Rebuilding a Chevy “Stovebolt” Six Part 4
Section 4: Reassembly
Last month the Weekend Wrench demonstrated how internal measurements can help you determine what work needs to be done on your engine. It explained ways to check for cracks in the heads and block and suggested methods to identify a good machine shop. This last step is especially important because the dialogue you establish with your machinist, and the quality of work he does, will go a long way toward ensuring smooth and long-lasting engine performance.
You will want to discuss such things as whether to install hard, insert valve seats in your engine’s head, (or block, in the case of a flathead) whether to install special, no-lead valves, and whether to install bronze valve guides. Chances are, unless you only drive short distances under light loads you will want your old valve seats machined out and modern valve seat inserts, designed for no-lead fuels, put in, or you may want to install no-lead valves. You dont need both new valve seat inserts and no-lead valves though, because either of them will provide the protection you need for daily driving with unleaded fuels.
Generally speaking, the installation of new valves is the less expensive way to go. Furthermore, some engines have rather delicate valve seats that cannot he machined to install inserts. Your best approach is to consult your machinist as to which is preferable for your application. While your at it, don’t forget to talk to your machine shop mechanic about bronze versus iron valve guides for your application as well. The softer bronze guides may be preferable for flatheads, which often have trouble getting enough oil to that portion of the engine. But for overheads, which generally don’t have that oiling problem, the harder iron valve guides usually are preferred because they’ll give you longer wear.
Regardless of whether you have checked the head, deck, and bearing saddles for cracks, your machinist should also conduct a Magnaflux test, just to make sure nothing was missed. During a Magnaflux test, the engine part is subjected to a strong magnetic field and small, metallic particles are sprinkled on the component’s surface. The small particles tend to collect on any cracks in the surface.
This test is far preferable to finding out about a hidden crack after you’ve had a lot of expensive machine work done and then assembled and reinstalled your engine. Chevy six heads are notoriously prone to cracking, so if you are doing one of these engines, have the head checked carefully, and find another one if your original has problems. Don’t forget to have your machinist balance your engine, too. Pistons vary in weight and should be matched up at a machine shop by drilling out small portions of metal until they are all equal. Crankshafts should be balanced with the flywheel and clutch installed, so the assembly will spin smoothly as a unit, with no vibration. Cams, crankshafts and connecting rods should be straightened as needed at the machine shop.
Buy the Parts
Once your machinist has told you which undersized bearings and what size pistons you will need to compensate for wear and machining (see “Does Your Engine Measure Up,” Part 3 – December 1995), and you decide which kinds of valves, or inserts you want installed, call an antique auto parts supplier and give them your laundry list. Don’t forget to buy a gasket set too. And just to be on the safe side, you may want to order a rebuilt water pump, oil pump and fuel pump as well. I purchased my engine parts through Egge Machine in Santa Fe Springs, California (address for Egge is listed in Part 2). Paint your engine’s parts before you start the assembly process.
If you are doing a Chevy six like our project engine, you will want to have the machine shop press new timing gears onto the cam and crankshaft. Also, if you’re doing a Chevy six, consider installing an aluminum crankshaft timing gear instead of the original fiber type if you are going to use the car as a driver. The aluminum gear will be a bit noisier, but it will hold up better. On engines equipped with a timing chain, you generally can install these gears by hand, so check your shop manual or machinist for your application.
Before reassembly, clean the engine carefully to remove machining debris. This also provides an opportunity to closely examine the engine and make sure the machining work was done correctly. Wash the outside of your engine with lacquer thinner, then, using a rifle bore cleaning kit, scrub all the little oil passages in the block: Dip the bore cleaning swabs in light oil then work them up and down the passages. Keep cleaning until all of the metal shavings and dirt from the machine work are removed. When your cotton swabs emerge white after inserting them all the way into the passages, the galleries are clean.
Now thoroughly clean the cylinder bores and bearing journals with rags and light oil. The grit left from the machining operations can cause your engine to wear rapidly. Any foreign substances will be carried throughout the engine by the oil, so shavings in the block can find their way to the bearings and ruin them too.
When everything is impeccably clean, paint your engine’s block, head, pan and other components, so when you assemble them the edges of the gaskets will not have any traces of paint. Judges and knowledgeable friends are likely to knock you down a few points if your car has painted gaskets. Cover flecks and mating surfaces with cardboard or stiff paper cut roughly to size. Since this type of painting generally is done with aerosol cans, you don’t have to worry about taping the cardboard in place. Before painting the sides of the head, set the valve cover, which is the same color as the engine anyway, in place to protect the rocker arm assemblies from overspray, and install an old set of spark plugs to keep paint off of the plug hole threads. Paint the side plate, bell housing and clutch inspection pan too. Let your parts dry a day or two before proceeding. Naturally, you’ll want to have the correct paint for your model, year, and engine type. An automotive paint supplier can help you in that regard.
Start checking the machine work by inserting the pistons, one by one, into their holes alongside a long, 1/2 inch-wide feeler gauge attached to a fish weighing scale. On the Chevy six engine, you need a gauge .0015 inch thick. For other engines, check your shop manual for the specified clearance between the piston and cylinder wall. Insert the feeler gauge at 90 degrees to the wrist pin. Now insert the piston and gauge until the piston pin is half way into the cylinder.
