John Scott Lewinski/CNET

MENOMONEE FALLS, Wisc.--To Harley-Davidson riders, the Powertrain Operations facility here just north of Milwaukee is hallowed ground. It's where the hearts of their favorite bikes start beating.

Engines for most of the Harley-Davidson line come alive here before they head to bike assembly plants in Pennsylvania and Missouri. To help promote Harley-Davidson's emerging Dark Custom line, CNET was invited to stroll through the plant as part of its Steel Toe Tour this past weekend to see how the company's V-Twin engines come together.

Harley-Davidson began offering the Powertrain Operations tour in 2010, escorting a small number of motorcycle lovers through the factory floor to see the various stages of V-Twin engine construction. The tour builds chronologically from the construction, refinement, and inspection of engine components, including cam heads and pistons, to the fly wheel assemblies and overall engine build. About 100 employees work in two shifts at Powertrain Operations alongside Swedish-made industrial robots that perform assembly and inspection.

The tour concludes with a look at the final engine constructions coming off the assembly line. Tour visitors get a chance to see how the engines look in an Iron 883, Blackline, Forty-Eight, or Street Bob bike. The experience might not be everyone's cup of tea, but for motorcycle nuts, it's a rare to look at the place where biker culture truly begins.

Like many factories, they wouldn't allow us to publish much of what we saw on the tour, but they did give us permission to post some photos. Come along with us as we look at just a few of the steps that go into the Harley-Davidson design and build process.