Bottom end basics for a blown small block Chevy engine build
383ci SBC Build, Part I
By Ryan Manson * firstname.lastname@example.org
When it comes to powering that project that you’ve been working on, there are basically three options; buying a crate engine, buying a junkyard motor, or building an engine from scratch. The first option is by far the simplest and for many of us makes the most sense. With a myriad of companies offering crate engines in every form, from benign 285hp 350s to 700hp LS engines, there isn’t much left to desire when it comes to motorvation. It’s literally a plug ‘n’ play situation, especially if the engine has been broken in and comes shipped with a dyno sheet to back the results. Couple that with a decent warranty and there really isn’t a better “bang for your buck” option when it comes to engine selection. The only downside to the crate engine option is that it can be a little pricey. For some of us, $15k is more than we’ll have in the whole project, so that supercharged LSA crate engine just doesn’t fit our budget. Those trying to pinch a few pennies can try their luck at the roulette wheel that is used drivetrain, but it’s “buyers beware” for the most part. That “low mile” LS engine that came from a rearended truck may in reality been some auto shop’s parts hauler driven by an overzealous, lead footed teenager that never saw an oil change in its 300,000 miles. That leads us to the last alternative, building an engine from scratch.
In the past, when it came to building a custom engine, it usually began with a used block, sent off to be machined and either built by the same engine shop or assembled personally by the owner. With one person ordering the parts (owner) and another dictating what the build required (machine shop), sorting through all the technicalities to get the right components the first time could be a daunting task. In a machine where tolerances are measured in the thousandths of an inch, incorrectly spec’d bearings, rings, or pistons could bring the entire build to a screeching halt.
It’s been a while since I built an engine, the Hemi in my F1 being the last, so I thought it time to throw down the gauntlet and build something. While I was mulling over my options, I realized that the last two engine shops that I used were no longer in business. Guys retire, businesses change hands, and the economy hits mom and pop shops the hardest sometimes when it takes a dip. Without a go-to option, I started to broaden my search for alternatives when it came to my engine build. While I could source a short block from a number of dealers, I couldn’t find one that served all my needs.
What I ended up finding however was an even better option; a completely machined engine block, available in a variety of bore and stroke combinations, via Summit Racing. That gave us all the specs we needed to then order up the corresponding rotating assembly that should result in a bulletproof bottom end (provided I don’t screw things up on assembly!). From there, it was a simple manner of choosing heads, valvetrain, ignition and induction components and we were off to the races.
Building an engine in this manner truly cuts the middle man out of the equation (sorry local machine shop) as the block shows up ready for assembly. Bored, line honed, decked, clearanced for the stroker crank, and with all the proper freeze and galley plugs installed, our engine only needed a quick cleaning for good measure before we got to work. Follow along as we showcase the products we’ll be using and how we hope to build a powerful motor with a vintage vibe.
Building a solid street engine isn’t rocket science, but there is a bit of math involved in order to optimize the performance and reliability of said engine. Whether you’re going with the aforementioned path and buying a machined block and matching components or you’ve farmed out a used block to the local machine shop and had them machine it to match your chosen components, it’s a good idea to measure everything before it all goes together.
One time-honored method to measuring bearing clearance was to use Plastigage. A strip of the colored stuff is placed in between the bearing and the crankshaft and the assembly is then torqued to spec. It’s then disassembled and the compressed Plastigage strip compared to the product’s provided scale. The result is then compared to the specs determined by the engine’s machine shop and judged as to whether or not the assembly is acceptable.
For most street-driven, moderate horsepower applications, this process is totally satisfactory as it’s only double checking the clearance that should have been machined into the assembly to begin with; it’s strictly done to ensure that the parts have been machined to the specs determined by the machine shop before assembly. It’s a fail-safe step and by far the simplest way to determine safe bearing tolerances, be they crank or rod.
Taking this process a step further requires the use of a few specialized tools. First, the crank main bearing journals are measured using a micrometer. The micrometer is then installed in a vise so that a dial bore gauge can be installed and zeroed to the crank journal’s diameter. Next, the dial bore gauge is inserted in each of the main bearing journals on the block (with the bearings installed and the caps torqued to spec) and the bearing diameter noted. The difference between this measurement and zero on the dial indicator is the bearing clearance. Checking each bearing assembly using this method allows the builder to note the exact clearance and adjust the tolerances to suit (installing over- or under-sized bearings for example).
Using this process ensures that each measurement is determined within the confines of the mic’s ability to accurately measure the crankshaft’s journal. It is possible to yield the same results by measuring the crank journals using the mic and comparing that number with readings taken from each bearing via the dial bore gauge and comparing the two. The problem with this method is that if either or both tools are out of spec a few thou, your clearance number will be affected and incorrect. By setting the dial bore gauge to the mic, it doesn’t matter whether either tool is out of spec as we’re only measuring the difference of the two via the dial indicator. Provided the measurements are taken correctly, it doesn’t matter what the actual measurement of either the crank journal or the bearing assembly is as we’re only concerned with the difference or clearance. CC
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