Rebuilding a Chevy Straight Six – Part 3

Rebuilding a Chevy “Stovebolt” Six – Part 3

Section 3: December 1995 (Prepping for the engine shop)

Last month in the Weekend Wrench we completed the disassembly of our project engine. We pulled the harmonic balancer, then removed the timing gear from the crankshaft. We checked the rod and main bearings for wear. We removed the cam, marked the pistons and the rod bearing caps, and reamed away the ridges at the tops of the cylinder bores. Finally, we pushed the pistons and rods out through the top of the engine block and removed the crankshaft.

What’s ahead for us will require a few machinist’s hand tools. They will help determine the amount of restoration work needed to revitalize our tired old mill. In the end we will trust a good machine shop to make the final calculations and run some tests to tell us what needs to be done, but by checking things ourselves, we will have a good idea of what the engine requires. What’s more, we may save some money by doing so.

To determine what your engine needs, you will need to take some critical and precise measurements. A few thousandths of an inch one way or the other between the cylinder bores and the pistons can mean the difference between a quiet, dependable engine, and one that may seize when warmed up or has piston slap due to loose tolerances. If the cylinders and pistons have more than .0011-inch clearance on an engine like this Chevy six, the pistons will be too loose and will wobble in their bores, as evidenced by a rapid clicking when the engine is revved. If the pistons are too tight in their bores, (under .0005-inch clearance) the engine will seize when the pistons expand as they reach operating temperatures. Either scenario (too tight or too loose) is bad news, and unless you check these clearances carefully while overhauling your engine, you won’t know about these problems until it’s too late.

Other parts need to be measured as well. If your crankshaft is worn, or out-of-round, its bearings will not hold oil pressure, may be noisy, and will be prone to failure. If your engine’s cam is worn, the valves may not open far enough or stay open long enough. If your cam bearing journals are out-of-round by more than .001-inch, the cam will need to be replaced or reground.

So how do you take the necessary measurements? It’s fairly easy, but it requires a certain feel and a few special tools. You’ll probably find what you need at your local tool store. After you’ve rounded up the tools, have an experienced machinist help you develop the correct feel and show you how to read the scales. If you don’t know a machinist, double-check your feel and accuracy by first measuring items of known dimensions, such as feeler gauges or pieces of machined metal stock. Make sure the object you are measuring whether you re practicing or doing the real thing is clean and the blades of your tool are clean as well. A tiny bit of dirt can easily throw off precise measurements.

Block

An engine block is seldom worn out. The exception may be if it came out of a Havana taxi cab. If a block is not cracked or corroded it usually can be machined to specs and even made better than new. I say it can be better than new because new engine blocks are green in the vernacular of racers and rebuilders. In other words, the casting process creates various types of stress in a new engine block that can take months to work out. These metal stresses combined with continual heating and cooling can result in warping of the block.

Your rebuilt engine also may turn out better than new because until recently most domestic manufacturers tolerated less than rigid quality control procedures on their engine production lines. Hot rodders and racers who wanted their engines to stay together at high RPMs came up with the accurate machining and balancing techniques that can improve the older powerplant in your vehicle.

But before we take measurements, let’s rule out some obvious problems. Are there any noticeable cracks between the cylinders? Are there any cracks around the main hearing saddles? Are the water jacket openings for the head corroded away? Are the soft plug openings (also called freeze or expansion plug openings) severely corroded and damaged? If you find any of these problems, get another block. Such things can sometimes be fixed, and if you are overhauling a Duesenberg straight eight you will certainly want to try, but with an engine as common as a ’57 Chevy six, finding a good alternative block is the best and cheapest answer.

Even if you can’t see any cracks in the bearing journals or cylinder walls with the naked eye, there still may be some. Check for smaller cracks by using a product called Spotcheck from the Magnaflux Corp. With the Spotcheck system, a cleaner is first used to prepare the surface. Then a penetrant in the form of a liquid red dye is applied followed by a white powder that makes cracks stand out. Spotcheck also can be used to discover cracks in heads, rods, pistons and crankshafts. It is especially good for nonmagnetic parts, such as aluminum pistons, that cannot be tested by the traditional Magnaflux methods used at most machine shops.

If your block is not cracked or corroded, chances are it is rebuildable. Next check the deck, the mating surface for the head. Start by cleaning it of all remnants of gasket sealer, carbon and dirt. Now place your steel ruler on edge along the length of the block. Shine your trouble light behind the ruler and if you see a gap or gaps, check them with a feeler gauge. If they are more than .007-inch wide at any point, the deck will have to be milled (planed flat). Place the straightedge across the engine deck it several different angles to complete this check for warping.

