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Engine assembly: Fitting rods and pistons

Engine assembly fitting rods and pistons

In our previous engine assembly article, we took a detailed look at checking bearing clearances for both the rod and the main bearings, checking the thrust dimension, installing the crankshaft, and torquing the main caps. In this segment, we’ll look at fitting piston rings, installing and locking (in place) floating wrist pins, and installing the piston-ring-pin-rod combination in the block. Essentially with this portion of the build complete, you’ll end up with a relatively complete short block, aside from the camshaft and cam drive.

Tedious but doable
Will you need special skills to reach this point? Absolutely not. Just like the other portions of the build, what you do need is the ability to check, double-check and triple-check your work. Sure, at times it can become tedious, but it’s also fulfilling. And, by the way, if you poll pro engine builders you’ll likely find they also find some of this stuff a bit monotonous, too. Follow along. This is a job you can accomplish.

Measuring and setting the piston ring end gap
Many modern high-performance piston ring sets are designed to be hand fit to each cylinder bore. These “file-fit” rings are manufactured 0.005-inch oversize (larger than the bore size), and it’s up to you to measure and set the end gaps. This only applies to the top and second rings in each ring set. The oil ring package cannot be file fit. Even in file-fit ring packages, the oil ring package is simply installed (more on this later). 

The reason a compression ring is file fit in a high-performance engine is because the gap has an effect on performance. Too large a gap and cylinder pressure leaks through the gap, causing the engine to lose power. Too small a gap and the ring ends will butt together as the engine heats and expands. Ring gaps that are butted result in engine damage.

Fitting the gap
You have to run a gap that’s as tight as possible without having the gaps butted. Most engine builders will tell you that 0.004-inch of end gap per inch of bore diameter is a good rule of thumb for the top compression ring. Certain second rings are set a wee bit looser. This is all dependent upon the ring manufacturer and their specification.

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To measure the ring gap, you first have to install the individual ring into a specific cylinder (we typically begin with cylinder #1 in the engine firing order). The ring must be squared in the bore. You can accomplish this with the end of a dial caliper (the depth gauge portion at the back of the scale), but it’s far easier and more convenient to use a commercial ring-squaring tool. Here, the ring is inserted in the bore, followed by the squaring tool. Since the tool is hollow (see the accompanying photo), you just pull the ring back up square to the face of the tool, and you’re done. You can check the gap.

To check the gap, a common feeler gauge is used. Insert the feeler gauge in the gap (if one exists) to determine the dimension. If you’re beginning with rings that are 0.005-inch oversize then there’s likely zero gap (and the rings probably overlap).

To reduce the ring size (increase the gap), you have to file fit the ring. This is done with a ring gap tool. There are some very fancy, sophisticated gapping tools available, but they’re equally expensive (some run deep into the thousands of dollars). A relatively inexpensive and easy-to-find alternative is available from Sealer Power and other companies. It simply has a hand crank that turns a rotary carbide, which in turn trims the ring end gap. You have to go slowly: filing a small amount, checking the gap in the respective bore, filing some more and so on. The idea is to effectively “sneak up” on the gap. If the end gaps are excessive then you’ll have to purchase a new ring (or ring set). Once the gap is set, we gently de-burr the end gap with another small, fine hand file.

In the filing process, it’s a good idea to fit each ring a specific cylinder. As a result, you’ll end up (for example) with a custom-filed top ring and custom-filed second ring for cylinder number one and so on down the list. We simply mark a large white poster board with each cylinder number and set the gapped rings aside for (later) installation on each piston.

Wrist pins and retainers
Keeping the wrist pin in check within the piston is critical. If the pin moves sideways, then the cylinder wall will ultimately be destroyed. There are several means to keep a pin in check, the most common being a pressed pin (in the rod) or a full floating arrangement with some form of lock ring. 

Pressed pins are regularly found in passenger car applications. Here, the pin is press fit into the small end of the connecting rod. The pin movement takes place between the wrist pin and the pin bore of the piston. Engines with press-fit pins must be assembled by an engine (machine) shop with special equipment designed for the task at hand. The process involves heating the small end of the rod. Next the piston is set in place over the rod small end and the wrist pin is pressed into place. It’s not a job for a home engine-builder.

Floating pins, on the other hand, are often found in high-performance engines. Here, the pin is allowed to float within the connecting rod small end. Instead of being pressed in place, the wrist pin is held in check with one of three types of locking devices: spiral locs, round wire locks or snap rings (regularly called “Tru Arcs”). Pistons are machined with retainer grooves (some for double retainers per side) that accept the locking device. The most effective device of the bunch is the Spiral Loc (essentially a flat coil of hardened steel), but it’s also the most difficult to install.

