Finally! An all-new Big Twin engine from Harley-Davidson! A bit late, but worth the wait! Since Harley doesn’t do this every day (or every decade for that matter) this is a Big Deal Big Twin!

For the purpose of this introductory explanation of what it is and what to expect from it, I thought I’d simply interpret and expand on, as best I’m able, the information H-D has provided on this powerplant, point by point, and explore how The Motor Company goes about delivering on the slogan for the Milwaukee-Eight (M-8): “Stronger, cooler, with more comfort and style.”

Starting from the top—literally—there are stylish new two-piece rocker boxes... an easy way to visually recognize the new engine. Under them is where the action begins and there’s a lot going on under the covers!
Starting from the top—literally—there are stylish new two-piece rocker boxes… an easy way to visually recognize the new engine. Under them is where the action begins and there’s a lot going on under the covers!

Introducing brand new Big Twin engines and associated mechanical entities, like primary drives, clutches, transmissions and such, on the bread-and butter-bikes only, is a bold move by The Motor Company. Face it, Touring models are the backbone of Harley’s model lineup, so to place this new engine exclusively in them—first—is a gauntlet thrown in the face of those naysayers who blather on about never buying the first year of anything. They say it’s risky business until the bugs are worked out. Harley is betting the bugs have been exterminated in advance. It’s a safe bet!

Beefy, ribbed rockers arms for the four-valves, sitting in four pedestal mounts in each cylinder, catch the eye first. Anyone familiar with Evo/Twin Cam arms will realize this design is both lighter and stronger, for quicker revs and durability at high revs. Operating smaller, lighter valves, against gentler cam ramps via beehive springs with lower pressures means greatly reduced loads, less friction, quieter operation and, more importantly, a lot more mileage between top-end overhauls.
Beefy, ribbed rockers arms for the four-valves, sitting in four pedestal mounts in each cylinder, catch the eye first. Anyone familiar with Evo/Twin Cam arms will realize this design is both lighter and stronger, for quicker revs and durability at high revs. Operating smaller, lighter valves, against gentler cam ramps via beehive springs with lower pressures means greatly reduced loads, less friction, quieter operation and, more importantly, a lot more mileage between top-end overhauls.

The major engineering decisions that make this engine new (and vastly improved) amount to these: twin-plug four-valve heads, single camshaft, gear-driven counter balancer, and… wait for it… crankcase venting to the transmission. There’s more, but mostly in detailed design, intended to enhance those main features.

Yes, even though crankcase breathing is routed to a transmission sump cavity, some air and oil will manage to make it into the heads. So, there’s this new one-piece breather valve located in each head. It’s more efficient than earlier designs and has a lot less to deal with in the first place. Call it cheap insurance.
Yes, even though crankcase breathing is routed to a transmission sump cavity, some air and oil will manage to make it into the heads. So, there’s this new one-piece breather valve located in each head. It’s more efficient than earlier designs and has a lot less to deal with in the first place. Call it cheap insurance.

The first 4-valve Harley heads since the board-track racers of the old days got those extra valves and those twin plugs because the powerplant has to be emissions compliant world-wide. It meets the toughest of those, “Euro 4,” with no trouble. That’s more than you could say for any previous Big Twin, but only tells half the story. What’s best about the four-valve, twin-plug, pent-roof combustion chamber can be summed up in one word… potential. The level of efficiency in this configuration is such that the engine makes its power in a very low state of tune. Killing a lot of birds with this rock is easy. Lower emissions, improved power delivery everywhere in the rpm range, fuel economy, and lower operating temperatures, to name a few. Those who want to turn up the heat (in both senses of the word) and use some of that latent potential are in luck! (We’ll examine that subject in depth in next month’s Motorhead Memo.)

The new heads also accommodate the cooling lines, although the oil-cooled heads and water-cooled heads have these mounts in different locations respectively
The new heads also accommodate the cooling lines, although the oil-cooled heads and water-cooled heads have these mounts in different locations respectively

The decision to use a single cam, regardless of how retro it appears on the surface, is actually the best way forward for this new four-valve, pushrod layout. Quieter because there are fewer parts thrashing around under the cam cover and because of gentler ramps, lobes and lifts… compared to camshafts that came before on Harley Big Twins. Because the flow in the heads is greater (by 50 percent, H-D says), massive amounts of lift are unnecessary. As is radical timing. Might take some adjustment in thinking for folks who intend to hot-rod these engines but it amounts to less wear and tear on the entire valve train as well as better results from milder cams with “specs” worlds apart from the “traditional.”

There’s also a genuine knock sensor attached to each head. (No more using the spark plugs for that purpose.) These sensors don’t just help prevent detonation. In combination with the twin spark plugs, they improve combustion efficiency by holding things within proper parameters, which, in turn, reduces engine heat under any given load. A win-win for rider comfort and clean-burn emissions.
There’s also a genuine knock sensor attached to each head. (No more using the spark plugs for that purpose.) These sensors don’t just help prevent detonation. In combination with the twin spark plugs, they improve combustion efficiency by holding things within proper parameters, which, in turn, reduces engine heat under any given load. A win-win for rider comfort and clean-burn emissions.

