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Valves: Shim type adjusters better?

Discussion in 'Technical and Troubleshooting Torque' started by MV, Jul 15, 2010.

  1. Well, I've used both & there are pro's & cons to each, but my big questions is:

    Are the shim type "self correcting"?

    What I mean by that is, valves will generally stretch, & the shims will generally come out of an engine thinner than when they went in, so do the two factors cancel each other out?

    Or is it just a "better" system for keeping your valve clearances in specs?

    Any thoughts?
  2. General consensus is that shim type valves frequently require no adjustment throughout the life of the engine, once the initial bedding in has happened, whereas screw and locknut require the occasional tweak. However, personal experience indicates that screw and locknut, on healthy, modern engines, also never needs to be touched after the run-in. I think it's more a function of modern materials and engineering tolerances rather than any inherent property of one system over the other.

    IMHO the perceived difference came about way back when because folk were, for instance, comparing the (then) modern Z1 with old-tech Triumphs and such. Apples and oranges regardless of their valve actuation methods.

    These days, I prefer screw and locknut on a theoretical basis (cheaper and easier when the do require adjustment) but wouldn't make a bike buying decision solely on that basis.
  3. You've never owned a CRF250/450 or any other modern dirtbike? Honda CBF250's are another one where the valve close up a bit between services.

    Titainium valved engines need regular checks to keep them in spec.
  4. Nope. I maybe should qualify what I said by making it clear I'm largely talking about UJM engines, not highly streesed, specialist bits of kit.

    Considering Honda have been happy in the recent past to sell bikes with lubrication systems that were 40 years out of date and considering their penchant for making engines out of Brazilian cheese, I would be reluctant to describe the CBF as either "modern" or "healthy" :wink:.
  5. Another reason shim setups are used is lack of reciprocating mass. Allowing more revs/hp. Screw and locknuts require rocker arms which add weight and complexity whereas shim under/over bucket have the cams operating directly over the valve stem. Less metal to move.
  6. True, and they (S & N) carry a lot of that weight a long way from the centre of rotation of the rocker which ups the moment of inertia that needs to be overcome/resisted by cam and valve springs.

    That said, seeing as there are S&N engines that will happily rev to 12,000 (and maybe more), I would regard this as being more a theoretical limitation than a real one for the majority of road engines.