Clearance approved

by Simon Hargreaves

Why it’s important to have room at the top, especially if you’re a valve

MCN reader Ron Smithson emailed with two questions: “Re: Honda’s CB500 range having a 600-mile valve clearance check; imagine the furore if a new Honda Civic (30,000 miles) was the same? Why’s a bike engine different?”

The second question has a straightforward answer: bike engines rev harder than car engines and have aggressive cam timing, which places more stress on the valvetrain. Engine internals that whizz round faster and get hit harder are worth checking sooner.

Valve clearance is the gap between the top of the valvetrain (the shim or bucket that tops the valve spring and valve stem) and the cam lobe (or rocker and tappet driven by the cam). The clearance has three purposes: it compensates for valve wear, reduces stress as the ramp of the cam lobe begins to bear, and allows expansion when hot (although the significance is debatable). Correct clearances are important to avoid overstressing the valvetrain and incorrect valve timing, which could lead to heat build-up and potential failure.

Many modern car engines (but not Hondas) don’t need valve clearance checks because they use hydraulic lifters – small, oil-filled cylinders with a piston and spring inside, acting between cam lobe and valve train. By alternating between spring pressure and oil pressure, they maintain zero clearance. Most bike engines don't use them because they aren’t as efficient as valve springs at high revs: the hydraulics are compromised at high cam speeds and lose control of the valve (valve float can be an engine-wrecker). Also, their weight negates using high performance cam profiles. They also have packaging issues; hydraulic lifters add height to the valve train. And they cost more.

However some low-revving bike engines can get away with hydraulic lifters: modern Harleys (except VRods), all Victorys, some big Japanese V twins, the new American Motus. And, ironically, Honda used them in their 1984 CBX750. It revved to 10,000prm and wasn’t noted for unreliability. Maybe they should fit them to the CB500 range.

Which brings us to Ron Smithson’s first question: yes, I can imagine the furore over the CB’s £300, 600-mile valve clearance check. The engine has double overhead cams acting on a bearing between the forks of a twin rocker arm, acting on two valves simultaneously. Unusual, but nothing to suggest a 600-mile clearance check. So MCN asked Honda why it’s necessary. Here’s their reply:

“Valve clearance checks and service intervals vary across our range. On the CB(R)500 series, after first service they currently do not need a further check until the 16,000 mile service. We remain confident overall ownership and running costs offer very good value over the life of the bikes.

“However for 2014 year model, due to changes in the production processes, the valve clearance inspection specified for the first 600 mile service is no longer required. As a result of changes in the production processes, there is a small planned increase on OTR price of these models.”

No word yet on how much that will be. £300, perhaps?

old blue
Offline
Joined: 08/05/2013

remember, these bikes were coming from new production
sites.

The first year bikes were being watched closely, and
Honda found that the engines were assembled well
enough that the 600 mile check on the valve clearances
was unnecessary.

mike

kharli
kharli's picture
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Joined: 24/01/2012

I have had only one valve clearance (an inlet) out of spec in near 40000 miles on my Honda nc700. A couple of exhausts have been loosened very slightly but were just in spec.
It would be interesting to hear of others experience over time on this subject ,

pittsy
pittsy's picture
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Joined: 06/08/2011

We're they concerned about the clearance increasing in 600 miles or decreasing? I'm struggling to understand what could wear in 600 miles?

Or, bearing in mind old blue's comments, were they simply concerned that they weren't set right in the first place?