The crankshaft main bores are the foundation of every engine block. The alignment of the main bores is important because their position establishes the centerline of the crankshaft and how the crank aligns with the cylinders, deck surfaces and camshaft. Misalignment in the main bores can cause binding and premature main bearing wear.

Misalignment occurs when the block shifts, usually as a result of thermal cycling over time. The repeated heating and cooling cycles can "relax" the block and create distortions that affect bore alignment. That’s why many racers prefer a "seasoned" block that has already experienced any settling that may occur.

Main bore misalignment and block distortion can also be caused by overheating, prolonged or extreme overloading (as in racing), or even improper torquing of the head bolts. The main bearings will show a wear pattern that gets progressively worse toward the center of the engine with the center main bearings showing the most wear or wiping. The cure is to check the alignment of the bores and the straightness of the crankshaft (one or both may be off). A bent crank should be straightened or replaced. If the bores are off, the main caps can be cut down so the bores can be bored or honed to restore alignment and size.

A block may also have to be align bored or honed if a bearing has spun, if a bearing cap has to be replaced because it is damaged or cracked, or if OEM two-bolt main caps in a block are being replaced with aftermarket four-bolt main caps to strengthen the bottom end of the engine.

Also important is the alignment of the camshaft bore in both pushrod engines and overhead cam (OHC) cylinder heads. Like the crank bores, the cam bores must be straight, parallel to the crankshaft and deck surfaces, and in OHC heads properly positioned with respect to the valves, too.

Alignment Checks

One of the first things that should always be inspected when rebuilding or assembling an engine is the alignment of the crankshaft and camshaft bores. The most common method of doing this is to place a straight edge in the bores and use a feeler gauge to check for misalignment.

How much misalignment is too much? It depends on the engine and the application. Light duty passenger car engine is not as critical as a high revving performance engine or a hard-working diesel engine. As a rule of thumb, most passenger car and light truck engines call for .002? or less of misalignment between all the bores, and .001? or less misalignment between adjacent main bores. For performance engines, you can reduce these maximum tolerances by half or more.

Another dimension to look at is bearing wear. Bore diameters should usually be within .001? of specifications to support the bearings properly, with no more than .001? out-of-roundness if the horizontal dimension is greater than the vertical dimension.

Also check for wear on the thrust bearing surface of the main cap. If worn or damaged, this surface should also be remachined.

Changes In Centerline

When main bores in engine blocks or cam bores in OHC heads with caps are worn excessively, removing the caps, grinding them down and align boring or honing the holes back to size with the caps in place can usually restore the bores. But this will change the centerline of the crankshaft or camshaft slightly, moving it further into the block or head unless corrective measures are taken to prevent this from happening (machining to the desired centerline location rather than relying on mandrels to center the tooling in the bores).

If a block is being align honed, the mandrels that support the honing bar are usually mounted in the undamaged end journals of the block. Centering pins in the middle of the bar are used to center the bar in the center main bore. Stock removal is usually limited to about .003? or less when honing.

With align boring equipment, pilots are used to position the bar. The main advantage of boring is that it can remove more stock and at a faster rate. It can also be done dry (honing requires oil or a lubricant). Changing the position of the bar will change the centerline of the bores and crankshaft.

Many engines can handle a few thousandths variation in the position of the crankshaft centerline but others cannot because of changes it causes in other critical dimensions such as the deck height of the pistons when the crank is at top dead center (which affects compression, piston-to-head clearance and valve-to-piston clearance). Performance engines and diesels are much more sensitive to centerline changes than light duty passenger car engines.

Because main bore alignment is so important, it should be the first thing that is machined on an engine. And it must be done accurately because most of the other critical dimensions center off the crankshaft.

OHC Heads

Warped heads are a common problem in many OHC engines, often as a result of overheating. Distortion is usually worse in the middle of the head where thermal stresses are greatest. A warped head throws the cam bores out of alignment and may cause the camshaft to bind. This can chew up the cam bores and journals, and may even cause the cam to seize or break. If a cam won’t turn freely in a head once the valve springs have been compressed or the followers removed, the head is probably warped and the camshaft may be bent. Check bore alignment and camshaft straightness to verify the problem, then repair or replace as needed to restore camshaft bore alignment.

OHC cam bores can also be damaged by lubrication problems. The cam journals are a long ways from the oil pump, and on cold winter mornings it may take the oil awhile to reach the journals. If the oil is too thick or if it has been neglected, oil starvation or loss of film strength may cause the journals to wipe or seize.

Damaged OHC journals with caps can be restored the same way as damaged main bores in a block by removing the caps, grinding them down and remachining the bores with the caps reinstalled. If the heads have no journal caps, the bores can be cut to oversize to accept bearing inserts or a new cam with oversized journals (if available).

