Ask MO Anything: Why Don’t More Motorcycles Use Hydraulic Valve Lifters?

We got this comment/question at the end of an article about Harley-Davidson’s new Revolution Max engine from Steve Sweetz, and it’s an excellent one:

Dear Steve,

You said it, and we couldn’t agree more except to add they’ve been standard on cars since way before the ’80s. The only reason we don’t complain more about valve adjustments is that we always give our borrowed motorcycles back to their manufacturer before it comes to that. And in that respect, some motorcycle manufacturers are better than others. My 2000 Yamaha R1 still hasn’t gotten to 26,600 miles for its first valve inspection, but then I never ride it. We know of at least a couple of Ducati fans who buy a new bike every couple of years, selling the old one just before its expensive desmo valve service is due. Caveat emptor indeed.

Maybe a little explanation is due first for our readers who grew up in the non-mechanical era. There needs to be a bit of free play between the things that move the intake and exhaust valves open – the lifter, or tappet – and the valve stem itself, to account for heat expansion and wear. That clearance shrinks over time, as the valves very gradually wear away their seats, and needs to be re-established. Simpler engines use screw-and-locknut adjusters. Most modern engines use shims. Generally a thinner shim is required to regain the factory clearance.

Valve clearance inspections are required at anywhere from 6,000 to 26,600 miles (for lots of Yamahas). On bikes that require valve inspections, somebody’s going to have to go in there with their feeler gauges to check and reset that clearance from valve to lifter to factory specs. It’s a tedious task that takes a while, some bikes more than others, and even requires mathematics if shims are involved. Shims are little round spacers that come in different thicknesses.

A shim kit, courtesy Wiseco. We’re talking tiny clearances varying by 0.05mm. I think that’s 1/20 mm. It’s an exacting task.

Hydraulic valve lifters

And in this corner, hydraulic valve lifters. These have been around for many years in automobiles, in Harleys beginning with the 1948 Panhead, and also in the Indian ThunderStroke engine which serves as our lead image. What goes on, basically, is that pressurized oil is squirted into each valve lifter, and sealed off in there as the lifter moves in its bore, thereby keeping valve lash, or gap, constantly at zero as the engine wears. The conventional wisdom is that hydraulic lifters can’t move quite as fast as mechanical, or solid, lifters, but plenty of people have been willing to make that small sacrifice for the greatly reduced maintenance requirements.

For engines turning less than around 5000 rpm, like most big car engines, hydraulic lifters were perfectly adequate. In olden times, HOT ROD magazine and others were packed with ads for solid-lifter camshafts, to open and close valves faster and higher for increased horsepower. Solid lifters meant you had to perform valve adjustments now and then to maintain the right clearances, but it was kind of fun when all you had to do was remove your V-8’s massive valve covers, lean on the fenders, and yank the pull tabs off canned beverages as you twisted the big nuts atop the rocker arms in your old pushrod engine. Thinking the job would be just as easy on my new 1986 Honda VF500F Interceptor was a valuable learning experience.

Solid lifters are better

Like so many things, it all depends who you ask. We asked Honda and got this from our media rep Colin Miller:

We have not had HVA (Hydraulic Valve Adjusters) in our units since the VT1100. That was the last one that I can remember. Some thoughts from my time on the service side and from others I talked to:

HVA are a good idea for some applications, but they do have drawbacks. They require the system to be significantly more complex due to the need to add actuators in the valvetrain system as well as a more complex oil supply system. At higher engine speeds, HVA systems tend to pump up and can cause valve float.

This can be a huge limitation for motorcycles since they often have higher engine speeds when compared to autos. HVA systems require the oil supply to be kept very clean to prevent build-up or debris from causing problems with actuators. Bucket-and-shim or screw-type adjusters take up less room, weigh less, and are cheaper from a design and manufacturing standpoint.

As for automotive, many Honda cars also don’t have HVAs. Even my 2004 Toyota Tacoma has shims that need to be checked.

Hmmm, we also remember the long-running Nighthawk 750 having hydraulic valves. The coolest were the early ones, the Nighthawk 700S produced from 1984 to ’86. It had hydraulic lifters along with a 10,500-rpm redline – shaft drive too. Honda kept on cranking them out through the 2003 model year.

Hydraulic is the way to go

Here’s the official word from Harley-Davidson on the topic, whose new Revolution Max engine it rates at 150 horsepower at 9000 rpm.

Harley-Davidson has used hydraulic lifters since the introduction of the 1948 Panhead, so we clearly have a lot of experience with this technology. For OHV (overhead valve) engines, like the Milwaukee-Eight V-Twin, and other OHVs, it is very common to use hydraulic lifters for the obvious benefits. Where it gets more difficult is with OHC (overhead cam) engines. Here again, Harley’s deep knowledge in this space helped tackle the engineering challenge.

The Rev Max gets a computer-controlled variable-valve timing system in addition to hydraulic lifters.

For the development of the all-new Revolution Max powertrain, Harley-Davidson engineers chose to incorporate hydraulic lash adjusters (HLAs) on each cam in order to eliminate maintenance, reduce noise, and accommodate more aggressive cam profiles with reduced ramp lengths. Harley-Davidson used advanced valvetrain simulation techniques during cam design and gave careful consideration to peak loads and valve lift to ensure the HLAs work with the cam. The cam profiles were also designed with consideration for the use of an HLA at high speed. Oil aeration, which is an enemy of HLAs, was minimized throughout the powertrain. The result is a high-performance OHC engine with the benefit of hydraulic lifters.

The only big V-Twin in in our recent Heavyweight Nakeds shootout, the KTM 1290 Super Duke R, peaked at 9700 rpm with 159 rear-wheel horses. Its eight valves will need checking every 18,600 miles. Meanwhile, the Harley Pan America’s new Revolution Max V-twin put out 134.5 hp @ 9200 rpm at the rear wheel on Mickey Cohen’s dyno, but its hydraulic valves should never need adjusting.

In fairness, lots of late-model bikes without hydraulics seem to hold their valve clearances very well: A quick look at a couple of forums reveals most KTM 1290 valves being in spec at check-up time, and that also looks to be true of the BMW K1600 24-valve inline six-cylinder at both its 18,000- and 36,000-mile checkups. Still, just checking those clearances can require lots of disassembly and therefore expense, even if none need adjusting.

In the end, it’s in the hands of the buyer: What’s more important, ultimate performance, or great performance with reduced maintenance?

On a personal note, I wet myself a little the other day when somebody online wrote that my new-to-me 20-year old Jaguar’s 32-valve V-8 would be needing a valve adjustment at its 100,000-mile birthday. Taking some deep breaths and checking the factory maintenance schedule revealed no such requirement. That’s because, according to another online expert who actually is one, “these engines are designed so that the valve-seat wear is roughly equal to the tappet wear, so that the clearances stay in status quo. If the car has delayed oil changes or other problems, this equilibrium can be upset and require repair, but it would be unusual. This is an area where Jag design has always been good.”

How smart is that?

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