Next check the taper in the cylinders. Engine cylinders that have accumulated many miles will be worn bigger near the top than at the bottom. That’s because the rings on the pistons slide up and down only in the upper parts of the bores. checking taper can be done with a bore-measuring tool or an inside micrometer. If the bore-measuring tool has a dial-type indicator, run it slowly up and down the bore and note the change in diameter. If it is greater than .005-inch, your engine will need to he rebored. If it is under .005-inch and your pistons are good, try cleaning the bores with a cylinder hone (available from auto parts stores), then install new rings on the pistons. If you measure your engine’s taper using an inside micrometer, measure near the top and near the bottom of each cylinder. Place the micrometer in each bore, then rock it back and forth a little to find the minimum straight-across measurement.

Next measure your cylinder bores for roundness. As engines run, pistons pivot on their wrist pins and can cause uneven wear at the sides of the cylinder bores. Also, most aluminum pistons are cam-ground to a slight oval shape with the widest dimension along the axis of the wrist pin. This is done because the steel wrist pin limits heat expansion. The areas of the piston perpendicular to the wrist pin are ground smaller to allow for greater expansion. As the engine heats up while running, these areas expand more because there is nothing to restrain them. All of these factors can effect cylinder roundness.

To check for roundness, take measurements near the top and near the bottom of each bore. Then measure at right angles to these first measurements. Your cylinders should not be more than .005-inch out-of-round. If they exceed that tolerance, have them rebored at a machine shop.

Pistons

If your engine block needs to be rebored, you will need to replace your old pistons with new ones in a larger size. The usual over-bore sizes are: .020-inch, .030-inch and .040inch, but let your machinist tell you which size to buy after he has determined how much it will take to clean the cylinders. If you are lucky and your cylinders only need honing, you may be able to use your original pistons, or if you can find a set of the .001-inch oversize pistons that were originally made for this situation, you can install them.

Crankshaft

If your crankshaft’s main bearing journals are out-of-round more than .001-inch or are tapered beyond the same figure, the crankshaft will have to be turned and the appropriate undersize bearings installed. Use a Vernier caliper or an outside micrometer to check the journals. Also make sure the journals are not out-of-round by checking at 90 degrees to your first measurement on each journal. Replacement bearings are generally available in .010-inch, .020-inch and .030-inch, but let your machinist make that determination.

Connecting rod journals need to be checked the same way. If they are tapered or out-of-round by .002 inch, they will have to be ground and undersize bearings fitted. Again, let you machinist make the call. Incidentally, don’t be tempted to file the connecting rod end caps to tighten the rod bearing clearances, as was done on older engines. Rapid bearing failure may result.

Camshaft

The lobes on your engine’s cam should be shiny and without any trace of grooves, discoloration or cracks. If these tell-tale signs of wear are apparent, the cam should be reground and Parkerized, a procedure where the cam is heated and then dipped in a solution that hardens the surface and protects against corrosion. If its bearing journals are .001-inch or more out-of-round, the cam will need to be ground and undersize bearings fitted. On a Chevy six, as well as many other engines, each bearing journal is slightly smaller in diameter from front to back in the block. It was designed that way so the cam can be installed and removed from the front of the engine. Check your shop manual for the correct diameters for these journals.

A Second Opinion

After making the measurements outlined above, you will have a good idea of what your engine will need, but don’t whip out your credit card just yet. First, take your parts to a good machine shop and have your findings verified. Let the machinist tell you how much your block will need to be bored and how much your crank will need to be turned so you can get the appropriate replacement parts. When you take everything in to be machined, don’t forget to take your flywheel and clutch as well. Chances are your flywheel needs resurfacing, and you will want the clutch and flywheel attached to the crankshaft during balancing so the whole assembly can be balanced as a unit for maximum smoothness.

How do you find a good machine shop? Start by asking members of local car clubs, particularly those familiar with cars like yours. Also check with local hot rodders. The chrome-valve-cover fraternity usually knows who does good work in the area. you might also try local garages and auto parts dealers for recommendations. Finally, visit the machinist’s facility before you entrust your precious classic parts to his care. Is the shop clean? Are the machines fairly new and in good working order? (Old machine tools can be notoriously inaccurate.) Do the guys working there show a flicker of recognition when you utter the name Studebaker, or do they only know about cars made after 1990? Determining the answers to these key questions can make a big difference in the outcome of your job.

Between now and next issue, we will be consulting our own machine shop and getting the verdict on our old Stove Bolt. Stay tuned, because we will talk about such things as setting up your engine for a diet of unleaded fuel, which kinds of pistons and bearings to buy, and valve guide selection as well.

By | 2018-07-15T14:22:39+00:00 May 3rd, 2018|Classic Chevy Trucks|0 Comments

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