The conventional way to install a set of Spiral Locs is to use a pair of small screwdrivers to literally spin the lock wire into place (into the groove machined in each end of the piston pin bore).

Basically, you start at one end (without the wrist pin or rod in place), turning in both Spiral Locs (one after the other). Next, the wrist pin and piston are assembled on the connecting rod. At that point, the remaining two Spiral Locs are worked into place. Most folks will tell you to keep a box of bandages handy with this job. Good advice.

Tru Arcs are essentially heavy-duty snap rings that install by way of snap-ring pliers. Round wire retainers aren’t that common, but there are tools available to install them (more in the photo guide). In any case, never reuse a pin retainer of any sort. Once used, they can become distorted (particularly from the removal process). Tension is lost and you run the risk of the retainer being damaged or destroyed during engine operation.

Installing piston rings
Once the piston-pin-rod combination is assembled, it is time to install the piston rings. Here, it’s a matter of winding the rings onto the pistons. In times past, piston rings were thick, higher tension affairs than we commonly see today. Some folks installed the rings on the piston by way of a special set of pliers. Today, with lighter tension (and regularly thinner cross section) piston rings, it’s easy to assemble them on the piston. 

Start with the oil ring expander. It doesn’t matter which direction it faces. Simply wrap it over the piston and insert in the oil ring groove. Next, carefully wind on the oil ring scraper rails. One goes to the bottom of the oil ring groove; another to the top. Each gap on the oil ring (expander and the pair of upper and lower scraper rails) should be positioned 120 degrees away from each other. You have to be careful with the expander since they have a tendency for the ends to overlap. Once installed, the oil ring package (expander and the pair of scraper rails) should be able to easily float from side to side in the groove. Double-check the expander before the piston is “pushed” into the bore (more on this later).

Next, the second ring is installed, followed by the top ring. If you examine the ring package, you’ll find a set of “pip” marks on both the top and the second ring. These pip marks are always installed facing up. Once installed, rotate the gaps so that they don’t coincide. Keep in mind, however, that rings actually spin within the piston groove. Keeping the gaps separated minimizes the amount of oil fouling that can occur during initial engine fire-up.

Installing the pistons and rods
Wipe the cylinder bores with a clean, lint-free towel and apply a small amount of conventional (non-synthetic) oil to the walls. We always apply a liberal amount of oil to the wrist pins and piston rings as well. There’s a caveat here though: You don’t need to drench the piston in oil. You only need sufficient oil to lubricate the rings as they pass through the piston ring compressor. 

There are two common types of ring compressors on the market: expander types where the tool is clamped over the piston and you tighten it in place and a tapered job where the rings are progressively tightened as the piston is pushed through it into the bore. In either case, liberally coat the compressor with oil (again, it doesn’t have to be dripping wet).

Rotate the crankshaft so that the rod journal is at Top Dead Center for the piston you’re installing. Place the piston/ring compressor combination over the cylinder bore. You have to be positive the piston is correctly oriented. For example, if the piston is domed, the dome is on the outside of the block or the valve notches are at the top (nearest the lifter valley). Additionally, be sure you’ve assembled the piston-rod combination correctly so that the chamfer on the connecting rod (and rod bearing) faces the fillet radius on the crankshaft. Finally (and as mentioned previously), double-check the oil ring(s) to ensure the expander ring end gaps haven’t overlapped.

If the connecting rod is equipped with studs, cover them with protectors (most auto parts stores sell inexpensive jobs) or use a length of rubber tubing over the studs. The idea here is to prevent the rod studs from damaging the crank during installation. We also coat the connecting rod bearing with assembly lube.

At this point, you should be able to push the piston and rod assembly into the bore, simultaneously guiding the rod so that it falls in place over the crankshaft rod journal. In most cases, you can hand push the piston in. In others, a very light tap with the handle end of a clean plastic hammer handle works. The need for excessive force means the oil ring isn’t installed correctly.

Rotate the engine on the engine stand. Coat the rod bearing (cap end) with assembly lube and install. You can torque to specs at this point, using three steps to sneak up on the final assembled torque. Slide the rod from side to side. It should move easily. Repeat the process for all of the pistons and connecting rods. Double-check the final torque figures (we usually make it a practice to check them three times).