Other repair options include finding a different core to rebuild, replacing the head altogether with a new OEM or aftermarket casting, or building up the worn or damaged cam bores by TIG welding and machining back to size. Finding good cores on some engines is nearly impossible, and replacing heads is expensive. That’s why OHC align boring has become an economical repair alternative for repairing and rebuilding late model OHC heads.

Warped OHC heads should first be straightened before align boring. Heads can be straightened by bolting to a beefy hunk of steel and using shims as needed to counter the distortion. The head should be placed in an oven at 450° to 500° F for six hours and allowed to slow cool for several hours afterwards. Straightening can greatly reduce or even eliminate the amount of metal that has to be removed to realign the cam bores and/or resurface the head.

Other methods of straightening aluminum heads include cold pressing the head (risky because you might crack or break the head), TIG welding on the top of the head to pull it flat, and spot heating with a rosebud torch to counter the distortion.

After OHC cam holes have been align bored, they should be finished by honing. Brush Research makes a special "Flex-Hone Tool" for this purpose (BC 1" 600SC 120MM AP x 26" OAL Flex-Hone Tool). The long narrow brush uses 600 grit silicon carbide beads and is designed to pass through two journals at a time. The brush does not remove stock but deburs the surface, removes sharp edges on the journals and oil holes, and improves the overall surface finish.

Machining Tips

When a block is align bored or honed, the main caps must be installed with the bolts or studs tightened to the recommended specifications. The oil pump should also be installed as this, too, may create some distortion in the main bores when the bolts are tightened down. Some racers may even bolt torque plates onto the top of the engine to simulate any additional distortion that may occur in the block when the heads are installed.

According to "Sunnen’s Complete Cylinder Head and Engine Rebuilding Handbook," the first thing you should do is measure all the bores (caps installed) and record their sizes. Next, remove the main caps, measure each one with a "cap checker" and write down the indicator reading for each cap. The cap checker is the only way to compare the radial distance and verify that you remove the correct amount of stock from each cap. Then grind the caps.

All housing bore diameters should be the exact same undersize dimension (.003? from the minimum housing bore diameter) before you align hone or bore the block. So after the caps have been reinstalled and tightened, remeasure the housing bores to make sure they all have the same dimensions. If everything is okay, select the appropriate mandrels for the housing bore diameter (if align honing), or pilots (if boring), install in the end caps and finish the setup as required depending on the type of equipment you are using.

One tip for machining bores on performance engines where aftermarket four-bolt main caps are being installed is to machine the bores in three-steps. First, bore the housings to within .030? of the desired size. Then bore again to within .005? of final size, and finish to size by align honing. Harder honing stones work best on cast iron while softer stones (such as J45 silicon carbide #150 grit) do better on bimetal applications where the block is aluminum and the main caps are cast iron, steel or powder metal.

Dennis Elmore of Speed-O-Motive in West Covina, CA, uses a Rottler F65A multipurpose machining center to do his align boring work. "We build about 600 to 700 performance motors a year for drag racing, street and circle track. We do a lot of small block and big block Chevys, a lot of 383 SB Chevy stroker motors. We use align boring because it is an accurate process that gives us more control than align honing. With honing, the stones just follow the hole that is already there. You can’t make corrections, and if you take too much metal out of the block it loosens up the timing chain. So we align bore and just touch the block. We machine most of the metal out of the caps as it should be done," says Elmore.

Do It Vertically

One of the things you have to watch out for when align boring or honing with a horizontal bar is bar sag. Because the bar is only supported at the ends, some sag may occur that affects the accuracy of the machining operation. One way to avoid this is to bore the block vertically rather than horizontally using special equipment designed for this purpose.

Randy Taylor of DCM Tech, Winona, MN, says his new "Vertical Align Boring" (VAB) machine eliminates bar sag altogether. The block is mounted upright in a vertical position so the bar is working in a vertical position rather than a horizontal position.

"It’s like pulling a rope up through a hole. There’s no bending or flexing because the bar cuts from the bottom up to improve accuracy,” says Taylor. The bar is 1-1/4? solid carbide steel and is very stiff.

Taylor says many align boring machines take up to 45 minutes to set up. The VAB machine can cycle floor-to-floor in as little as 15 minutes. The machine uses a single point fixed cutter, and can use the crankshaft centerline or the front or rear journals to cut holes offset if necessary. The machine can handle engine blocks or OHC cylinder heads, and has a relatively small footprint that takes up less floor space in the shop than traditional align boring or honing machines.

DCM has an exclusive marketing agreement with Sunnen to sell their VAB equipment. Taylor says a completely tooled machine sells for around $15,600. For OHC capability, add another $2,300.

Tim Mera of Sunnen Products Co. of St. Louis, MO, says Sunnen also sells a portable align boring machine, the POB100, that is often used by performance engine builders who are installing aftermarket main caps. The machine sells for around $6,000. The POB1200 version of this machine can also do cam bearings for oversize bearings or roller cam bearings.

