueprinted, and, when asked how much he spent, he or she mentions something in the area of, say, $1,000 to $3,000, it’s obvious that a blueprint has not been performed. Blueprinting requires skill and expertise, and is very time and labor intensive. That doesn’t come cheap.
Before any machining takes place, flaw check the block to make sure that no cracks are present. Any cracks or damage must be addressed by repairing the damage or finding another block. Also soncially test each cylinder wall to verify wall thickness and determine whether it can be overbored. This needs to be done before any machining and crack repair is done.
Minimum acceptable wall thickness may vary depending on the block, but in general, you should have at least .200-inch thickness after boring and honing. If the walls are too thin, they can distort enough to result in ring blow-by or even eventual cracking. Keep in mind that a too-thin cylinder wall may be corrected by overboring and installing a quality cylinder sleeve (again, this can vary depending on the type of block).
Once the block has been deemed serviceable, the first order of business involves checking and correcting the main bore if needed. In addition to making the main bore straight and round, it needs to be centered per its design. Specialty guide fixtures are available, but today, with the increasingly common use of CNC machining technology, the appropriate software can handle this duty. Once the main bore is verified as correct, all remaining machining basically uses the main bore centerline as the reference point.
When the block has been deemed serviceable, the first order of business involves checking and correcting the main bore if needed. Or, the block can be machined to correct any anomalies on a CNC-milling machine (with generic programs for specific blocks, bore sizes, and deck heights, or by custom-programming by the CNC operator). Accurizing fixtures are essentially precision templates, or guides, that establish proper geometric centerline locations and angles. If a shop isn’t equipped with a CNC machine, these fixtures allow precision machining using traditional shop equipment boring machines, multi-axis milling machines, etc.
The sensor probe is slowly run along the cylinder wall (in a number of clock positions and from top to bottom) while monitoring the gauge readout.
Before measuring and/or align honing the mains, all caps must be fully installed (at spec’d torque value). If the block has additional side bolts (as in the GM LS block, or vintage-Ford FE side-oiler block, for instance), these must be installed as well. Fully tighten the primary main cap fasteners before tightening the cap side bolts. This is critical: The side bolts must be fully tightened to spec before measuring or honing, since they do affect main bore geometry.
If you plan to use main studs (instead of bolts), install the studs finger tight, then add a small bit of preload. Be sure to follow the stud maker’s specs! Do not overtighten the studs into the block. There’s no need to (since nut tightening achieves clamping load), and by overtightening studs, you can easily create a splayed stud problem. For instance, do not double-nut the studs and try to tighten to the main cap torque spec.
Block Inspection Check List
Here are the steps to perform a flaw detection to check for cracks:.
Use a sonic tester to measure and record all cylinder wall thicknesses. A minimum of three height locations and at four clock positions should be used. Record the data and use it for proper overboring.
Inspect and correct, if necessary, every threaded hole in the block for thread integrity and condition.
Align hone the block. This creates the centerline that everything else references.
Cut the decks to obtain a square block, where both decks are the same height from the main bore centerline and parallel to the main bore centerline.
Measure and correct cylinder bore spacing and angle. When correction is necessary, perform an overbore or overbore correction sleeve installation. The sleeves are resized to original or planned bore diameter.
Measure and correct camshaft bore centerline. If correction is required, overbore the cam tunnel and use the necessary oversize-OD cam bearings.
Check and correct all lifter bores for centerline location and angle. Correction may require overboring, installation of bronze liners, and resizing for proper lifter clearance.
Utilizing specialty main bore and cam bore inserts (various diameters are available for specific blocks) and a dedicated micrometer, it’s easy to accurately measure the distance between main bore and camshaft-bore centerline distance (measuring from the outside of each bore adapter and subtracting one-half of each guide’s nose diameter). (Photo Courtesy BHJ Products)
Production engine block decks may be off specification by a few thousandths of an inch; in some, by as much as hundreds of thousandths. This means that both decks might be too high or too low or one deck may be higher than the opposite deck. Here’s a case in point: A recent Pontiac 455 build started with an aged 1973 455 block. Although the OEM spec for deck height is listed as 10.210 inches, this block’s left deck measured 10.2545 inches, which is .0445 inch taller than spec. The right deck measured 10.2456 inches, which is .0356 inch taller than spec. Luckily, the excess material allowed correction precisely to the desired 10.210-inch spec. In this case, both decks were too high, and had a different deck height at each bank. As is, the OEM excess in deck height decreases compression ratio. Also, the uneven heights from left to right and front to rear results in different compression ratios per cylinder and from bank to bank. Yes, the engine would run, but it wouldn’t be running at optimum efficiency.
