How to ascertain T.D.C
T.D.C stands for Top Dead Centre and this is the point where the piston moves up the top of the bore and just prior to the piston moving down again, i.e. the piston as the the full extent of its travel. What makes this tricky is that the piston remains at TDC for a few degrees, so establishing the true T.D.C is a little tricky to find unless you know the procedure.
With the Rover engine and probably quite a few others, T.D.C is not when the top of the piston is level with the top of the bore, it never quite reaches the top! It is only a fraction, it equates to a volumn of about 15cc, so not a lot. I think this is to allow for extra valve to piston clearance maybe, alluminium expands at twice the rate of steel for the same temperature.
You can turn the engine by hand until piston 1 is at approximate T.D.C. If you have the timing cover off you can do this by aligning the camshaft and crankshaft gear wheel marks. The Camshaft mark at the six o'clock position the crankshaft mark at twelve o'clock and this should be true T.D.C but there is scope for error with this method. The best way to ascertain true T.D.C is desribed below.
This procedure is with the heads removed.
Starting turning the engine to move the piston up the bore so that the camshaft and crankshaft timing marks are at the six and twelve o'clock position as described above. This is the starting point.
Fix a DTI guage above piston bore 1, try if you can to have the DTI in the middle of the piston as this eleviates any sideways movement of the piston, as slight as it should be, will cause inacuracies. Zero the DTI guage.
Always turning the engine the same way; clockwise, turn until the DTI guage reads 0.050" after T.D.C. Make a mark on the crankshaft timing wheel as a reference point.
Continue turning the engine clockwise until the DTI guage reads 0.050" before T.D.C. Make another mark on the crankshaft timing wheel as a reference point.
The mid-point between these two marks is the true T.D.C.
Now position on this true T.D.C and set the DTI to zero and do another rotation to ensure that it hits zero at the maxiumum travel.
For instructons on how to perform timing as part of a camshaft upgrade click here