Wrap the engine in the plastic storage bag. You’ve completed this segment of the build.

Tools used:

  • Piston ring filing tool
  • Piston ring squaring tool
  • Tapered piston ring compressor
  • Crankshaft socket-half-inch drive
  • 1/2-inch drive ratchet
  • 1/2-inch drive torque wrench
  • Deadblow hammer
  • Feeler gauge set

Photo guide

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Filing piston rings to fit isn’t new. It’s been a performance item for more than three decades. It’s not a tough job either, but you have to take your time. We use this ring filer from Sealed Power to get the job done. There are much more exotic machines out there, but this one accomplishes the job for little cost. The downside (in comparison to some of the three-digit-cost jobs) is that you must grind small amounts, check, regrind and so on until the correct gap is reached.

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In the article, we mentioned the use of a ring-squaring tool. Here’s the B & B Performance model in action. To recap, the piston ring is installed in a given bore. Next, the ring-squaring tool is placed over the bore (and into it – the tool has a machined step on the ID). You simply pull up on the ring, squaring it against the tool.

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Once the ring is square in the bore, you can check the gap. Here, a common (but high-quality) feeler gauge is used to check the gap.

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Once the gap is set, it’s a good idea to very lightly de-burr the ring with a small, fine-tooth file. What you want to do is to gently remove the sharp ends the filing process creates.

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With the rings filed, it’s time to assemble the piston-wristpin-connecting rod package. Key components shown here are the pins, Spiral Loc retainers and, of course, the piston.

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Here’s a closer look at the Spiral Loc pin retainer along with the lock ring groove machined into the CP piston. The idea is to wind the lock into the groove.

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In this photo, you can see the rod, piston, pin and a special tool that’s designed to ease the pain of Spiral Loc or round wire retainer installation.

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During assembly, you have to ensure the piston and connecting rod is properly oriented. This means the rod big end side (and bearing) with the large chamfer must face the crankshaft fillet radius. The piston notch(es) must also be oriented correctly.

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As you can see, marking the piston surface with the bore number and the direction of installation (arrow faces forward) makes for easier assembly.

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Lubricate the pin and install. Follow up with the appropriate remaining wrist pin retainers and the combination is ready to install.

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Next is the installation of the ring package. It sounds difficult and there are plenty of old tales regarding it, but it’s not that difficult. Today’s low-tension rings make installation rather simple. The article offers full details on the process.

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The oil ring expander is installed followed by the oil ring scrapers. The only trick here is to make sure the ends of the expander don’t overlap.

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The second and top rings can be installed next (in that order – you install rings from the bottom up). Note the hollow “dot” on the ring? That’s a pip mark. Pip marks are generally installed pointing up. The top and second rings usually have them.

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Here’s the assembled piston ring package. Prior to installation, it’s a good idea to separate the respective ring gaps (top, second and oil). Most engine builders concur that a spread of approximately 120 degrees between the gaps is right (a third of the way around the piston).

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Lube the ring package using conventional (not synthetic) motor oil. We use assembly lube on the bearings. Keep in mind you don’t need to shove the rod into a pail of oil. Sufficient oil to keep the works from binding during installation is all that is necessary.

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In this photo, the piston package has been inserted into a tapered ring compressor (this is a very nicely machined high-quality piece, as is their ring-squaring tool). What it does is slowly compress the rings on the piston as they go down into the bore. This tends to eliminate ring hang up as the piston is installed. We always double-check the ring orientation (including the oil ring expander) as it goes through the compressor. It’s also a good idea to lube the ID of the tapered compressor with engine oil.

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This photo shows the piston skirt and rod inside the cylinder bore with the compressor on the block deck surface.

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You can often push the piston through the compressor with your fingers, but in some cases, a very light tap with the (clean) end of a hammer handle can be used to get things moving.

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On the bottom side, you have to carefully guide the connecting rod over the crankshaft journal. The red fluid is engine assembly lube. Keep in mind it is quite easy to damage the crank, particularly when using connecting rods with studs. More info is in the main article above.

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Tighten the rod bolts (or nuts, depending upon the configuration) and then torque to specification. Typically, we use three steps to work our way up to the required torque figures.

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While not covered in the article, it’s a good idea to check the connecting rod side clearance. To do this, a feeler gauge is simply inserted between the respective connecting rod pairs. Factory service manuals regularly spell out a tolerance range for this measurement.

Read the next article in this series and find out how to build the head gasket and install cylinder heads

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