Decking machines the head-gasket surface of the block flat, and with a BHJ fixture, the ma
While an overbore is part of any true rebuild, many engines are overhauled, keeping the standard pistons and simply honing the cylinders and reringing the pistons. These back-alley builds are usually done when money is too tight for a true rebuild. Generally, on a high-mileage engine, the cylinders are honed or deglazed with a drill-mounted tool. Also, everything is cleaned, new rings are loaded onto the old pistons, and everything is buttoned back up. The success of a reringed engine depends upon the amount of bore wear and taper that already exists. If the wear is limited to a couple thousandths of an inch, a fairly long life and decent performance can be expected. If the wear is much over .005 inch, though, it may run OK for a while, but don't even consider reringing a worn block with .010 inch or more taper.
Next to an overbore, decking is the most common machine-shop operation on a block. The flat surface of the block, where the heads bolt on, is the deck. Decks have to be flat to evenly clamp the head gasket and prevent a head-gasket failure. This surface can be machined to achieve the required flatness and surface finish to hold a gasket. With the high conformability of today's head gaskets, decking is not always necessary, but a check should be made by your machinist to make sure the surface isn't too far out of shape. While the purpose of decking in a basic rebuild is primarily to renew the surface, in a high-performance build, decking is often employed to insure the decks are perfectly parallel to the crankshaft centerline and indexed at exactly 90 degrees to each other per specifications. Factory machining can be sloppy in this respect. Also, decking in a performance build is done to machine the surface to a desired specification, such as setting the surface exactly even with the piston top at TDC (top dead center), or zero deck.
Head milling is also a normal part of having a cylinder head reconditioned. The cut can be
The next most common machine-shop practice is an align-hone or align-bore. During align-honing, the main bearing caps are cut down a few thousandths of an inch, making the main journal openings smaller. Then the caps are torqued into position on the block, and a hone is run through all five main-bearing bores, insuring they are in perfect alignment. Is align-honing necessary? Unless there are signs of unusual wear in the old main bearings, the answer is usually no in a basic rebuild. A drawback of align-honing is that the procedure moves the centerline of the crankshaft higher in the block, which can cause a loose timing chain if the honing is excessive or performed carelessly. Align-boring is a similar procedure, although here, a cutting tool, rather than a hone, is used to true-up the bearing bores. Align-boring is usually reserved for specialized requirements, such as fitting high-performance aftermarket main-bearing caps to the block.
Heads, You Win
The cylinder heads are just as important in sealing the combustion pressure as the block and rings. The first level of headwork is a basic valve job. During a valve job, the heads are disassembled and cleaned, and the sealing surfaces of the valves and seats are machined to provide a fresh sealing surface. The heads are then reassembled with new valve-stem seals. From there, it gets more complicated and expensive, depending upon the condition of the heads. Don't be surprised if the valve guides are worn, since they've usually had it by the time a valve job is required. Renew the valve guides by boring the original hole the valve resides in and installing a sleeve. New valve-guide sleeves come in thick- and thin-wall bronze of various grades, as well as iron. It is common for valve-seat wear to be bad enough to cause the valve to recede into the head or be "sunk." In this case, a performance loss will result unless the seat is brought back up via the installation of larger diameter valves or a seat insert. On the subject of new valves and inserts, it's not uncommon for the valve stems or faces to be worn beyond useable limits, and older heads will benefit from hardened exhaust-seat inserts for use with unleaded fuel.