The Importance of Checking Valve Clearance

andrew Maintenance, Tutorials, Video 2 Comments

Can You Feel It?

My local mechanic who I spoke to gave me his opinion on the subject. As a man who has seen many engines pass through his workshop, he said most bikes don’t need their valve clearances checked for a long time (I think my bike had 27k on the clock at the time) and that I would know if and when they needed adjusting, as the engine wouldn’t feel right, it would sound wrong and wouldn’t be performing well. He told me that if it was still pulling like a train, the chances are they are fine.

Basically, if the valves are doing their thing, the pump is working correctly and all is well. I know that performance drop off is a gradual thing, but you should be able to tell if things are out of alignment with your bike.Throughout my life as a spanner wielding biker, I always encourage people to have a go at their own bike maintenance, and that includes the valve clearance adjustments. Get yourself a service manual (Haynes and Clymer, or the official manufacturers workshop manual, although these assume a decent amount of training of its readers whereas the home mechanic guides are more elementary), take a couple of weekends off, enlist the help of a friend, and ideally someone with a box of shims or a local bike dealer you can borrow or buy them from (getting genuine shims is expensive, and not at all instant, so its good to ask about and find someone with a good supply of them). While were on the subject it’s worth mentioning that the workshops won’t want your load of 1.80’s while you take their 1.625-1.775.

Most bikes engines tend to tighten over time, and these guys have to buy shims from the parts suppliers like everyone else. Once you’ve used their shims, replace them with new ones. Most modern Jap/European sports bikes use 7.48mm diameter shims. Get out there in the sunshine and be a man for once. You’ll feel much better as a result. Take the money you’ve saved and buy yourself a shiny new lid.If you’re servicing your bike properly, and that means by the manufacturer’s schedule, then of course you should check your valve clearances. Is it likely to be worth the cost the first time around? It depends how you look at it. Checking valve clearances is part of servicing and tuning up your machine. Just because you don’t have to adjust them, doesn’t make the exercise a waste of time. If you find they’re all within spec but on the limit, then you’ll expect to change them next time around. 

You'll need a set of Feeler Gauges

You’ll need a set of Feeler Gauges

You can give your bike to a workshop to check, but there’s nothing more annoying than handing over £400 in labour and having nothing to show for it, sometimes I wonder whether they don’t just take a 16,000 mile bike for a blast round the block, and if it seems ok, just say they were fine. Hopefully they won’t also charge you for it too.

I recently bought a ‘runner‘ which was in reasonable shape, but had never had the valves checked. I suspected as much too. At low revs the bike felt fine, but above about 6000rpm, the engine became rough and vibey, and didn’t pull as strongly or as willingly as I thought it should. After riding the bike home, I took it apart and measured the valves. 10 out of the 16 valves were out of spec. The bike was a V-four Honda with 42k miles on the clock. All of the out of spec valves were toward the tight side, some exhaust valves by as much as 0.1mm. PAIR ducts had also become choked up with soot. Spark plugs were in reasonable condition.

Following the adjustment, which brought the valves back in towards the upper end of the adjustment (only one shim was retained), the bike was revolutionised. Pulling strongly throughout the rev range, and exhibiting that eagerness to rev that a motorcycle engine should have. I also felt far happier bouncing that engine off the limiter, knowing it wasn’t about to go pop.