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Do-It-Yourself Projects
Engine Assembly, Part 3: Fitting Rods and Pistons
By Wayne Scraba/
Printable version
Degree of DifficultyModerate
Estimated Time300 minutes
300 minutes
Fitting Piston Rings, Installing Floating Wrist Pins and the Piston-Ring-Pin-Rod Combination in the Block -
The last time around, 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.
Crankshaft Journal in an Engine Assembly
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, poll pro engine builders and 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 upon performance. Too large a gap and cylinder pressure leaks through the gap and the engine loses power. Too small a gap and the ring ends will butt together as the engine heats and expands. Ring gaps that are butted results in engine damage.
So what’s the answer? 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.
In order 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.
Ring-Squaring Tool
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 deburr 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.
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