Re: TDC II

[DeWitt Harrison <de@aztek-eng.com>]

On Fri, 11 Dec 1998 12:10:46 EST, Scott Justusson wrote:
>>[ ... ]
>> 	And the timing gear on the cam contributes to the confusion.  I have
several of >>the 10v gears in the shop.  Lining up the keyways and looking at
the teeth >>provides an interesting insight.  The teeth do not line up.  Some
are the same, >>some are 1/2 tooth off, which means some will "automatically"
be off by a certain >>amount.  Just wish I knew which cam sprocket was
"correct".
>>[ ... ]
>Ok, good point, lets list the potential variances to TDC in a given audi motor
>[ ... ]
>Bottom Line:  TDC is approximate at best.  To really get an "exact" tdc of the
>crank, you would need to have the flywheel attached, the transmission on, and
>the head off the car.  To translate that to camshaft tdc, you would need to
>have the head on the block with the belt attached, and measure the cam lobes
>for tdc.  The cam tdc would be a constantly floating target however, cuz each
>time you changed the belt/water pump or even adjusted belt tension, cam tdc is
>affected.
>[ ... ]

Finding bottom end TDC need not be all that much trouble. A precisely gauged
feeler
through the spark hole works very well. There are some designed for exactly this
purpose but it shouldn't be hard to rig something from a depth gauge. Just
be sure to
be rotating the crank in the normal direction when taking a reading to avoid
errors
due to bearing slop. A couple of runs at it should tell you were "0" really is.

The point about cam TDC slop is excellent. If you've got an adjustable cam
sprocket,
it would seem highly appropriate to reestablish cam TDC every time the cam cover
is off. If you just go by the fixed marks, you may well be wasting the price of
that particular motor goodie.

DeWitt Harrison
Boulder, CO
88 5kcstq