TDC...is it neccesary....

[ken farr]

Hey everyone:

  I was thinking ( usually followed by crashing sounds and breaking glass ), when I adjust my valves on my B1100, I check, make sure I am TDC and that I have no tension on the pushrods.  That is I can spin the push rods, showing they are not under any compression.
  My thoughts are this, does it matter if I am at TDC or even BDC, or even anywhere else in the cycle, as long as the push rods are not on the cam lobe?
  I don't plan on changing my routine ( again crashing sounds and breaking glass ), but I can't really see why it would matter, again, as long as the pushrods aren't ridding on the lobes.

Thanks in advance:

kjf

( next post, new washer )

[pressureangle]

Yes it does matter. Many cams are ground with a low spot on the back of the base circle to facilitate oiling the lobe and tappets. If you set lash on this low spot you'll be too tight running. TDC assures the valves are closed when they most need to be, regardless of the clearance on any other places.

[kckershovel]

Right TDC is the safe way. I have never encountered a problem with very close to tdc. I'm not anal about it as long as I am within a few degrees and there is no tension on the push rod that's always been good enough for my things. I always err on the side of loose adjustment as well. I don't mind a little noise and better loose than tight. 

[Antietam Classic Cycle]

TDC on the compression stroke.

[Kev m]

As others have said, the important thing is valves are fully closed.

It doesn't have to be EXACTLY TDC Compression for that cylinder, you can be off a degree or two...

...but BDC or somewhere in between...or TDC Exhaust all could/would be problematic.

[kckershovel]

Ken , if you are getting loose tappets at anywhere other than really close to TDC something is outta whack .

  Dusty

    Not necessarily. There will be times the exhaust valve will be fully closed with the intake valve open and vise versa. Those would be instances where the pushrod will have lash not at TDC compression stroke. It's just to risky to try and adjust them there and really not necessary. I have played around with that idea on various engines and decided that the little bit of time that might be saved after coming up with your own pattern would not be worth the risk of being wrong/off a little bit.

    The Harley was easy to observe this as the valve lash was set to zero by feel. After the valves were set the proper way. I would rotate the engine slowly while rolling the pushrod back and forth between my fingers. There were many locations that has the same zero lash feel. TDC compression stroke there was a big window. Intake and exhaust stroke had a small window if that makes sense?