Building an engine is vastly different than assembling one. One involves proper fitting of parts—the other, merely bolting things together. Today Davin takes you on a deep dive into one of the key parts of engine building, whether you’re aiming for performance or basic conveyance: compression ratio.

The concept is simple: The amount of space inside the cylinder when the piston is at bottom dead center compared to the amount of space inside the cylinder when the piston is at top dead center. As anyone who’s built an engine will know, however, that which is easily summed up is not always the easiest to execute. Accurately calculating compression ratio takes a bit of math, a bit of knowhow, and some antifreeze (seriously).

Davin starts by obtaining the size of the combustion chamber in the cylinder head, using a plate of glass and burette to measure its exact capacity in cubic centimeters. With some basic math, he calculates the cylinder volume, and all that is left is the volume added or subtracted by the piston dish or dome. In the case of our Jeep’s 4.6-liter engine, the piston is dished. While Davin could measure that additional area exactly the same way he did the combustion chamber, he decides instead to set the piston a small distance into the cylinder and grab a pencil to do the math and get the final number.

Compression ratio determines a few very important characteristics of an engine. The one Davin is most concerned with is this straight-six’s ability to run on “pump gas.” To Davin, that means 87-octane fuels. For an engine to run on fuel that volatile without any knocking, it is best to target between 9 and 9.5:1 compression ratio. He calculates that this particular Jeep’s engine compression would be around 8.5:1 as it sits now. Not ideal.

He devises a simple fix: Use a thinner head gasket and take a quick trip back to the machine shop to shave the deck height just a smidge more. With those changes made, this engine will be ready for final assembly, a process which Davin can begin with confidence, knowing that all the parts will play nicely together and that the mill will run as he expects it do when it’s time for first start-up.

To see the rest of the assembly and other tips and tricks our resident engine builder, be sure to subscribe to the Hagerty Youtube channel.

Thanks to our sponsor, an auto parts retailer founded in 1999 by automotive engineers with two goals: Liberate information hidden behind the auto parts store counter (by listing all available parts, not just what one store stocks or one counter-person knows), and make auto parts affordable so vehicles of all ages can be kept reliable and fun to drive. Visit to order auto parts online 24 hours a day, 7 days a week, and have them conveniently delivered to your door. Need help finding parts or placing an order? Visit RockAuto’s Help pages for further assistance.

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Enjoy Jeep stories, opinion, and features from across the car world - Hagerty Media

When Kate Cook was 15 years old and beginning to search for her first car, her father, Eric, imposed only one requirement: Whatever Kate chose, it had to be four-wheel drive.

At the time, Kate’s family lived near Lake Arrowhead, a resort town tucked into the San Bernardino Mountains high above the surrounding deserts of Southern California. Known since its earliest days as a popular summer getaway for well-to-do Hollywood and Los Angeles business types, Lake Arrowhead and its surrounding hamlets—at least, once away from the immediate lake shore—have long offered an affordable change of pace for those seeking to escape the frenetic pace of city life. And unlike L.A., Lake Arrowhead experiences winter, complete with snow and ice. Eric was determined that whatever car his daughter chose, it had better get her home safely after she navigated those twisty mountain roads.

Even before her dad set his conditions, Kate had her heart set on a Jeepster Commando. She had seen them around the mountain and loved their shape. But how to find one that had not been completely disfigured? Aftermarket modifications tend to find their way to old Jeeps. After much searching, Kate and Eric located a 1966 Jeepster in the nearby desert town of Barstow, California, Upon bringing it home and digging into it more deeply, however, they discovered that they were the proud new owners of a rolling time bomb. Thus did Kate’s education in auto mechanics (and in some respects, adult life) begin.

Aaron McKenzie

Eric, who spent his career as a mechanic in the Air Force, taught Kate how to sift through every inch of the Jeepster as they diagnosed and remedied its many maladies. At the time they started to rebuild the engine, however, Kate did not even know how to change a tire or check the oil. She’d have to learn.

In the midst of this process, Eric and Kate’s mom, Cynthia, both fell ill with serious and debilitating health problems. Now unable to physically work on the Jeepster himself, Eric nevertheless came out to the garage with Kate everyday and, through his words, talked her through the process of reviving the 4×4. Both Kate and Eric are inveterate to-do list makers, which Kate—only half-jokingly—suggests is their way of trying to impose order on a chaotic world. Whatever its psychological roots, the systematic approach is certainly a habit which helped keep her focused and clear-headed through her parents’ illnesses.

“There was no timeframe for when my parents would get to be feeling better, or even if they ever would,” says Kate. “But with auto mechanics, there are steps and then you’re done, so with those lists I could check things off and that gave me a sense of stability.”

With the Jeepster on the road and her parents’ health improving, Kate eventually found her way to art school in Texas, where the vehicle once again shaped the course of her life.

Aaron McKenzie

“Around town I kept seeing this old-school Jeep, and then one day I saw this blond girl driving it,” says Kate’s husband, Morgan. “I thought, ‘Wow, I need to meet her!’”

Now in their fourth year of marriage, Kate’s and Morgan’s life together now frequently revolves around adventures in their Jeepster. Most of the time, owing to the old rig’s quirks, those adventures do not go as planned. That’s perfectly fine with Morgan and Kate.

“Most of the time we try to get to certain destinations but if we don’t, that’s okay,” says Morgan. “The trip is the trip, not the destination.”

Aaron McKenzie

Not surprisingly, this Jeepster is more than just four tires and a pile of metal for Kate, who now makes her living as an automotive artist and pinstriper. The Jeepster and the structure it brought it to her life taught her perseverance, helped get her through her parents’ illnesses, and ultimately led her to her husband. Little wonder, then, that Kate has no intentions of parting with this Jeep.

“I would never sell this Jeep,” says Kate. “If I ever couldn’t drive it, I would just park it out front and plant flowers in it. It’s just a time capsule for me and for my own family. It’s so sentimental to me.”

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