Smoother V-twins are the reason for counter-balancers in the first place. The Twin Cam “B” got ’em in a chain-driven form to tame vibes in the solid-mount Softail chassis, but it was a piece of add-on engineering…and not without niggling faults. The Motor Company knew they could do better and now they have. This is a definite design improvement, if for no other reason than it cuts reciprocating weight, simplifies and reduces parts count, is quieter and shall not fail or flail at high rpm (although for a Harley that’s 5500–6000 rpm!). Then there’s this: The Motor Company can obviously, easily, change the balance factor to suit different applications.

Finally, we get to look underneath the head at those twin 12mm spark plugs, parked at each end of the pent-roof chamber, straddling two 39.8mm (1.567 inch) intake valves and two 30mm (1.18 inch) exhaust valves. Mind you, each head has an individual port for each of the four valves! There’s lot’s of velocity as well as increased volume gains with that feature. Helps explain the superlative throttle response of the M-8 engines!
Finally, we get to look underneath the head at those twin 12mm spark plugs, parked at each end of the pent-roof chamber, straddling two 39.8mm (1.567 inch) intake valves and two 30mm (1.18 inch) exhaust valves. Mind you, each head has an individual port for each of the four valves! There’s lot’s of velocity as well as increased volume gains with that feature. Helps explain the superlative throttle response of the M-8 engines!

The thing is, speaking of applications, why put this counter-balanced engine into a rubber-mount chassis? It seems fair to say, that after the M-8 engine has spread to and through the entire Big Twin lineup for a couple of years, we just might see some new chassis to hold it. What if The Motor Company has an improved Touring chassis in the works? Or, maybe the day will come when all “long” primary models go away in favor of short primary redo’s of the Softail and Dyna “platforms?” And, dare I hope, if the Dyna goes away, it might it be replaced with a versatile, smooth, great-handling, light Big Twin model, that combines the best of the Dyna and the FXR… only goes both one better? Change the balance factor to suit a solid mount frame and away we go!

New lifters run in new, anti-rotational guides, which keep the wider roller portion of the lifter pointed in the right direction, keeping wear and tear, friction and noise to a minimum
New lifters run in new, anti-rotational guides, which keep the wider roller portion of the lifter pointed in the right direction, keeping wear and tear, friction and noise to a minimum

Anyone tired of so-called “oil carryover” can rejoice in the fact that oil no longer goes anywhere near the air cleaner. Instead, it is routed into a cavity in the sump underneath the transmission. May not sound like much, but it’s a kind of quiet revolution in that age-old bugaboo of Big V-twins with small crankcases: oil control. The massive amounts of air and oil churning within is caused by the huffing and puffing of pistons pumping up and down, creating first pressure then vacuum inside. These pulses need to be vented properly. In the past it has been done directly from the cases, then up through the cylinder heads. Never, since 1936, has it been completely tamed. The proof has been found inside air cleaners for decades. After hard running, oil, in theory separated from air in advance, has still managed to make the occasional mess where you don’t want it… all along the right side of the motorcycle. No more… period! An under-appreciated improvement that can not be overestimated.

Here we see two images of the new 55mm, two-piece induction module. The fuel injectors you see in black on the blue picture, which are indexed in the manifold, have a dual-spray pattern, to ensure that each of the two intake ports receives its own properly metered “spray.” The MAP (manifold air pressure) input sensor for the ECM plugs into the white connector, pictured on top of the manifold. And, yes, the black module in the picture, as fitted to the M-8 engines, is made of plastic to save weight. This has raised an eyebrow or two, but modern plastics can take as least as much abuse and heat as aluminum, as well as being a more accurately made part.
Here we see two images of the new 55mm, two-piece induction module. The fuel injectors you see in black on the blue picture, which are indexed in the manifold, have a dual-spray pattern, to ensure that each of the two intake ports receives its own properly metered “spray.” The MAP (manifold air pressure) input sensor for the ECM plugs into the white connector, pictured on top of the manifold. And, yes, the black module in the picture, as fitted to the M-8 engines, is made of plastic to save weight. This has raised an eyebrow or two, but modern plastics can take as least as much abuse and heat as aluminum, as well as being a more accurately made part.

Except for the lifters, all this stuff lives on top of these new cylinders. The barrels with their cast-iron liners are front and rear designated, and mounted on metal base gaskets, so they’re not interchangeable as in previous Big Twins. The stud hole spacing is wider as well, with deep spigots and crankcase contact areas slightly increased in the bargain. The name of the game here is stability. These are not going to distort or shape shift like older designs.
Except for the lifters, all this stuff lives on top of these new cylinders. The barrels with their cast-iron liners are front and rear designated, and mounted on metal base gaskets, so they’re not interchangeable as in previous Big Twins. The stud hole spacing is wider as well, with deep spigots and crankcase contact areas slightly increased in the bargain. The name of the game here is stability. These are not going to distort or shape shift like older designs.

These features are the foundation upon which Harley-Davidson has erected its latest and greatest Big Twin engine. But there’s more to it than that. More than meets the eye. So much more, we can’t get to all of it in one issue. (So, the second part of this dissection of details… will be in the next issue of THUNDER PRESS.)

OK, next we’ll get to the “bottom” of the new Milwaukee-Eight. In other words—to be continued…

Click here to read part two!