The Honda service schedule says valve clearances should be inspected every 25000 miles. My car had 73k on the clock when I did the job so it was definitely due!
Apparently if the gap is too narrow the valves can become burned/damaged which is serious & can mean big repair bills.
If the gap is too wide this is less serious with the main symptoms being an annoying loud tapping noise and loss of power although some people say it can also cause valvetrain damage.
According to what I read, wear makes the gap narrower not wider – don’t know if this is right but I suspect so – please see below.
Honda specify the following clearances
2.0 - intake 0.21-0.25mm, exhaust 0.28-0.32mm
2.4 – intake 0.21-0.25mm, exhaust 0.25-0.29mm
I raised the front of the car onto axle stands and took the front offside wheel off. Then removed both plastic engine covers, the coils and dipstick, undid the 2 pipes and tube connected to the metal cylinder head cover, took off the 6 cover bolts, washers and plastic lugs, then removed the cover.
The cylinders each have to be set in turn at top dead centre to do the adjustment, by using a 19mm socket on the crank bolt from inside the wheel arch.
Setting the cylinders in firing order 1 3 4 2 is done by moving the timing mark on the right hand cam gear visible from the right of the engine to 9 o’clock 12 o’clock 3 o’clock then 6 o’clock respectively.
What needs adjusting to set each clearance is a screw surrounded by a locknut. There are 4 valves per cylinder, 2 intake nearest the front and 2 exhaust nearest the bulkhead, so 16 screws in all. So I needed to loosen the nut, turn the screw to leave the right gap using a feeler gauge, then tighten the nut to torque, 20Nm on the 2.4, 20Nm intake and 14Nm exhaust on the 2 litre.
My valves were out of specification because on all 16 valves the gap was too small as I couldn’t get the feeler gauge in at all. Honda do say inspection every 25k miles as stated. As I understand it though, garages will usually ‘inspect’ by listening to the engine, if they do it at all, so it had probably never been done.
The workshop manual says to adjust the clearances so there is “slight drag” on the feeler gauge. So I took this literally by loosening the screw so there was no drag, then tightening until I felt the first signs of drag, just a rasping feeling on the blade of the gauge, then closing the locknut. I thought this was what the manual was telling me to do.
However when I started the engine it was clacking so much it sounded like a bag of spanners. The valvetrain tapping/clacking was audible even inside the car. I went for a test drive and the power loss was noticeable. I knew this wasn’t right, so I decided that it would need doing again. Not great because it had taken me ages.
I did a web search and found a lot of forums discussing this issue of what “slight drag” means. They give the consistent message that in fact definite, distinct drag on the feeler gauge is needed. Some also suggest trying the next size up (one thousandth of an inch, as these are usually US forums, which is 0.025mm) and saying if that will go in then it is set too loose. I settled the issue by asking a guy at work who used to be a mechanic. He confirmed that the drag has to be firm. He made the point that the screw doesn’t crush the feeler gauge, so therefore when you can’t move it at all, that is spec, so anything looser is going to be slightly more.
Second time round I did the job a lot, lot quicker. I tightened each screw using a feeler gauge in the middle of the range, 0.23mm intake and 0.27mm exhaust, to the point when it wouldn’t move, then backed it off just enough so it would only just move freely, about an eighth of a turn, then fixed the locknut. Then I turned the engine a complete revolution and checked every one again. I got the gauge to go back in all of them except one so I backed that one off slightly. I then tried putting a gauge 0.03mm bigger in some and found it wouldn’t go. I also tried a gauge 0.02mm smaller in some of them i.e. minimum Honda spec, and it was a bit looser but still fairly firm. So then I put the cover back on.
This time when I started the engine up it sounded great. No tappet sound at all from inside the car and good under the bonnet. The power I had previously is back. On acceleration it feels noticeably smoother than it did before the first adjustment. And most important of all, I know that unlike before, the gaps are within spec.