a dti on it's own does not tell true tdc.
try this: # fit a timing disk to crank and a dti to top of piston # fit a pointer, pointing at dti markings # turn crank clockwise until dti indicates tdc # carefully rotate crank clockwise, noticing that the dti does not move immediately
-this rotating movement of the crank is the dwell while the piston changes direction, before moving back down the bore.
This 'non movement of the piston' can be measured on the timing disk and if you halve the measured number of degrees you have the actual tdc for the engine.
Another method is to use a piston stop. # fit timing disk, pointer and piston stop # rotate the crankshaft until the piston comes to a stop, against the piston stop # zero the timing disk # rotate the crankshaft in a reverse direction, noting the pointer as it counts the number of degrees of rotation # continue until the piston comes to a halt against the piston stop # note the number of degrees of total rotation # subtract the number from 360 degrees and you have the number of degrees that the piston has been prevented from moving. # halve this figure and you have true tdc ie: if you have a figure of 18 degrees then rotate the crank back to your original zero mark, remove the piston stop and rotate the crank a further 9 degrees. You have arrived at true TDC.
Brian P.
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