If you ever want to get scared, check out what a crank looks like at high RPM. If you’re interested, the machine to do this with is called a Spintron. What you’ll see is a large, heavy, supposedly rigid chunk of steel squirming around like crazy. You’d swear any metal walking around that much would soon fail.
In some cases, it’s exactly what happens. Common failures include bearing wear, spun bearings, excess crank wear, cracks or breaks in the crank/block/main caps, and more. Although you can never completely eliminate all the causes of these failures, you can certainly reduce them – it’s what successful race teams and engine builders do all the time.
Among the most common ways to improve the bottom end of any engine is to make sure the main bores are straight and at the proper spec for diameter and shape. The machine operation to correct any defects is called align honing. It’s a job requiring good knowledge of what you want to accomplish, how to get there, and experience with the equipment that allows you to make mid-course corrections and adjustments to get the perfect job done. When you do it well, it’s a sign you’re the right machine shop to perform operations that will keep your customers’ engines running when others fail.
I say this because in the last few years I’ve seen both excellent work and some really crappy work. There are at least a couple of shops I know of that I wouldn’t let near any of my projects – not because they don’t want to do the right stuff, but because they don’t know
how. The engine you’ll see in this article is a good example. It was align honed at one shop and then (to the embarrassment of the owner) shipped to another to get it re-done. The first shop (name withheld to keep lawyer weasels away) managed to machine the main bore at one end a full two thousandths larger than the other. Given the tolerance is two TENTHS of a thousandth for this kind of work, you can understand why the block had to go to Dependable Machine in Overland, MO (St. Louis suburb). There, Joe Simon took over and did the job you’ll see here.
If you don’t know what to expect and how to tell if it’s right, how will you catch bad work before it’s too late? That’s why it makes sense to have enough knowledge to do a great job and to cover your butt.
It turns out this block had most of the problems you can face. First, the main bores were not within spec. They should all be the same and within the range given by the bearing manufacturer. In this case the main bore diameter is shown to be 2.9370" to 2.9380". Most machinists will shoot for the center of this range and end up with 2.9375". The maximum
tolerance allowed is .0002". Starting from the front, this block read 2.9380", 2.9375", 2.9375", 2.9370", and 2.9360". Ouch! When Joe was done all mains measured up at 2.9375".
However, before this could be done there was another problem. The #2 and #3 main caps did not register properly in the block. What this means is that where the caps fit into the notched area of the block on either side, there was clearance instead of a snug fit. It is important to have a good tight register because this keeps the caps from walking around side to side. A loose cap exaggerates the problem of crank stability and can mean early failure.
The solutions include using a flat, blunt chisel and hammer to slightly move the metal of the block register inward. This is common practice and if done carefully is acceptable. NEVER,
use a punch or other sharp object as this creates stress risers and could result in a cracked web. Another way is to knurl the cap sides. I don’t care for this as the result is less than full metal-to-metal contact and so less strong and stable. A third way is to replace the cap. Replacements are available for many engines. The one we chose was to select from a large
supply of used caps for replacement that was both tight and close to the same profile inside diameter. Joe tried about 20 different caps until he found the best initial fit. When the honing is done it will correct any small discrepancies.
Another problem we found was that the bolts did not fit the threads smoothly. When you torque the mains down, the specs rely on clean threads without burrs or defects and bolts to match. The threads were dirty so Joe cleaned them with a small stainless steel round wire brush chucked into a drill motor. The brush threaded in and out, removed all the gunk and left the threads intact. The bolts were used, stock stuff and it was apparent they were not in prime shape. We replaced them with new high performance bolts as should have been done at first, and used the recommended lube on both threads and washer faces to get the correct torque readings.
At this stage the caps were all trimmed to the same height so we started with similar rough bores and allow the honing to create a good, round, properly-sized set of main bores all aligned on the same centerline. It’s important to have the caps the same dimension so the starting place is about the same when the hone is engaged.
Honing consists of running a long bar hone through the mains in a controlled manner so all caps get the same cut. The trouble is that not all mains have the same surface width under the bearing shells so often enough the hone will leave one main a little larger than the others. Small changes in design or metal consistency can also cause this. Further, if the block is honed from just one end (probably the case with the first shop’s work) you get ascending or descending bore diameters from one end to the other.
Solutions to this effort at standardizing the bores and coping with different cut rates include honing from one end and then switching to hone from the other end. The honing is done in short steps and bores are measured after each cut to determine just exactly how the hone is performing. That allows you to adjust as needed. Another trick is to loosen a cap if you want to reduce the cut on it while cutting the others. If when measuring you discover one bore diameter is increasing too fast, you loosen it, leaving the others torqued tight and the cut rate slows on just that bore. When the others are brought up to the same level, you finish with all caps torqued.
Appearances can be deceiving as well. For example, you may find the parting line area of the block and/or cap appears not to be honed. This is both common and acceptable. If you think about it, what it means is material was removed only where needed. The critical areas are at the top and bottom of the bores where the vast majority of stress is concentrated. If an already correctly sized bore (side to side) is increased while the dimension top to bottom is increased by honing, you would end up with an oval bore that can’t be repaired.
Shape is important. While it’s OK to have the metal untouched at the parting lines, you want the bores round. Measurements are taken side to side as well as top to bottom to verify this. In addition, the bores are measured both at the front of the bore and at the rear. This will verify the bore is not tapered end to end. Checking at three points, (front, center, rear) will tell you if the bore is barrel-shaped.
What you want and what Joe did so well was to get the bores all defined as rings with consistent diameters, no barrel or taper, all along the same centerline from front to rear and all at the same diameter. This will mean the bearings will all fit with proper crush, clearances around the entire circumference of the bearing bores will be consistent, and no stress from improper alignment will contribute to failures. Trust me, it’s definitely worth it to get this aspect of engine building right.