Keep this important point in mind: The OEM spec for block deck height should not be considered set in stone unless all other OEM engine dimensions and materials are used. If so, you must use the exact same crank stroke, the same connecting rod length and material, the same piston compression height and volume, the same thickness and material of head gasket, and the same volume and shape of cylinder head combustion chamber. Be sure to use the same intake and exhaust valve diameter and length and the same rocker arm ratio as specified in the OEM engine. You also mock the short block together (crank, rods, and pistons) to measure the pistons’ top deck location at top dead center (TDC) relative to the block deck. If the deck height is too short, the pistons can hit the heads. If the deck height is too tall, you decrease compression.
When test fitting, be sure to consider the installed (crushed) thickness of the head gaskets. Be sure to use exactly the same brand and type of head gasket for test fitting that you plan to use during final assembly.
You want to be as close as possible to zero clearance between the piston dome and cylinder head during engine operation. However, obtain static clearances, because the clearances decrease as the engine operates because of heat expansion and metallurgical stress. The minimum clearances you should generally have in static state (with the engine not running) between the piston and combustion chamber at TDC are:
• Performance street application with steel connecting rods: about .040 inch
• Racing engine with steel rods: about .045 inch
• Racing engine with aluminum rods: about .060 inch
Remember that these approximated clearances are minimum clearances.
With this in mind, it’s important to understand how block deck height relates to your crank, rod, and piston dimensions. In order to achieve a zero deck height (so the piston lies flush with the deck at TDC) consider crankshaft stroke, connecting rod center-to-center length, and piston compression distance.
A crankshaft’s stroke refers to its total stroke from a rod journal at TDC all the way to that journal’s bottom dead center (BDC). When determining where the top of the piston deck will be at TDC, consider one half of the crank stroke because you’re only concerned here with how far the crank is pushing the piston upward. Connecting rod length refers to the distance from the big-end centerline to the small-end centerline, not the overall rod length. Piston compression distance (CD) refers to the distance from the centerline of the piston pin bore to the piston’s flat deck surface.
A CNC cutter spot faces lifter bore roofs in preparation of lifter bore centerline correction. With a programmed CNC machine, no indexing fixtures are needed for lifter bore correction.
Here’s the formula for finding block deck height at zero deck:
1/2 crank stroke + rod length + piston CD
For example, the crank stroke is 3.500 inches, the rod length is 6.000 inches, and piston CD is 1.500 inches. When you plug the numbers into the formula, you get a 9.25-inch deck height.
(3.500 ÷ 2) + 6.000 + 1.500
1.750 + 6.000 + 1.500
9.25
If you want the piston top flat to be, say, .015 inch below deck, you add .015 inch. In the above example, the finished block deck height needs to be 9.265 inches. The decks may also be out of plane. You could have decks that are low at the front and high at the rear, high at the front and low at the rear, low inboard or low outboard, etc. In other words, decks may be flat, but they might be “crooked.” For corrective machining, you need to index from the crankshaft main bore centerline to make both decks the same height (and the proper height) and 90 degrees to the main bore centerline in both front/rear and inboard/outboard planes.
A blueprint illustration reveals all of the block dimensions according to the engineering design. Some points of reference may or may not be correctible, but a factory spec drawing allows you to double-check many critical dimensions to verify various areas of your OEM block. (Illustration Courtesy General Motors)
Obviously, the cylinder bores must be round and the correct diameter for the intended pistons and rings. Also the centerline placement of each bore and the angle of each bore must be accurate. If not, they must be corrected in order to blueprint the block. When OEM blocks are made, mass production line tolerances may allow for the bore centerline to be placed slightly off-center, and the angle of the bores may be slightly offset from front/rear and inboard/outboard, which slightly deviates from the original engineering design.
