I'm putting this in Vintage instead of tech due to the age of the bike in question. Here's the short version of the story; 1971 Honda sl100, let my buddy ride the bike on a parade "lap for history" at Mid-Ohio. He returned from said lap in the crash truck, bike has now got a bent & broken intake valve (see attached photo). Noticed immediately that the screw/locknut valve lash adjuster was awol from the intake valve follower, missing parts were later found at the bottom of the crankcase. The bike had been ridden around all weekend with no unusual noise or hard starting. My buddy says the bike just "shut off" on him. Here's my quandary - did my friend simply over-rev a 40 year old engine to death or did some numbskull (me) not tighten the adjuster locknut? I don't usually make torque mistakes, but if the adjuster was tight, how in the hell did it end up in the engine? I'm not really trying to assign blame - just trying to ascertain if I screwed up so I can avoid this sort of thing in the future. Any opinions?
I would bet on the adjuster coming loose. I had the same thing happen at mid - o to my old SR500 in the vintage superbike race. I have been paranoid ever since when I do a valve adjustment. I was sure it was torqued.
fatigue failure of the valve stem at the retainer groove is my guess, the groove is a stress concentrator
Adjuster loose / valve stem broke - which was first ? Anyone's guess. I fixed a CM450T this summer that had never had it's valve lash checked in 10k miles. My guess was it ran out of lash, then the adjuster loosened up & dropped out, quickly followed by dropped/bent valve which then got pounded back up by the piston & split the end of the guide. Got a schooling on Honda valve guide replacement that might come in handy later. Check the seat end of the guide closely.
Thanks for the input guys! So it sounds like the vote tally so far is 1 for yes, 1 for no and 1 for who knows? That's about what I figured, chicken and the egg. BTW, can you believe that Bike Bandit & Honda want $38.62 for one intake valve? I just picked up another entire bike for $60. Ebay is replete with spares but right now they're all coming from Thailand & I don't want to risk it. I already bought one set of valves that are the wrong size. Well, actually only one valve isn't right. Care to guess which valve? Intake, the one I need, of course.
