Engine Of The Week: Twin Cam Harley-Davidson Motorcycle Engine Kit

Engine Of The Week: Twin Cam Harley-Davidson Motorcycle Engine Kit

These kits increase the power and performance of factory motorcycle engines such as Harley-Davidson Sportster, Evolution and Twin Cam engines.


For the past 36 years, Dan Fitzmaurice and the folks at Zipper’s Performance Products have been helping other engine shops deliver more power and performance to customers through the development of its motorcycle engine kits. These kits increase the power and performance of factory motorcycle engines such as Harley-Davidson Sportster, Evolution and Twin Cam engines.

“We have a few dealers who we might supply complete units, but most all of it goes out the door as kits,” says Dan Fitzmaurice, owner of Zipper’s located in Elkridge, MD. “The kits include cylinders, pistons, heads, and cams, which make up the basis of the package. Some are sold with induction and a tuning module as well. The heads have been CNC ported. The cylinders have been professionally sized. The pistons are spec built to the engine matching combustion chamber. Compression ratios are figured out. In many cases we even provide calibration for the tuner. When it’s all assembled, it’s a cinch to get it done and delivered to the customer.”

Zipper’s develops these kits to go on the factory base of engines, and the most popular kits are for the Harley-Davidson Twin Cam engine.

“We develop the kit and we do a lot of testing,” Fitzmaurice says. “We come up with calibration. We also test the riding or racing experience. We sell that as a package so that engine shops can make money without having to do a lot of work sorting out what works with that engine such as height, induction, compression ratio, etc. We make a variety of kits for different displacements, models and how much power they put out.”

Zipper’s develops and CNC-ports heads in-house. The shop also grinds its own camshafts in-house. Essentially, aside from pistons, valves and gaskets, Zipper’s is making most of the parts in its kits right in Elkridge, MD.

“We develop everything in-house along with a bunch of other ancillary parts,” he says. “We use Cometic to source gaskets in a lot of our kits and we use a couple piston manufacturers to make a spec-built piston for our specific application. We don’t make valves, but a lot of those we use off the aftermarket shelf.”

In order to develop much of the engine kits in-house, Fitzmaurice has built-up quite the array of tools and machines.

“We have CNC lathes, CNC mills, cam grinders, universal grinders, tool equipment, honing machines, etc.,” he says. “It’s a 35-year collection.”

These Twin Cam engine kits are made in a way that in some cases the kits could deliver 30-, 40-, or 50-percent more power depending on the need of the shop that is buying the kit.

Red Shift Performance Cams

“A Screamin’ Eagle 110 from Harley-Davidson makes 88 hp,” he says. “We have one package that ups that to 110 hp. Another boosts it to 130 hp, and a third one puts out 150 hp. Those are big jumps. We try to do it in phases so shops have something to choose from that fits a certain customer. We structure it in a way that shops can study the power curve and the price point and see what fits their customer.”

Offering those different levels and options for other engine shops to choose from is exactly why Zipper’s insists on developing kits in-house.

The most popular Zipper kits are the 103cid, 107cid and 110cid kits. That’s the bulk of the market, according to Fitzmaurice.

“Our focus has always been powerful and reliable,” he says. “Durability is very important to us because we are all racers. We used to race professionally in the ‘80s and ‘90s. Winning races and being there in the end was always about being able to consistently make the power run after run and have durability.”

In those days, Fitzmaurice and his racing buddies didn’t have a lot of time to work on their own equipment because they were always servicing customers. They had to make things reliable, and have carried that over as a Zipper’s mantra.

“We want shops to say this worked out so good we want to buy another one,” Fitzmaurice says. “Repeat business is the key to business. So we haven’t strayed from that, and it drives all of our development work we do.”

Engine of the Week is sponsored by Cometic Gasket.

Article courtesy Engine Builder.

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