Quote:
Originally Posted by Lynn Eades
The only way to sort out TDC is to measure a given piston height before TDC and the same after TDC, marking both accurately and dividing it.
Dodge put a screw in plug in the cylinder head, so that you could put a rod down to the piston. Henry didn't feel the need.
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Yes that's certainly the ideal method Lynn, but Henry made things difficult for us in more ways than one! Not only did he give us no access plug like the far more considerate Dodge brothers, he also offset the crankshaft! One of the peculiarities of the flathead is that the crankshaft is not directly in line with the cylinder bores, it's offset towards the right bank by about 1/4". It's a clever idea which effectively increases the stroke momentarily when it's most useful, ie. during the power stroke to generate more torque, while at the same time reducing piston sidewall force when it's at its greatest. Of course, just like a stroked motor it can only be taken so far, before the conrods start hitting the bottom of the cylinder wall. What this offset crank means for us is that the piston height differs between the upstroke and the downstroke, for example it's almost 1/4" lower at 90 degrees BTDC than it is at 90 degrees ATDC. Which means that when we mark the crankshaft pulley with the piston at the same height before and after TDC, then bisect the two marks, we end up with a false TDC. Once again this is something all the so called flathead experts never bother to warn us about! Fortunately it's quite easily compensated for, by simply moving the TDC mark 1.5 degrees clockwise.
Funny you should mention the access plug in the Dodge head Lynn, it's something I gave serious consideration to on one of my tired old flatheads. The plan was to drill a 1/8" hole through the head, missing the water jacket of course, so I could use a bit of 1/8" high tensile rod to gauge piston height. It's a highly accurate method when used around 90 degrees from TDC, because the piston moves 66 thou per degree either side. That's about 1/16" which is easily detectable by eye, which means that when you factor in the 1.5 degree offset your TDC mark will be accurate to well within one degree. The next step was to correlate TDC with the timing gear marks, so I wouldn't have to drill holes in all my motors! The guinea pig motor could then be plugged with a bit of oversize rod hammered in the head, which could be weld repaired if the motor ever got rebuilt. In the end though I thought I'd try the volumetric method first, and it proved to be highly accurate, although quite tricky to use. The problem is you have to work near TDC itself, where the piston only moves 1 thou per degree either side, and only 4 thou at 2 degrees, and 8 thou at 3 degrees. It's easy enough to detect with this method, because it amplifies piston movement by a factor of nearly 200, but there's a "dead spot" either side of TDC where the piston doesn't move at all for several degrees, owing to crankpin and piston pin clearance. That means you have to turn the crank incrementally both ways to mark the dead spot and then bisect the two marks. There's gotta be an easier way!