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Valve clearances: typically they decrease or increase?

Discussion in 'Technical and Troubleshooting Torque' started by mattb, Apr 11, 2012.

  1. G'day All.

    I have read and heard contradicting things about this over time. Do valve clearances typically increase or decrease over time? (Maybe different systems shift in contradictory ways which explains the disagreement? I've got shim and buckets; screw and lock nut; and pushrod systems; but I don't keep a diary of where they were at last time)

  2. Typically they can go either way. It might be slightly more common that they increase, but closing up happens all the time too. Some engines do have a propensity to do one or the other. It's probably a little safer having them open, because they start making a noise before doing themselves any major harm. By the time you can hear something isn't right with a closed valve, it's usually burned.
  3. Yeah I think we might of had this debate before. I was of the opinion that they always increase, but it seems some experience decreasing. Has something to do with alloy heads and the valves recessing.
  4. Valve recession closes the gap, rocker or shim wear opens it, IIRC.
  5. As said above, it depends where the principal wear is taking place. Another reason for gaps to close up is the valve stem stretching just prior to the valve head coming off.

    Not so much the case with modern metallurgy, but older air cooled engines with inadequate finning round the exhaust port can be prone to it. Sidevalve singles hitched to laden double-adult sidecars were particularly keen on it on the hot, slow August Bank Holiday trip to the seaside once upon a time :D.
  6. Helpful replies. Thanks.
  7. With the engine in the DR650, it is a screw and lock nut system, easy to work on. The intake valves don't really move much over time because they are cooled by incoming fuel. The exhaust valves tend to get hot and close up over time, I plan on checking both sets of valves each 10,000 kays, they were also done before 2,000 kays by Jeffery Honda and I've done them once myself since then, about 20,000 kays on the bike.
  8. Interesting. Mine are still within spec with nearly 50k up. I don't baby it much either.
  9. honestly I don't know if mine have ever really budged either, when I do them I just open up the rocker covers, loosen the screws and put them to the thickness of the gauges. maybe I should just try jamming the gauges in.
  10. Always check before adjusting otherwise you may adjust for no reason.

    And don't do what I once did and adjust my valves to mm instead of thou". Wan't until I checked my spark plugs till I realised the mistake.
  11. I've only ever had inlets tighten up. This however, is on really hard working engines. A diesel mechanic I know also tells me the same thing. Seems @rse about I know, but that's how I've found it.
    Common consensus seems to be air filter effectiveness seems to be a big factor in valve health also.
  12. they can move both ways, however shims usually close up.
    Unless its an old Z750 Kwaka then the valve heads shrink and seat in the port rather than the valve seat.
  13. well I suppose if I did that it would have saved me about 2 hours lol