Align Honing – Engine Professional

Tips on achieving main bore alignment

By Mike Mavrigian with Sunnen

The crankshaft relies on a free-spinning path for smooth rotation and proper bearing oil clearance. When main bores are not in perfect alignment, the potential arises for uneven bearing clearances and bearing damage, isolated hot spots due to added friction and parasitic power loss.

Engine block main bores may require corrective attention for a number of reasons: the main bores may have shifter during use, due to casting warpage, excessive stress, and/or engine over-heating. Main bores may have been damaged as a result of main cap fracture, crankshaft failure or main bearing failure. If stress, overheat or other damage is evident, consider upgrading to stronger billet main caps. When main bore service is required, this may involve either align boring or align honing. The term “align” refers to obtaining a straight, consistent bore centerline from front to rear of the main bore.

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What’s the difference between align boring and align honing? Align boring is required when main bore damage is extensive or when the main cap to block is badly mismatched. Align honing is used when minor issues are present, such as bores being slightly oversized, individual bores are tapered, or when the bore is out of round. Regardless of the need for align boring or align honing, the goal is to return the main bores to the original/correct bore size in order to accommodate the desired crankshaft main journal bearing oil clearance.

Both align boring and align honing approaches will move the crankshaft centerline up…slightly closer to the camshaft centerline. When the main cap and block saddle are equally radiused, where material is removed from both the main cap and block saddle, this is a “centralized” main bore, where the parting line is at the 3 o’clock and 9 o’clock positions. As a result, depending on the amount of decreased crank to cam centerline distance, this may require the use of a slightly shorter timing chain, or larger timing sprockets.

INSPECTION
In order to determine the need and extent for corrective action, the block and main caps must first be thoroughly cleaned. Inspect the block and caps for cracks. Examine the main caps for signs of fretting, which would indicate that the cap(s) have been moving in relation to the block (this may be due to crankshaft flex). Fretting damage involves metal being transferred from the block to cap or cap to block. Engines that have experienced severe operation such as heavy loads or detonation can be prone to fretting damage. Check that the caps all fit tightly in the block. If caps fit loosely, the cap sides may be knurled to achieve a tight fit, or replaced with new billet caps that do fit tightly.

Start with first measuring the original main bore. This will determine how much to remove from each main cap. To begin, mark each cap with the amount of material you wish to remove. The first step is to cut one face of each main cap. Only cut enough to clean up a three-point surface contact. This will then be the side that is placed against the back fence of the clamping fixture. This will assure that the caps will be cut at 90 degrees. We recommend a 0.003-0.005″ reduction. This will drop the centerline of the crank centerline a miniscule 0.0015-0.0025″. To save time, we recommend the use of the CCG-100 cap checker. This allows you to check the amount of stock removed from the cap without retorquing it onto the engine block (to potentially find that you did not remove enough). TIP: On wide rear main caps such as small block and big block Chevy blocks, remove 0.001″ less than on the other main caps. For example, if caps 1 through 4 are -0.004″, then cut only 0.003″ from the rear main cap.

TRUING THE MANDREL
Truing of the align hone mandrel is important. Since stones on this mandrel will last for honing 300 to 400 blocks, truing should not be a problem unless you have a wreck, forget to cut the rear seal area on a 400 Chevy small block, or don’t over-stroke properly.
Here are a few basics to make an align hone job go smoothly: If a new mandrel is not properly trued in, you won’t be able to achieve a round hole or end up with a main bore that is free of taper.

Obtain a junk block or a piece of steel tube that is approximately the same length of a typical V8 engine block. You shouldn’t be able to move stones or shoes forward and backwards in the mandrel. If stones and shoes are loose, make sure that the tension blocks are snug and then tap them lightly with a brass rod and hammer. Then tighten the tension block completely. When installing the CH-75 driver on the RY mandrel, be sure that the two drive lugs on the back plate engage with the mandrel. If you fail to do this, the mandrel will be hard to push and will get easier when you pull during honing. This means that you’re actually driving the mandrel with the wedge, causing a multitude of problems.

All main caps must be cut so that all main bores have the same undersize (within 0.0005″). If the rear main is wider than the others (as with small block Chevy), it can be left 0.001″ larger than the rest. For the purpose of truing the mandrel, the caps can be cut 0.006″. The large amount of resulting out of round will help with the truing process.

Place the engine block on the honing machine with the rear main faces the drill motor. Slide the mandrel into the block, setting the overstroke. This is critical to obtain a straight bore.
When doing a three main block, the stones and shoes should never pass completely through the opposite end main bore. For a block that has more than three mains, pass the edge of the stones and shoes halfway between the first and second main housings on each end of the block, which should ensure a straight bore. If you are having trouble getting taper in one end, you can adjust the overstroke accordingly. You can remove taper by adjusting overstroke. To correct an “hour glass” issue, decrease overstroke. To correct a “barrel” shape, increase overstroke. For example, if the block is tight (smaller) on the drill end, pull the mandrel further out of the block and push the mandrel a shorter distance into the block.

Keep in mind that today’s V6 and inline-4 blocks are too short for the standard 10-stone mandrel. For these applications, use an 8-stone mandrel. Too many stones and shoes will cause you to fight taper and bellmouthing. A general rule of thumb is one stone for every two inches of block length.

