A rotating assembly kit promises the hard part of a bottom end in one box: a crankshaft, a set of connecting rods and, usually, the pistons that ride on them, chosen to work together. That promise is real, but the word "kit" is stretched to cover very different boxes. Some hold three parts. Some hold eight. Two listings can even share a title word for word and still ship different pistons. This guide goes through what a rotating assembly kit actually contains, how to read each line of the parts list, and what you will still be buying after the box arrives.
Three different boxes sold under one name
The complete assemblies in our catalog right now all come from Eagle, and their product descriptions fall into three groups. Knowing which group a listing sits in tells you more than the price does.
| What's in the box | What the description lists | Examples in our catalog |
|---|---|---|
| Crank, rods and bearings (no pistons) | Billet crank, H-beam rods and King XP rod and main bearings (the EJ descriptions add 4340 steel and 3/8" ARP 2000 rod bolts) | Subaru EJ20/25 3.110" assembly, EJ20/25 3.268" assembly, Honda K20 crank, rods and bearings |
| Crank, rods, pistons, rings and bearings | The same crank, rods and bearings, plus forged pistons and plasma-moly file-fit rings | K20 with -3.5cc 86mm pistons, EJ25 with -4.5cc flat-top 100mm pistons, EJ257 forced-induction stroker |
| Everything above plus a damper and flexplate | Cast crank, I-beam rods with 3/8" ARP 8740 bolts, hypereutectic pistons, rings, King SI bearings, damper, flexplate | Small block Chevy 5140 I-beam kit, Eagle balanced kit with 4.000" crank and 157-tooth flexplate |
The first group is for builders who already have pistons in mind, or who want a specific piston brand. The second gives you a matched set down to the rings. The third, so far only in our small block Chevy listings, goes a step further and includes the parts that bolt to each end of the crank.
Reading the parts list, line by line
The crankshaft: material and stroke
Our import-engine assemblies all use a billet crank, in 4340 steel wherever the description names the material. The small block Chevy kits use a cast crank. The number in the title is the stroke: 3.661" on the K20 assemblies, 3.799" on the Honda F20/22C, 3.750" and 4.000" on the Chevy kits. The EJ listings show why that number matters. Eagle sells EJ assemblies with a 3.110" crank and with a 3.268" crank, both on the same 5.137" rods, and its EJ257 forced-induction assemblies, built on the 3.268" crank, are titled "Stroker." A different stroke moves the piston to a different height in the bore, so the crank, rods and pistons only work as the set they were sold in. That is the main advantage of buying them together.
The pistons: bore, volume and alloy
Piston assemblies come in several bore sizes for the same engine: 86, 86.5 and 87mm on the K20; 99.5, 99.75, 100 and 100.5mm on the EJ25; 92.5 and 93mm on the EJ20; 4.030" on the Chevy kits. Choose the size your block will actually be finished to, which means measuring first and talking to your machine shop. Order pistons before the bores are measured and you risk buying the wrong size.
The cc figure describes the piston crown. A negative number such as -3.5cc, -15.5cc or -26cc is a dish (Eagle calls several of them "inverted dome"), and a positive number such as the K20's 13.6cc is a dome that sticks up into the chamber. Dished pistons lower compression and domed pistons raise it. Your boost, fuel and tune decide which you need, so settle that with your tuner before you pick a part number.
Then check the alloy, because the title won't always tell you. We list two EJ20 -16cc 92.5mm assemblies with identical titles: one ships Mahle pistons forged from 2618 aluminum, the other Mahle pistons in 4032. Two EJ25 3.110" -22cc 100mm listings split the same way. Across the range you'll also see Traum, Diamond and CP pistons, and hypereutectic KB pistons in the Chevy kits. Piston alloy changes how the piston is fitted to the bore, so tell your machine shop exactly which pistons are coming before it finishes the cylinders.
The rings: file-fit or standard gap
Most of our assemblies ship plasma-moly file-fit rings. File-fit rings are supplied so that the builder sets the end gap on each ring for its own cylinder, which takes a ring filer and some patience. The Chevy 350 2-piece assembly ships standard-gap rings instead. If you don't plan to gap rings yourself, budget for the shop to do it.
The bearings: series, not size
The import assemblies list King XP rod and main bearings; the Chevy kits list King SI. The descriptions name the series but not the size, and bearing size has to suit both the crank and the block's main bores. Check clearances during assembly rather than assuming them. Our guide to engine bearing sizes explains how STD and oversize bearings are labeled.
The rods: length and bolts
Rod length appears in every title (5.470" K20, 5.137" EJ, 5.700" and 6.200" Chevy), along with the beam type. The import assemblies use H-beam rods, with ARP 2000 bolts wherever the bolts are named; the Chevy kits use I-beam rods with ARP 8740 bolts, and the 350 2-piece kit specifies press-fit rods. We cover what those callouts mean in H-Beam vs I-Beam Connecting Rods, so we won't repeat it here.
Damper and flexplate
Only the small block Chevy kits list a damper and a flexplate, and the 4.000" kit specifies a 157-tooth flexplate. A flexplate pairs with an automatic transmission, so check it against your car before you assume it's a part you can use.
What none of these boxes contain
Even the most complete assembly stops at the moving parts. Plan for the rest before the block goes to the machine shop, so nothing holds up final assembly.
| Still on your list | Why it matters |
|---|---|
| Pistons, pins and rings | Only if you bought a crank, rods and bearings assembly. They have to suit the stroke and rod length of that assembly. |
| Head gaskets | One per head, matched to your finished bore. The Five Details That Decide Which MLS Head Gasket You Order covers bore, thickness and sides. Browse Head Gaskets. |
| Head studs and block fasteners | No assembly lists head studs, and none lists main or case fasteners. See Head Stud & Bolt Kits. |
| Machine work | Measuring, boring and honing to suit the pistons you chose. Only the small block Chevy kits say "balanced" in their descriptions. On any other assembly, ask whether it has been balanced before you plan around it. |
| Block reinforcement | A forged rotating assembly doesn't brace the cylinder walls. On an open-deck block, that's a separate job done on the bare block. |
| Gaskets, seals and timing parts | None of the descriptions mentions them, so price them separately for your engine. |
A rebuild kit can be a different kind of box altogether. Our BMW N54 Rebuild Kit has no crankshaft, so the factory crank stays in the engine. Instead it bundles forged Mahle pistons, Manley H-beam rods, King bearings, a head gasket, an ARP head stud kit and EPOXY54 with the Install Kit. Read the contents list rather than the word "kit," and you'll know which jobs are covered.
Where the block fits into the order of work
Two of the engine families above, the Honda K20 and the Subaru EJ25, are open-deck aluminum blocks. More cylinder pressure from boost reaches the rods and crank, and it also pushes on cylinder walls whose tops stand free in the water jacket. That's why a forged rotating assembly and block reinforcement so often land on the same build sheet. Our EJ257 rebuild parts list shows where each one goes in the order of work.
EPOXY54 gives open-deck blocks closed-deck strength without machining. It has 21,000 psi compressive strength and a thermal expansion rate closely matching aluminum, and it's a DIY install in 5 steps, done on the bare block before the crank, rods and pistons go in. It can be applied before or after machine work. It supports the cylinder walls of a sound block, and it is not a crack repair. For cast iron small blocks, there's the EPOXY54 Cast Iron Block Epoxy Kit (5L).
Shop the parts
Browse every assembly in Rotating Assemblies, or start from your platform with Honda K20 or Subaru EJ25. If you'd rather choose the crank and pistons separately, see Crankshafts and Pistons. Not sure which listing matches your block or your power goal? Email support@epoxy54.com before you order.