Conference vicki::boats

    OK, I'll demonstrate my ignorance.
    
    What is TDC? And BTDC?

    tdc stands for top dead center.  Btdc stands for before top dead
    center.  This is the crankshafts reference to the timing marks when
    the number one cylinder (in the firing order) is at the top of it's
    stroke with both the intake and the exhaust valves closed.

re .0:

As I recall from my motorcycle racing days when I when through the same 
calculations: you must know the rod length (from wrist pin centerline to 
crank journal centerline). Not what you wanted to hear, I guess.

	Let 'x' = the distance below TDC where 'a' (the BTDC angle) is achieved
(i.e. how far off TDC you must move the piston vertically).

	x = stroke/2 - ((cos a) * stroke/2)

				Al

    re .5	Thanks Al, I knew I'd done it once before but had a
    mental block about the con rod length having to be in there somewhere,
    though I thought I'd got it to cancel out somewhow.  I think this
    is an approximation that only holds for small angles and reasonable
    ratios of con rod length to stroke, but since thats the range of
    interest it will probably work just fine.

    	Reg
    

    Reg, 
    could it be this simple???
    
    If you know the stroke length (and you claim to), van you not then
    calculate in simple math degrees by distance moved? For example:
    
    Assume a 3.75" stroke. Always, your stroke will take exactly 180
    degrees. If you divide 3.75/180, you get the actual distance of
    stroke per degree of crank rotation. in this example, one degree
    of rotation equals .020833..." 
    
    I can't see why we need to go into trig'.
    
    Mike

    It's not linear.  Piston will move up faster at mid stoke than at
    bottem or top.
    
    Nelson

I beleive the formula would be


Distance from TDC    =     stroke - ( cos(deg from TDC)*stroke)
                           -----------------------------------
                                          2

    
    	Oh, its very non linear.
    
    Consider THE SHORTEST theoretical con rod possible, stroke/2,
    this is theoretical because it would jam solid at 270 degrees from
    TDC, since there would be no way to shove the piston back up, it
    would just be pressed against the cylinder wall.  Starting
    from TDC the crank turns 90 degrees and the piston is at BDC, then
    the crank turns another 180 degrees before the piston moves at all,
    the last 90 degrees take it back up to TDC.  Compare this to a very
    long con rod (relative to the stroke), the piston/crank rotation
    relationship is very similar between 90 - 270 to what it is between
    270 - 90.  Somewhere between these ridiculous extremes is a practical
    con rod length, probably 1.x times the stroke, that has acceptable
    side thrust and engine height (the most obvious trade off).  I still
    believe that the con rod : stroke ration matters, but only for large
    angles.  For ignition timing we're talking about <10 degrees (static),
    so the approximation is fine.
    
    	Reg
    

Come on folks we are supposed to be an engineering company. The 
poor guy with the gage only has to drop in a rod mark the bottom, 
mark the top then divide away. This technique is used in the real 
world on diesels everyday to set the timing of the injector pump.

In order to be super accurate the dial indicator can be used with 
a known extension to get to BDC.
Happy timing.
norm

    If you are really worried about the length of the con rod and
    do not wnt to rip apart the motor, how about going to a auto
    parts store and requesting to see a con rod for you engine.
    Measure it and hand it back to the guy, with a thank you.
    
    Mike
    

    
    All this talk of measuring a spare connecting rod, complex
    mathematical formulaes, etc. has made this into more of an
    issue than it really is. Besides, any slight error in your
    measurements or calculations could throw your timing setup
    way off.
    
    With the dial indicator you already have half of the tools
    needed to do the job right. Degree wheels are not that expensive,
    (less than 10 bucks I believe) and if you can't afford one
    any decent engine builder would have one you could probably
    make arrangements to borrow.
    
    Rick W.
    

    If you just want to set up the ignition timing then just forget
    the formulas; go to a shop and buy a timing lamp. The timing marks
    are usually on the flywheel or on the crankshaft.
    
    E T