Centering the mandrel in the main bore is critical for achieving perfect roundness. Cross holes in the mandrel body are featured to insert dowel pins. The mandrel should be placed in the main bore with the center cap removed. Then locate the mandrel and pins so that they are resting in the parting line of the center main bore saddle. Expand the shoes until the mandrel is solid in the bore, with no rocking. Make sure that the pins are still resting on the parting line. Then remove the pins and install the center main cap. This method of centering takes for granted that the parting line is the centerline of the bore. This is not always the case (in Cummins blocks for example) or if a block may have been previously line bored. To insure proper centering, especially in racing applications, you should double check mandrel centering with a dial indicator. This can only be done near the drive end of the mandrel between the stone and stone wedge slots and the driver. The mandrel should be set up as if you were ready to hone, including stone pressure. Position the stones at 12 o’clock and set the indicator to zero.

Next, jog the mandrel around until the stones are at 6 o’clock and check the indicator reading. Total indicator runout at both 12 o’clock and 6 o’clock should not exceed 0.005″. Un-centered mandrels can lead to egg-shaped holes or holes with four sides. Egg-shaped holes are caused by shoes being under-expanded. Square-shaped holes are caused by shoes being over-expanded.

Stone pressure (in addition to mandrel centering) is the most critical factor in generating good bores. Most shops tend to use too little stone pressure. The stone pressure should be between 5 and 6 on the pressure dial. Too little pressure will cause the mandrel to be unstable in the bore, which can result in taper or out-of-roundness. This is a common problem, as it’s easier to hone when using light pressure. Adjust the oiling system so that the entire length of the mandrel receives proper lubrication. Wet all of the stones, then turn the oil off. When truing the mandrel, oil should only be used to form an abrasive slurry. When truing, you want to wear the stones and shoes parallel to each other. If you continue to use the honing oil while truing, you will end up slowing the truing process and honing the bores larger. If you have a rod honing machine, lift the foam pad and scoop out a decent amount of abrasive sludge that accumulates in the tray. Liberally smear this onto the stones and shoes. This will help increase wear on the stones and shoes, speeding up the truing process. Stone release is set to 90 degrees, applying the proper stone tension.

Begin honing, making sure that the mandrel travels from stop to stop. Initially, the hone will seem to bounce. This is due to the large out-of-round condition and with no oil being used. Measure the mains to check that they are all cutting at the same size. If one or more main caps have become larger than others, re-cut those caps so that all of the caps have the same undersize.

Continue honing. After a short period of time, oil can then be turned on to rinse the stones. Then check all bores for size and taper. The mandrel will be trued in when it is cutting all mains the same size with no taper, and the shoes show a non-interrupted wear pattern for the entire length of the mandrel. Note that the shoes do not have to show a wear pattern on the entire shoe width. The wear pattern should be evident on both sets of shoes. The process must be repeated until there is a non-interrupted wear pattern.

THE ALIGN HONING PROCESS
The main caps are first cut on the sides in order to square the side of each cap to the parting line. We need a flat surface on the cap sides that is 90 degrees to the parting line. Once square, the cap’s parting line surface is cut on the cap cutter, typically removing about 0.003″ or more. The same amount of removal is required for each cap.

Install all main caps to the block
Lightly oil cap bolts and torque to specification.
Measure all main bores to verify that they are all within 0.0005″ of each other and at least 0.002-0.003″ smaller than the desired dimension, providing enough material to be honed out.
Mount the block to the honing station.
Place the honing mandrel through the main bores and attach the drive motor with a flexible coupling.
Set the stops so that the honing stones just exit the end bores on each end of bar travel.
Extend the honing stones and shoes and adjust pressure.
Position the oiling bar overhead of the caps.
Run the honing mandrel back and forth ONCE.
Remove the honing bar and measure all bores with a calibrated precision bore gauge.
Bores are to be checked for size after each pass of the honing mandrel.
Note that the end bores tend to grow in size faster than the middle bores.
If check measurements indicate that the bores at one end are increasing in size at a faster rate than the other end, reposition the block end for end. This keeps the honing stones and main bores true.
If one bore is increasing in size faster than others, consider loosening that cap to stop removing material from that bore.

TIPS

Never replace only one stone or shoe. When stones are damaged, or when you’re only using 8 stones on a 10-stone mandrel, everything must be done in sets. All stones and shoes should be numbered and replaced in that order.
Never loosen or break torque on a main cap if/when it gets to size before other caps. If a main reaches desired size before other mains, re-cut that cap and continue.
Actual honing time should be less than TWO minutes.
When dealing with a Chevy 400 small block, you must cut out the rear main seal with a ridge reamer or special tool. Then align hone the block. You can also purchase a rear main seal to compensate for the larger seal bore.
Make certain that the register between the block and main caps are solid. If the fit is loose, use a cap knurler to build up the sides of the cap.
To correct an hour-glass condition, decrease mandrel stroke. To correct for a barrel shape, increase stroke.
When installing main caps in preparation of honing, always use the same cap fasteners that will be used during final engine assembly. Avoid using “spare” bolts for align honing and then switching to the “good” bolts for assembly.

Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2026EPQ2/#p=36