Corrective machining, which is covered in Chapter 3, can relocate the cylinder bores to achieve exact centerline location and cylinder wall angle. As a result, removing material in order to accomplish this means you use oversize pistons and rings. But because most performance builds involve increasing displacement anyway, this is a moot point. The same holds true for lifter bores in overhead valve engines. The lifter bores may also be corrected, relocating their centerlines and angles. Oversizing the lifter bores in order to make these corrections simply means that bronze lifter bore liners are then installed and honed to the required diameter.
Machining decks to the proper height and angles equalizes and angle-corrects the base for cylinder volume. But you also equalize crank stroke, rod length, piston compression height, piston dome volume, and cylinder head chamber volume. By correcting cylinder bore centerline (and bore spacing), you place the bores and pistons in a no-compromise, as-designed, on-center travel path, which reduces operating friction and stresses. By correcting lifter bore centerline location and bore angles, you improve valvetrain efficiency. It all boils down to reducing friction and wasted energy, which translates into better performance and longer engine life.
Core shift commonly occurs during the casting process at the manufacturing level. It’s not uncommon for cylinder bores to shift slightly beyond engineering specification. This is one of the primary reasons for thoroughly accurizing the block by determining where these shifts have occurred and machining to correct these out-of-spec centerlines. Out-of-spec tolerances may pass the manufacturer’s acceptance range for common street applications, but for high performance and racing you want to correct these issues in order to achieve a high level of precision, both to accommodate increased horsepower and to aid in engine longevity.
The purpose of oil restrictors is to reduce the amount of oil at the top end of the engine, so more oil is delivered to the rod and main bearings. These restrictors are threaded plugs that are installed in the oil passages, which feed oil to the lifters, valves, springs, and rockers. Windage drag, which is drainback oil accumulating on the crank counterweights, is also reduced. Opinions vary with regard to restrictors, and the need for them also varies depending on the type of engine.
In general, restrictors may be used with solid (mechanical) roller lifters, but should not be used with hydraulic lifters or with flat-tappet lifters, since they both need more lubrication. Also, while restricting oil to the upper end may be fine with full-roller rockers, stock OEM ball/pivot rockers need more oil delivery, so don’t install restrictors if you’re using ball-type rockers.
Depending on the type of engine, restrictors may be installed at the rear of the block. These are installed deep inside the lifter-valley oil gallery holes or in the lifter valley. In either case, thread tapping is required, so if restrictors are planned, the tapping and subsequent cleaning must be done prior to final wash and block assembly. The size of the oil holes in restrictors varies from about .040 to .065 inch. Again, this depends upon the application and is quite often based on the opinion and experience of the block machinist/engine builder.
The large oil drainback holes in the lifter valley of a V-type block obviously allow oil that was delivered to the upper end to drain back to the sump. However, if something goes awry at the top end, such as valvespring failure, valve failure, keepers falling loose, etc., the resulting fragments can travel across the lifter valley and drop into the oil sump. To avoid this, it’s common for race engine builders to epoxy a piece of screen over these drainback holes. If you decide to do this, make sure that the block surface is clean and dry to provide good adhesion for the epoxy. If the screen pops loose, it can cause damage by interfering with lifters or by working its way down to the cam.
Many OEM blocks have ragged, unfinished drainback holes and/or slots that have jagged casting flash edges. It’s always a good idea to grind these edges smooth, not for the sake of appearance, but to ensure that no flashing pieces break loose in the future.
Residual oil that has done its job at the upper end needs to return to the sump in a timely manner. Because older cast-iron blocks commonly had fairly rough, raw casting surfaces in the valley, it was/is common practice to smooth out the surfaces or coat the valley with a high-build paint to fill the casting pores and promote faster drainback. Today’s castings, especially quality aftermarket blocks, tend to have finely finished surfaces. In the case of the GM LS engine, there is no lifter valley.