"Except for the Vertical Align Boring system, align boring and honing hasn’t changed much over the years. Many people are still using the same basic equipment and mandrels. One improvement we’ve made is to offer bronze guides for longer shoe life. The most important thing is getting the mandrels trued in before you start," says Mera.

More Boring Equipment

Anthony Usher of Rottler Mfg. in Kent, WA, says Rottler has two machine models for line boring automotive and diesel engines: the F80 and F65 series multipurpose machining centers. The F80 machines can handle blocks up to large earth moving and natural gas compression engines such as CAT and Waukesha, while the F65 machines are designed for smaller automotive engines and ideal for performance racing work," Usher explains.

"Boring is easier compared to honing as the boring tool is set to final size and all the main bores can be machined in one automatic cycle, tolerance and repeatability is within .0001? to .0002? maximum,” explains Usher. “The alignment of the bores is perfect, eliminating errors such as bar sag associated with bar type machines. The cutting tool is supported by precision roller bearings resulting in excellent surface finish and round holes."

Usher says one thing that is great with Rottler’s system is that every bore is the exact same size and perfectly round, "so when the customer measures his block, he is always very happy with the accuracy and quality."

"Large diesel blocks such as a CAT3516 V16, which is nearly 100 inches long, can be set up and line bored complete in less than 2 hours, floor-to-floor," says Usher. "This type of job can take one to two days on conventional manual operated machines. And to further help the whole process, the machine works unattended while boring the complete line allowing the operator to walk away and attend to other work while the machine completes the automatic cycle.

When line boring performance blocks, the crankshaft to camshaft center-to-center distance can be accurately controlled and actually measured and set on the machine as high performance timing chains and belt drive conversions require precise distances. Rottler recently introduced a new design set-up fixture which can handle OHC and in-line blocks. Usher says some customers are using this set up so that they can line bore the main and cam lines in one set up resulting in perfect parallelism between both lines. Speed-O-Motive uses this system to main line bore and machine clearance for stroker crankshafts in one set up. The F65 system allows line bore jobs to be completed in less than 30 minutes floor-to-floor.

Tom DeBlasis of Silver Seal Products, Trenton, MI, says his company makes a portable OHC boring bar system that can be used on a wide variety of domestic and import OHC cylinder heads. The full-blown package has a jobber cost of $3,600, but the basic setup (bar, cabinet, bushings & cutters) can be purchased for under $1,000. The system uses a hand drill to power the bar. Different sized cutters and bushings are required for different OHC applications. DeBlasis says his boring bar is quick and easy to use, and makes more economic sense for small shops who can’t afford a large align boring machine.

BHJ Products in Newark, CA, makes a "CTA1" Cam Tunnel Alignment Fixture for setting up align boring equipment. The setup fixture is used with existing align boring equipment to establish the location of the boring bar in the block. Al Cabral of BHJ says they also sell a line boring bar and fixture that can be used with an existing milling machine. "This approach is cheaper than buying a dedicated line boring machine. It can do main bores and cam bores in blocks but not cam bores in OHC heads."

Ray Meyer of RMC Engine Rebuilding Equipment in Saginaw, MI, says his company has a new portable align boring system for doing main bores and cam bores in engine blocks. It can be used with a Bridgeport mill or any other machine that can accept a right angle power adapter. Its best feature, says Meyer, is the ability to maintain set distances between the crankshaft and camshaft centerlines. It uses a hanging rod to set the center-to-center distance. The price is $8,995.

Meyer says align boring is growing in popularity. "We’re getting more calls for align boring systems today, mostly from performance engine builders." He estimates about 70 percent of the market for align boring equipment is performance engine builders with the remaining 30 percent being restoration work. "You can save a lot of hard-to-find blocks with align boring."

Meyer says he hasn’t seen much demand yet for OHC align boring equipment because many production engine rebuilders are offering complete OHC heads ready-to-go in exchange for old heads. This has reduced the need for small shops to align bore OHC heads.

Dale Feller at Winona Van Norman, Wichita, KS, says his company has been selling the same align honing equipment for years – Italian-made combination machines that sell for around $20,000 to $25,000 and can be equipped to do blocks or OHC heads.

Ed Vaughan of Peterson Machine Tool/Kansas Instruments in Shawnee Mission, KS, says Peterson sells several align boring machines made by Berco in Italy. The "BT-6," which sells for $14,995, is more of an OHC head boring machine, but it can also do blocks. The larger BC4 and BC5 machines can do both but are primarily designed to do blocks. The machines use inserted tools and set up is simple with dual indicator alignment. Vaughan says with this equipment honing is not necessary after boring. "When the hole is bored, it’s done," he says.