Crankshaft: We don’t know if the crank webs are welded or still pressed-fit. If welded, then the crank-slip defect will be resolved. If not, trouble could very well be on the horizon. The crankshaft has been totally redesigned increasing flywheel inertia 20% without adding weight.
Pistons: They are light weight design, flat top, with low-tension bore-conforming rings and compression ratio is higher. The piston rods are larger and stronger.
Camshaft: Yes, there is a cam chain and a cam chain shoe that can wear out and cause a catastrophic engine failure. However, like the Indian Power Stroke engine the inner cam chain and tensioner shoe is eliminated. You can easily check the existing exterior cam chain shoe for wear simply by removing the cam cover. If the shoe is worn, it is very easy to replace the shoe with a new one without the use of special tools. This is a wonderful improvement and for all intent and purposes the cam chain defect has been generally considered cured and fixed. You still need to inspect the shoe condition, but even a non-mechanic can do this and even replace a worn-out shoe. Very easy to do! There is no need to pull the cams or oil pump to replace the shoe. This is great news! There is only one camshaft in the M-8 engine further simplifying the cam chest mechanics.
Lifters: The valves are operated by solid lifters, but like the Twin-Cam engine, with a twisted operating valve train. This may still be a problem if the cam-actuated solid lifters stick in their bores and cause pistons to strike stuck open valves. Time will tell if the lifter problem is solved. The engine has solid lifters which means there is an adjustment screw on the rocker arms near the valve stem. Harley-Davidson claims the valves never need to be adjusted for the life of the engine due to the valves being light in weight so there will be no valve recession. This is yet to be seen and proven in the real world with valve recession almost certain to occur as the valves wear against valve seats. However, if a valve adjustment were ever needed it would be very easy to do not having to deal with engine disassembly like other engines with solid lifters or ride directly on the cam shaft lobe relying on (under or over) valve shim cups which is a royal pain to adjust the valves. At least the M-8 engine has an adjustment screw to adjust the valves like the tappet system of old. It would be worth your attention to periodically check the valve clearance so as not to burn a valve, say every 25,000 miles would be fine. There could be some sort of a compensation system employed so no valve adjustments are needed? Nobody knows at this time. The only hint is the valves are so light in weight they will not wear the valve seat or the valve face (recession). I find it hard to believe the factor valve adjustment will last the life of the engine. Has Harley-Davidson created a miracle of no valve lash on solid lifters? We’ll see. I hope it is true.
Foreseeable Problem With Rocker Arms – When the engine is new the lifters, pushrods, rocker arms and valve stems will work just fine. The problem begins when the rocker arm loses stability due to uneven wear of the point of contact with the valve stems. There’s no adjustment available to compensate when one of the two intake (or exhaust) valves wear unevenly compared to the other valve (remember, one rocker arm operates two valves). A loud tapping sound will be made with a loss of power. The hydraulic lifter will not compensate for this unbalanced situation. Will a valve burn? No. Just a tapping sound will be produced as the rocker arm slaps against the valve stem. It’s not a major problem and the tapping noise will not harm the engine. To correct the problem? Wait until the engine needs a top-end overhaul then reset the valves in the cylinder head, replace the rocker arms, push rods and that should bring the engine valve train back into new condition.
Primary Drive: The primary chain case is much more narrow which eliminates that sloppy, long-leveraged drive-side crankshaft. Reliability will increase here, no doubt about this along with a redesigned and stronger redesigned compensator sprocket that should not fail like those in the past. There’s an automatic primary chain tensioner that has had problems in the Twin-Cam engine and could keep on tightening the chain which can cause fast chain and sprocket wear and even overheating main crankshaft and clutch roller bearings. Need to watch for this problem if it has not been fixed. The tensioner just keeps on tightening the chain each time you wallop the throttle upon hard acceleration like when passing a car. The spring-loaded tensioner never loosens the chain to maintain a proper chain tension. A hydraulic system would be a fix for this. You may be able to install the old manual chain adjustment system (but I am not certain if this can be done on the new M-8 engine).
Cylinder Heads: Increased cooling fin depth keeps engine heat under control. Exhaust ports are shorter also lowers head temperature. Water and/or oil ports, depending on engine model, keeps the engine running cool. Each cylinder head has two intake and two exhaust valves which flows more air and fuel for increase engine power and obtain better fuel mileage. Two spark plugs per cylinder creates a faster and more even combustion burn (just like a top-fuel nitro dragster). Engine has an advanced knock-sensor system that reacts faster and an automatic compression release for easy and much faster effortless engine starting even when the engine is cold.
Breather System: Much improved as the crankcase air flow is now directed into the transmission case instead of directly into the air cleaner. This should stop the oil priming problem (oil spilling out of air cleaner) on all prior V-twins have historically had. Once the pressure has bled off into the transmission case the unpressurized vapors can now be bled-off-controlled to be burned in the cylinders. There is one potential problem as these blow-by gasses can be very acidic and transmission components could corrode creating a new problem. Generally, I would not worry much about corrosion issues as engine/transmission oil have acid controlling alkaline chemicals to counteract the acidity. Just keep the oil clean – especially when laid up for the winter in storage – and all should be well.
Fixed Defects: At this time, the engine appears to have fixes that will make the Milwaukee-Eight engine more reliable, more powerful and more fun. But there are other problems that may be hidden from our eyes at this juncture in time. The valve train is heavy above the lifters so there is potential for trouble of valve float or metal fatigue issues. Cylinder heads may crack for all we know over time due to changes in the design. Let’s see what develops and hope nothing fails in the real world. At least Harley-Davidson is addressing many of the defect problems that ruined the Twin-Cam engine.
Cam Chain Tensioner: The cam chain will wear out the fiber composite tensioner shoe as it did to the Twin-Cam engine. If you do not inspect the tensioner it can wear down to metal-on-metal contact with the cam chain and it will clog up the oil pump and seize the engine. If this happens at highway speed it could kill you as it locks up the rear wheel and you fail to pull in the clutch in time. So, the M8 engine is still defective in this regards, but the cure is simple; inspect the condition of the shoe every 30,000 miles and replace it every 60,000 miles or every 30,000 miles just to be safe. Replacing the shoe is so easy, just remove the cam chest cover, unscrew the shoe retaining bolt, install the new shoe, reinstall the retention bolt, torque it to specification, replace the gasket and cam chest cover. As easy as installing a new spark plug! The old Twin Cam engine required a slew of special tools and some major assembly riders could not do on their own. You can replace the cam chain shoe on your M8 engine easily. Just remember one thing, if you forget and ignore inspecting of the cam chain shoe condition it will wear out and bite you hard as it catastrophically destroys your wonderful M8 engine. Don’t forget the inspections!
Reliability: The Milwaukee-Eight engine will never surpass the reliability of the Sportster 1200 Evolution engine for that V-twin engine is proven to be incredibly defect free. So much so, that many Harley-Davidson dealers do not even keep an inventory of Sportster engine parts. Imagine that! If you need a engine or transmission part they order the parts from the factory. Just ask any Harley dealer how many engine repairs they do on Sportsters and they will respond with a depressing answer. They don’t make money on Sportster engines breaking down. My 2012 1200cc Sportster XL Custom has 70,000 miles on it and it runs like new. Doesn’t burn oil and sounds quiet like it was when new. But the M-8 engine is way better than the Twin-Cam engine.