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Engine Rebuilding Tips |
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ENGINE REBUILDING If the cylinders are worn, they’ll have to be bored or honed to accept oversize pistons and rings. If not, you can run a glaze breaker down the bores and do the work yourself. If you don’t have valve and seat refacing equipment, you’ll have to send that out, too. Worn guides can be reamed out, replaced or relined in-house with a few special tools. But jobs such as head resurfacing, line boring, crank refinishing, etc., will have to be farmed out. Find a reputable local machine shop that you can use for this type of work. Another reason for doing your own engine work is to control the quality of parts and work that goes into the engine. This is something you can’t control when you buy an engine from an outside source. It may be top quality, or it may not. But you don’t want to find out "the hard way." The truth is, some remanufacturers reuse a much higher percentage of parts than others do, obviously for cost savings purposes. You can also save by buying the parts you need in an engine kit rather than individually. A kit gives you everything you need in one box and reduces the chance of mismatching parts. The parts in a kit usually include bearings, rings, pistons, timing chain and gear set, valve seals, gaskets, oil pump, camshaft, lifters and other miscellaneous parts. You can usually get OEM or better quality parts in most kits, which may be better than the parts found in some remanufactured engines. One aftermarket supplier of engine kits now offers a 100,000 warranty (including labor) on all of the parts in its premium engine kits - which is a better deal than you’ll find on almost any replacement engine, new or remanufactured. CRANKSHAFT BEARINGS Dirt contamination often causes premature bearing failure. The underlying cause may have been a missing air filter, air leaks into the crankcase (missing oil filler cap, PCV valve, etc.), or not changing the oil and filter often enough. If the engine has a "spun" bearing, it’s likely the bearings were starved for oil - possibly as a result of a failed or badly worn oil pump, an obstruction in the oil pump’s pickup screen, or too low an oil level in the crankcase (leaky gaskets or seals). Excessive heat can be another cause of bearing failure. Bearings are primarily cooled by oil flow between the bearing and journal. Anything that disrupts or reduces the flow of oil not only raises bearing temperatures but also increases the risk of scoring or wiping the bearing. Conditions that can reduce oil flow and cause the bearings to run hot include a worn oil pump, restricted oil pickup screen, internal oil leaks, a low oil level in the crankcase, aerated oil (oil level too high), fuel-diluted oil from excessive blowby or coolant-contaminated oil from internal coolant leaks. Misalignment is another condition that may indicate the need for additional work. If the center main bearings are worn more than the ones toward either end of the crankshaft, the crankshaft may be bent or the main bores may be out of alignment. The straightness of the crank can be checked by placing it on V-blocks, positioning a dial indicator on the center journal and watching the indicator as the crank is turned one complete revolution. If runout exceeds limits, the crank must be straightened or replaced. Main bore alignment can be checked by inserting a bar about .001 inch smaller in diameter than the main bores through the block with the main caps installed and torqued. If the bar doesn’t turn easily, the block needs to be align bored. Alignment can also be checked with a straight edge and feeler gauge. A deviation of more than .0015" in any bore calls for align boring. Line boring must also be done if a main cap is replaced. The concentricity of the main bores is also important, and should usually be within .0015". If not, reboring will be necessary to install bearings with oversized outside diameters. Connecting rods with elongated big end bores can cause similar problems. If the rod bearings show a diagonal or uneven wear pattern, it usually means the rod is twisted. Rods with elongated crank journal bores or twist must be reconditioned or replaced. Uneven bearing wear may also be seen if the crankshaft journals are not true. To check the roundness of the crank journals, measure each journal’s diameter at either bottom or top dead center and again at 90 degrees either way. Rod journals typically experience the most wear at top dead center. Comparing diameters at the two different positions should reveal any out-of-roundness if it exists. Though the traditional rule of thumb says up to .001" of journal variation is acceptable, many of today’s import engines can’t tolerate more than .0002" to .0005" of out-of-roundness (always refer to the specs). To check for taper wear on the crankshaft journals (one end worn more than the other), barrel wear (ends worn more than the center) or hourglass wear (center worn more than the ends), measure the journal diameter at the center and both ends. Again, the generally accepted limit for taper wear has usually been up to .001", but nowadays it ranges from .0003" to .0005" for journals 2" or larger in diameter. The journal diameter itself should be within .001" of its original dimensions, or within .001" of standard regrind dimensions for proper oil clearances with a replacement bearing. If a journal has been previously reground, there’s usually a machinist’s mark stamped by the journal. A 10, 20 or 30 would indicate the crank has already been ground to undersize, and that further regrinding may be out of the question depending on how badly the crank is worn. |
