Introduction to Rebuilding Mopar Engines - Engine Rebuild Basics

In this installment, we tackle how to prepare the block, how to choose the right cylinder heads, the differences in crankshaft and piston qualities and the procedures required for them, and rod reconditioning. Later on, we'll discuss rings, the quench factor, and engine balancing.

I have read several articles in your magazine on how to rebuild engines. Most of these articles assume the reader is an expert on engines and knows what the writer is talking about. I've also read several books on Mopar engine rebuilding and still can't figure out some of the things the writers are talking about. Since I don't plan to rebuild the engine myself, I really need a detailed list of what I should do to my '70 Barracuda 340 four-barrel engine. I would like to have it rebuilt to stock specs, but I don't know what questions to ask, nor do I know what exactly should be done. To reduce the risk of receiving poor quality work, I'd like to know these things before I take the car in for the rebuild. Your help is greatly appreciated.Lawren MinorVia e-mail

Lawren, rebuilding a 340 to stock specs was covered from top to bottom in the December 2000 through February 2001 issues of Mopar Muscle. We showed how to tear down a greasy junkyard engine, how to inspect it, what parts to get, and what machining was required for a low-dollar but solid buildup to original specs. Then we took it to the dyno, ran it in pure stock form, and bolted on an extra 110 or so horsepower with four aftermarket additions that anyone can do. We can't get any more detailed than that.

While it's easy to get caught up in the serious end of the engine world, we realize there are Mopar fans at various levels of experience looking to wrench on their toys. We know some of our tech is pretty hardcore for those new to engine building. Really, it doesn't take long to get up to speed, just starting with the basics and working up. With that in mind, we'll get to "splainin" some of the things to know and think about when digging into an engine for a rebuild.

Block Basics
The block is the foundation of any engine, whether the goal is a stock rebuild or a radical high-performance piece. First on the list of things to consider are the cylinder walls. Cylinder wear tends to be tapered, with the greatest amount of wear at the top of the piston-ring travel and little toward the bottom. The more wear and taper, the more wiggle room the piston has in the bore. As the piston becomes unstable in the bore, sealing against both combustion pressure and oil is compromised. Making matters worse, in a tapered bore, the piston rings move in and out in their grooves, following the taper. The result is accelerated wear in the piston-ring groove and further deterioration of the rings' ability to seal.

Reconditioning the cylinder bores requires a trip to the machine shop to have the block bored out and honed to a larger dimension. Then, corresponding larger pistons and rings must be purchased to match. The engine will gain a couple of cubic inches from the larger bore, but more importantly, the cylinder will be in a perfectly round condition to seal as well or better than new. An overbore, the purchase of new pistons, and having them fitted to the rods is the most expensive part of a basic engine buildup, but for good-as-new performance, it's money well spent.

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.

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.

Milling cylinder heads is a machine-shop operation similar to decking the block, but is done on the flathead gasket surface of the heads. Milling can be just a clean-up cut to ensure flatness and surface finish, or a heavier cut to decrease the combustion-chamber volume for a higher compression ratio. If a heavy mill job is going to be performed, the heads will sit lower on the block, and intake alignment becomes an issue. The intake-manifold surface of the heads (or the intake itself) will also need milling to compensate. Another consideration is that heavy milling will move the rockers in closer to the cam a like amount, which may cause the pushrods to have a less than ideal fit, since they will effectively become too long by the amount milled.

For high-performance or race use, the list of tasks to do on cylinder heads just keeps growing. If dual valvesprings are required by the cam selection, the stock-style valve seal will no longer fit, requiring machining for a guide-mounted seal. If high valve lift is also part of the plan (i.e. a large camshaft), the tops of the valveguides will need to be cut down to gain clearance; although fortunately, this machining operation is typically done in conjunction with the valve-seal operation. Depending upon the valvespring selection, the spring seat may also need machining to properly locate the spring. Now, for a real performance effort, the cylinder heads do more than seal-they flow air, and airflow is power. Porting heads quickly adds to the complexity and cost of cylinder headwork, depending upon how far it's taken. With the availability of new aftermarket cylinder heads, replacement is often a much more economically viable path.

Crank Call
The crankshaft is what conveys the reciprocating motion of the piston to the rotary motion that burns rubber at the rear wheels. In a simple rebuild, the crank is inspected for wear, and if in good condition, the journals can simply be polished and the crank reinstalled. If the journals are showing wear, they can be ground undersize by a crank grinder, creating a fresh machine surface for the bearings to ride on. Oversize bearings are available to match the undersize journal diameter, typically to .030 inch.

