Valve Clearance Adjustment on DOHC

mister_hMarch 13, 2006

I have a motorcycle with DOHC 16-valve 4-cyl engine. The valve clearance needs to be checked/adjusted every 5000 miles according to the owner's manaual. I've done once before, and now I'm doing it for the 2nd time. When I measured, the clearance has become tight again (naturally).

If I set the clearance to the higher side of the spec, what are the disadvantages or problems I might see? For example, the spec on the manual is 0.005" to 0.008". If I set it to 0.009", would there be any running problem or permanent damage to the valve or valve seat on the head?

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No; but neither would I do this.
I would present this to the motorcycle forums.
What were your measurements?
If still within tolerance (or within a thousands) you could stretch the 5K to 6K, maybe...

    Bookmark   March 13, 2006 at 4:52PM
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I'm familar with the older Honda screw and locknut valve adjustment. I think that you may have bucket and shims adjusters since your engine has 4 valves per cylinder.

In many cases, the lash for the intake valves is less than for the exhaust. Check your manual to see if there is a differene between the intake and exhaust valves. More allowance is needed for the exhaust valve as compared to the intake owing to its greater operating temperature and length growth.

If the clearance is still within spec at the 5000 mile mark, you should be able to run another 5000 miles. However, you said that the clearance had diminished and was wondering if you could set it a little higher to extend the adustment period. I would not recommend this. Presumeably, the recommended lash setting is the result of many test hours during engine development and the manufacturer's experience with similar engine designs in the past. The perfect lash setting would be zero under all conditions, but this is not possible, so some lash must be allowed at cold engine to keep the lash from disappearing when hot and working hard. When the lash is too tight, the valve is prevented from sealing tight and a burned valve may result, and misfires may occur. it would seem that one should err on the high side, but this may not be the best.

When the lash is too much, tappet noise is present and the impact between the lifter and valve parts increases. This has a greater effect on a high revving motorcycle engines than, say for a tractor engine turing at 1800 rpm. I suspect the red-line on your engine is in the 10,000 rpm range not that it can be held at this speed for any length of time. Too much lash at the top end speed is bad. So, stick with recommended lash. If my hunch serves me correctly, .005 to .008 inch would be for the exhasut valve with .005 inch max for the intakes. These numbers are for short valves and will increase with valve length.

The reduction in valve lash indicates the the valve head/seat is wearing faster, or the valve is sinking into the seat, or the valve is stretching faster, than the wear at the cam end of things.

So, measure periodically and adjust only when necessary. Such is life with solid valve actuators and high speed motors. Valve lash tend to be more for air-cooled engines than for liquid cooled.

    Bookmark   March 13, 2006 at 11:20PM
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They're less apt to tighten up now that its got some miles grinding, pounding on the valve face and seats,, they're broke_in Set it midway, be happy.

    Bookmark   March 14, 2006 at 7:58AM
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Thanks for your comment!
I will just set it to the middle of the spec and see how it goes. Hope it won't be changed too much when I check next time.

    Bookmark   March 14, 2006 at 11:10AM
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