If you decide to apply a coating to your lifter valley, the block must be absolutely clean and dry (serious hot-tank wash). The most commonly used coatings include Glyptal, an electrical armature winding coating, which is highly resistant to oils and acids. All machined surfaces, threaded holes, lifter bores, etc., must be plugged or masked to avoid paint contamination. In my and many others’ opinion, the time and effort required as well as the risk of dried paint particles breaking loose just isn’t worth it, considering the small increased rate of oil return. You’re better off by simply grinding/smoothing any noticeable surface defects/lumps, sharp edges, etc., and ignore the paint idea.
Whenever you deviate from “stock,” in crankshaft and rod selection, you must check for rotating and reciprocating clearance at the block. Specifically, if you choose a longer crankshaft stroke and/or beefier aftermarket connecting rods, clearance checking is an absolute must. The common clearance issues occur at the bottom of the cylinders and at the oil pan rail areas. Mock-install the crankshaft to the block with clean and lubricated main bearings, and using the same main caps that you plan to use in the final build. For interference checking, there’s really no reason to fully torque the main caps at this time. Simply snug them to about 20 ft-lbs.
Slowly rotate the crank and, starting from the front, watch each crank counterweight as it moves close to the block (near the pan rails). You should have about .060 inch clearance between the counterweights and the block at the tightest locations. If you see component contact or too tight of a clearance, mark the block location using an ink or paint marker. After all counterweight clearances have been checked, remove the main caps, crank, and bearings; then remove block material as needed with a high-speed die grinder. Clean off the metal particles and re-install the crank and perform the clearance check again to verify.
When test fitting the crankshaft (with main bearings installed and lubed), check crank counterweight-to-block clearance, especially when using a stroker crank. This increased-stroke crank in a Pontiac 455 block had a fairly tight clearance between the forward area of the front counterweight and the block, just above the oil-pan rail. Once the crank was removed, hand grinding removed a small bit of material from the block to achieve a comfy .050-inch clearance at this tight spot.
During this test fit, you can see that the rod bolt is contacting the block. The area to be relieved was marked as a reference. The interference area of the block must be relieved to obtain a bare minimum of about .060-inch clearance.
Aftermarket performance blocks commonly provide a bit of additional rod clearance (more than an OEM block), since the block manufacturer is aware of the possibility that a longer stroke crank may be used. This Dart 351W block already has a convenient notch for big-end rod clearance. Depending on your specific crank and rods, some additional clearancing may still be needed, but it is minimal.
If using a stroker crank and/or a high-lift camshaft, don’t forget to check for possible rod-to-cam lobe interference. This photo (taken without cam bearings or cam) helps to illustrate the potential for clearance issues. With cam bearings, camshaft, crank and rods, and timing gear test installed, carefully rotate the crank while observing cam lobe clearance. It may be difficult to see, but possible with a skinny inspection light. If clearance is a problem, you may be able to remove material from the rod big ends or move to a cam with lower lobe height. This is why aftermarket blocks are often available featuring a raised cam tunnel.
Once you’re sure that the crank clears the block properly, install one rod/piston set, starting with the number-1 cylinder. There is no need to install rings at this time.
Slowly rotate the crank, check for the rod big-end clearance to the block during a full-stroke travel. Rotate the crank a full 360 degrees. Again, minimum clearance of the rod to the block at any given point should be about .060 inch. Typically, if you do find a rod clearance issue, it is likely in the rod bolt/rod cap shoulder locations. Don’t be tempted to grind material from the rods; remove material (if needed) from the block.
Also, check the clearance between the crank counterweights and the bottom of the piston skirts. If clearancing is needed, the crank counterweights can be lathe-machined (prior to crank balancing). Mock-install the camshaft and timing assembly and check the rod big end clearance to the camshaft, especially if a high-lift cam is used (this is the reason that aftermarket performance/race block manufacturers often offer a “raised cam” block, which places the cam bore higher, farther away from the crank centerline). The cam bearings must be in place in order to properly center the cam in its bore.
After carefully removing block material with a handheld electric grinder (and after washing the block and reinstalling the crank and rod), clearance was again checked. It is a more-than-healthy clearance of approximately .080 inch. When checking rod clearance, you must be careful to rotate the crank slowly as you observe the rod approaching the block. In some cases (depending on the block), only a couple of spots may require clearancing. In some cases you can get lucky, with no additional clearancing needed, or you may need to provide clearance for the majority of rod-to-block areas. Just take your time and check each and every location.