Mopar engines were equipped with either forged or cast cranks. Forged cranks are made by forging steel in permanent molds, while cast cranks are sand castings poured from cast iron. Forgings are higher in strength and vastly superior in terms of ductility. That being said, Mopar cast cranks have also proven themselves to be reliable in service, including typical street/strip and even sportsman race applications. Steel cranks require hardening to provide sufficient surface hardness for journal life. And for the most part, the factory used a deep penetrating induction-hardening process, which remains after an induction-hardened steel crank is ground. Some rare factory race cranks, however, were Tuftrided, meaning they underwent a shallow hardening process; a regrind on one of these cranks will require the part be reheat-treated.

For years, factory cranks were about the only choice for anything but an all-out race-engine build. These days, there is a variety of aftermarket replacement and race cranks available. One advantage of going new is the crank hasn't been subject to fatigue, and metal fatigue leads to crank failure. The crankshaft is subject to repeated load and unload stress cycles and has a finite fatigue life based on the number of cycles and load. Although it may seem fine through a typical magnaflux inspection, a crank that has seen severe duty over a long period of time may be near the point of fatigue failure. Aftermarket crankshafts can be had in various grades of iron and steel, constructed from castings, forgings, or billet to suit practically any need or budget. There are also several stroker-style cranks available for both big- and small-block Mopars. A stroker crank increases the stroke of the engine, adding significantly to the cubic-inch displacement. Strokers have become popular in performance-engine building; after all, as the saying goes, there's no substitute for cubic inches.

Packin' Pistons
We discussed in detail pistons and piston choices in "Piston Science" elsewhere in this issue, so we'll touch on the basics here. Like crankshafts, pistons are manufactured by either the forging or casting process, and again, forging gets the nod in durability. Various grades of aluminum are used in both forged and cast pistons, again delineating the grades of durability. Pistons will be part of the process if an engine is being rebuilt, and it pays to anticipate future mods when making a selection. Building a mild 440? Who knows, maybe a little nitrous will be in the cards a few years down the road for a little added oomph. About that time, you'll be glad you stepped up to a set of forged slugs instead of the cast cheapies.

There is a variety of design specifications available in pistons, the most important of these being the compression height and head configuration. Compression height basically determines how high the piston ends up at TDC, which is important for compression ratio as well as clearance. Piston-top configuration varies greatly, depending upon the requirements of the engine. Dished pistons have a depression built into the top of the piston to lower compression ratio. Flat-top pistons are simply flat across the top; although valve notches are incorporated into high-performance designs to provide clearance for high-lift and long-duration cams. Domed pistons actually rise up into the combustion chamber to squeeze at a higher compression ratio. Reverse dome or kick-up quench dome pistons are designed to provide a proper quench clearance in an open-chamber head.

Some other points to ponder include the piston-ring size for which the ring lands are cut. Stock engines used 5⁄64-inch thick compression rings, while thinner 1⁄16-inch compression rings have long been popular in performance application, and thinner rings are now widely used by OEM manufacturers. The piston determines the ring thickness, so be aware when ordering parts. Pistons can be fitted to the rods with the pin pressed into the rod, or floating with the pin retained by locks at the end of the piston's pin bore. If rods designed for floating pins are used, get pistons machined with grooves to accept locks.

Rodin' At Random
Connecting rods connect the pistons to the crank, and they take some abuse in a high-performance engine. If the stock rods are being reused in a rebuild, the rod's bearing bore at the big end (crank) is checked for roundness and size. If outside of specification, the rod and cap can be ground slightly at the parting surface, and the bore honed to resize the rods. New high-strength aftermarket bolts are often installed during the process. Some Mopar engines, such as early small-blocks, used floating piston pins, which ride on a bronze bushing in the small end. These bushings need to be checked for wear and often need replacing.

Like crankshafts, rods have a finite fatigue life, depending upon the level of loading they have seen in the past. Various modifications have been routinely performed on factory rods to increase their durability. Shot-peening is a controlled process in which the rods are blasted with steel shot to add a compressive layer to the outside of the rod, increasing fatigue life. Racers would also polish a rod's beams in an effort to eliminate stress risers that can initiate a fatigue failure.

Aftermarket rods used to be outside the realm of a basic rebuild but are now affordable enough to consider in any engine build. Between the cost of resizing, new bolts, bushings, and any additional strengthening mods, the money invested in reconditioning a set of stock rods closes in on the price of aftermarket pieces. Aftermarket rods come in grades of forged steel from cheap to downright costly, and for the most part, quality goes up in scale with price. Consider the application and power level targeted; opt for enough rod to do the job, as a compromise here can cost the engine.

A successful engine rebuild begins with the fundamentals, which is what we have presented here. Be sure to tune in for Part Two, where the topics will be rings, quench, and balancing. Armed with this info, you'll have all the basic knowledge you need to start your own engine rebuild.