Engine rebuild kits have all the replacement parts for overhauling an engine without going to the store to get gaskets, special measuring tools, or miscellaneous parts. They come neatly packaged, clean and tidy. But it’s important not to let that packaging lull you into a false sense of security. The appearance of matching sleeves, pistons, rings, and bearings can cause frustration and agony if not properly measured and installed in a workshop environment. Following certain assembly procedures will reaffirm how wonderful these engine kits are. This paper is certainly not exhaustive in the engine assembly procedure, but will, hopefully, point out…
a few of the pitfalls that might be bypassed under the guise of a ‘rebuild kit’.
Engine cleanliness isn’t next to godliness, but it oughta be. Removing all the oil and built up grime on engine parts will ensure the longevity of any rebuild project. Sending out the head, block, and major grease-carrying components to be professionally hot-tanked will remove ages of built up grime and internal rust. Smaller parts can be cleaned in the shop using a solvent tank, then dried, inspected for integrity, and laid on top of a clean rag (an old sheet works good for this). When the engine comes back from the machine shop it should, ideally, be put on a rotating engine stand and prepared for reassembly. Use an air (air compressor) gun to blow off the engine block along with all the openings and bolt holes to remove any residue. A clean solvent rag can be used to wipe off the internal surfaces to remove any film left over from the tanking process. Hopefully the machine shop has reinstalled the casting plugs (sometimes called freeze plugs) and the oil galley plugs.
The first item of business in the kit is to install the cylinder sleeves. It is recommended that the machine shop install these, but, if they didn’t, here is the best way to do it. First, stick them in the freezer for a couple of hours. This will cause the metal to shrink just enough to slip into the cylinder bore a little easier. Place the block on a hydraulic press and line it and the ram up with the sleeve. Place some cylinder sealer around the upper end of the sleeve, install an adapter on the top of sleeve which will mate to the ram and quickly but gently press the sleeve into the bore. Don’t stop midway to answer the phone or talk to the dog as the natural heat from the block will quickly cause the sleeve to expand thereby increasing the risk of breakage. Only stop when the sleeve is flush with the top of the block. Go ahead and repeat the process with the other sleeves. If, like most ill equipped barn shops, you don’t have access to a press, you can use a block of wood and a large sledge hammer. Make sure and get an extra set of hands to hold everything reasonably straight; and tap lightly but firmly with the hammer and watch your hands.
Now that the sleeves are in place the next step is to match the pistons to each cylinder. Take the manual and locate the piston to cylinder wall measurement. Generally it should be around .004 to .007 sidewall clearance. Then locate an industrial supply house (or a very well stocked automotive store) and purchase a very long feeler gauge, commonly called a ribbon gauge. Get at least two sizes, one about .002 and another about .004. Intall the piston in the bore and see if the thinner feeler gauge will slip in next to the piston. The proper fit is when the ribbon gauges can be pulled from between the piston and sleeve with a specified inch-pound pull using an inch/pound scale. If the shop doesn’t have the scale you need to gauge it with a mildly firm pull; the feelers will slip freely out from between the piston and cylinder sleeve without binding. You should check the piston at the top and at the bottom of the cylinder for proper clearance in case the sleeve is tapered. (Don’t assume that if the piston freely falls through the bore that the clearance is adequate. If the clearance is inadequate the cylinder bore (or sleeve) will have to be honed and then rechecked. Take the time to do it right.
Each piston will come with a certain number of piston rings. Unwrap the rings and lay them next to each piston in the order of installation. Now it is time to check the ring gap. When the ring is in the cylinder bore it has to have a specified compressed gap; in other words an opening between the ring ends. Consult the manual for this spec. Generally, it is considered to be .002-.003 per inch of cylinder bore diameter. Install the first ring in the bore, take the piston, invert it, and gently slide the piston in the bore pushing the ring approximately half way down the sleeve. Take your feeler gauge and measure the end gap clearance. Compare with the specs and determine any necessary changes. Insufficient clearance will require that the ring gap ends be filed down to tolerances. Take a file and mount it vertically in a vise. Take the ring and, holding it firmly in both hands, draw it downward over the stationary file. Remove some metal and check the ring in the bore again. Repeat as needed. Follow this procedure for each ring making sure to note each gap specification for each particular ring. After the gap for each ring is corrected then take each ring and check the ring to corresponding ring groove in the piston. There should be ample room for the proper feeler gauge between the ring and groove. Again, check the manual for this spec. Too tight a fit will keep the rings from proper rotation and movement as the piston moves up and down. Make sure and place each ring in corresponding order with each piston groove; check the ring for a mark indicating the correct side up. They are now a matching set. This is an important step in the engine assembly because as the rings heat up they will also expand. Insufficient clearances will cause extreme scoring of the cylinder wall, ring breakage, and potential engine seizure.
The next step in assembly is to check the piston pin to bushing tolerances. Once again, this is a very close tolerance and should best be done by a competent machine shop. An insertion pressure of pin to bushing will be slightly looser than that of the pin to piston. When dealing with clearances of .0002 or less it is important to have the proper measuring tools. A pin too tight or too loose will cause a piston and rod to literally break apart under the stresses of motor operation.
It is recommended that the head be reworked in its entirety by the machine shop. Tools for magnafluxing the head for cracks, pressing guides in and out, getting the correct angles on the valve seats, setting the proper valve to head recession setting, measuring and milling warped surfaces, amongst other things are really beyond the scope of a doing a lasting engine rebuild on the home shop level.
Crankshaft and camshaft measuring, grinding, and polishing is also out of the realm of the home shop. It is wise, though, to always double check the machine shop’s work with a feeler gauge, dial indicator, and/or plasti-gauge. It is important to check the crank end play with a feeler gauge and also to check each bearing journal with plasti-gauge before starting the reassembly procedure. It’s amazing how many of these specs can be found in the manual when you know what to look for.
One additional point worth mentioning is to make sure that the oil pump is primed before buttoning up the bottom of the engine. It is recommended that the pump be disassembled and packed with a lithium based grease commonly called ‘white lube’. If the pump cannot be conveniently disassembled then take the white lube and squeeze it into the intake port of the pump. Rotate the pump in the direction that the engine would rotate the pump and continue to squeeze the grease into that port until the pump gears are adequately primed. It is imperitive to have oil pressure in the motor before it is actually started up. Priming the pump will facilitate rapid charging of the oil circuit.
These points are to encourage the mechanic using an engine rebuild kit to make sure to check and verify the tolerances of all the moving parts. Just because the kit looks good doesn’t mean that everything will go together the way it should. Taking the time to check all the above tolerances will ensure that the kit will do what it was designed to do: make the engine run right the first time.