Never assume anything. Just because the block’s main bearing saddles have oil feed holes, it doesn’t necessarily mean that these holes properly align with the oil holes in the upper main bearings. Test fit the new upper main bearings to the block and check for oil hole alignment. If necessary, the oil holes in the upper bearings may be enlarged to achieve correct alignment. It’s easier and quicker to do this than enlarging the saddle holes. Hole enlargement must be performed carefully to avoid damage to the bearing, and make sure that the new hole is carefully deburred to remove any high spots on either side of the bearing shell.
Whether you’re modifying the upper bearings or not, once you obtain correct oil hole alignment with each upper bearing, mark each bearing for block location (number-1, -2, etc.) so that each bearing is installed in the same location during assembly. Mark each bearing backside with a felt-tip marker, or place each bearing in a clearly labeled plastic bag. In addition to checking for oil hole alignment, run a small-diameter rifle brush through each main saddle oil passage to make sure that there are no obstructions.
Check each threaded hole in the block for thread cleanliness and integrity, especially on a used block. A hole with contaminated or burred threads prevents you from achieving proper torque values and clamping loads during assembly. Prior to, during, and after block cleaning/washing, run the appropriate-size rifle brush through each threaded hole, along with a hot soapy water solution, followed by a rinse and a blow-out with clean compressed air.
When checking or correcting slightly burred threads, do not use traditional cutting taps; they are designed for removing metal. Instead, and especially on the most critical threaded holes, such as cylinder head holes in the block decks and main cap bolt holes, use a dedicated “chaser” tap. Chaser taps are designed to re-form threads instead of cutting threads. These taps are available through quality automotive machine shop supply companies such as Goodson Tools. Using a standard cutting tap, depending on the condition of the existing threads, can remove enough material to weaken the threads.
Threaded Inserts
If any less-critical threaded holes are stripped, cross-threaded, etc., use quality thread inserts to repair the hole. In basic terms, there are two common types of thread inserts: helically wound stainless steel (Heli Coil and other brands of the same style) and solid steel or stainless steel. In either case, first drill the damaged hole oversize following the spec provided with the intended repair insert. Then tap the hole, using the specific cutting tap supplied with the thread repair kit. Screw the thread repair insert into the newly tapped hole.
Install a helically wound insert using a specialty driver tool supplied in the kit. When fully installed with the top threads located immediately below the top surface, remove the driver tang at the bottom of the insert. The insert has a driver tang that’s notched, which provides a stress point. A quick tap with a skinny, flat punch knocks the tang loose, which must then be removed from the hole. Install a solid-wall insert that has internal and external threads.
Solid inserts, depending on style, may or may not require application of a thread-locking compound to prevent it from backing out. Other styles may require staking at the top using a small, sharp punch.
There’s nothing wrong with saving damaged threaded holes with inserts. Sensible applications include mountings for water pumps, oil pan, bellhousing, timing covers, etc. Thread-repair inserts also offer good methods of saving otherwise damaged holes for areas such as valve cover bolt holes, exhaust manifold/header flange bolt holes, intake manifold bolt holes, and spark plug holes.
High-tech threaded inserts are also available for high-stress areas, such as cylinder head bolt holes. Lock-n-Stitch makes reverse-angle-pitch threaded inserts that apply greater clamping force between the insert and bolt hole walls. This type of insert can actually result in stronger threaded connections than the original tapped holes.
The only real debate regarding thread inserts involves using dissimilar metals (for example, stainless steel inert in an aluminum cylinder head, especially in spark plug holes). Some builders claim that dissimilar metals can result in different rates of expansion and contraction during operating temperatures. My own experience with threaded inserts has always been successful. Others claim to have experienced problems with fastener loosening or spark plug performance. My opinion is that stainless steel inserts in aluminum components is a good idea, since the harder stainless steel material (as opposed to the parent aluminum) greatly helps to avoid galling or thread stripping issues.