Pull out the gauge using the fish weighing scale. If the scale reads between 7 and 18 pounds, your pistons are within tolerance. If your scale reads lower than 7 pounds, your pistons are too loose. If your gauge reads higher than 18 pounds your pistons are too tight. If they are too tight, you can probably have the machinist hone the cylinders a little so they will fit. If they are too loose you may need to have them knurled to size or go to the next larger size piston and have the engine bored again.
Don’t be tempted to put your engine together if the tolerances arent right. If the pistons are too tight, they may seize when the engine reaches operating temperatures, or at the least the engine will be very hard to start when warm and the pistons will wear quickly. On the other hand, if your pistons are too loose, you will have annoying piston slap and your engine will burn oil.
Check your piston rings in their lands on the pistons using a feeler gauge to insure that they have the correct clearance. On the Chevy, the fit around the compression rings should be between .0020-inch and .0035-inch. The oil ring is okay if it has no less than a .010-inch clearance. Also check your compression ring opening gaps by slipping the rings into a cylinder bore one by one and squaring them with a piston inserted upside down into the bore. The gaps should be .007-inch to .0020-inch when measured with a feeler gauge.
Before you begin assembly of your engine, pick up a shop manual for your car and study the procedures. Each engine is different so no set of general instructions will completely suffice. Also work methodically and carefully and keep in mind that when it comes to building engines the old song title “Little Things Mean a Lot” very much applies.
With that in mind, start by smearing the cam bushings (these usually are pressed in at the machine shop) with a thin coat of assembly lube, then carefully slide the cam into place. Be careful not to bump and damage the bushings. Roll the wick seal into the rear main bearing using a round dowel and working from the ends to the center. Using a sharp single-edged razor blade slice off the excess seal exactly level with the mating surfaces. You can use a piece of wooden dowel the diameter of the rear main bearing to help press the seal into place and to hold it while you are working on other things.
Clean the end cap bearing journals as well as those in the block with lacquer thinner. This is important because one bit of grit will cause a deformity or bump in the bearing shell which will cause heat, friction, and early bearing failure due to insufficient clearance. In addition oil or grease on a bearing saddle will make it more likely for the bearing to spin in its journal and may upset tolerances as well. Slip the main bearing shells into place in the main bearing saddles and end caps.
On Chevy sixes, be sure to install the upper bearing halves so the smaller of the two oil holes will be toward the camshaft when the bearing halves are rolled into place. If you are working on another kind of engine, check your shop manual for the correct procedure for your car.
Once they are in place, smear the bearing surfaces with a thin coat of assembly lube, or motor oil, then get a friend to help you set the crankshaft gently back into place, making sure that its gear teeth mesh with the timing gear on the cam and that the little timing marks on the gears line up properly. Next, smear a little assembly lube on the bearing surfaces of the main bearing caps, and tighten the special, self-locking pal nuts. Never use ordinary nuts on main bearings, as they would come loose in service and wreak havoc with your engine. On our Chevy six, three fresh .002-inch shims need to be placed under either side of the intermediate main bearing end caps. Then the main bearing caps are evenly torqued in place. This is accomplished in three stages working in rotation until theyre torqued to 100-110 foot pounds. (This spec is for the Chevy six – your engine may vary.)
Turn the crankshaft using your hands. You should just be able to rotate it with some effort. If it turns easily, it is too loose. If it won’t turn at all, it is too tight. If you have either problem, check your bearing clearances with Plastigage. Be sure to wipe the bearings clean, then follow the instructions on the package. Bearings that are too loose will not hold oil pressure. Bearings that are too tight will not get enough oil. Either situation will ruin the bearings.
Before installing the pistons and rods, check the end play in the crankshaft by forcing the crankshaft all the way forward or aft, then slipping a feeler gauge between the bearing end cap and the base of the crankshaft counterweight. The reading should be between .003-inch and .009-inch. If it is not within tolerances, replace the rear intermediate main shells and check again. Also, on the Chevy, check the lash between the timing gears with a feeler gauge. It should read between .004-inch and .006-inch. Install the rings using a ring expander, and be sure to arrange them so their gaps are in the correct positions according to your shop manual. The three-piece, oil-saver oil rings are a good idea on most engines.
Wrap the big ends of your rod bearings with rags and tape so they will not score your cylinder bores or damage your crank journals while you are installing the pistons. Coat the cylinder bores with motor oil. Using a ring compressor, slip the pistons into their bores, making sure they are facing the correct direction. (On a Chevy six the piston pin clamp must face the camshaft side of the engine.) Gently push or tap the pistons down into their bores but be careful not to damage your crankshaft by bumping it with the big ends of the connecting rods.
Again clean the bearing saddles on your connecting rods with lacquer thinner making sure to remove all grit and dirt. Slip the insert bearing shells into place then coat them with assembly lube. Install the bearing caps making sure the numbered side is toward the camshaft on the Chevy or follow your punch reference marks if your engine has no reference marks. Now torque the end caps using only self-locking nuts to 35-45 pound-feet of torque. Tighten them evenly in three stages.
Look over the job carefully and make sure you haven’t forgotten anything and be sure to wrap your engine in heavy plastic when you quit working for the day so no dirt or grit can invade.
Next we’ll install the valve train and head.