TDC, Top Dead Center, is when the piston is at the exact top on the compression stroke - on the 4 cycle engine, the crankshaft rotates 2 times for each cycle of the engine - each of the pistons reach TDC on the compression stroke and again on the exhaust stroke - due to the layout of the engine, #1 and #4 are at TDC at the same time - when #1 is up on compression, #4 is up on exhaust and conversely when #4 is up on compression, #1 is up on exhaust - at the same time, #2 and #3 are at BDC or bottom dead center, one on intake and the other on power - the camshaft(s) rotates one time for each cycle of the engine, thus the gear on the camshaft which is exactly twice the diameter of the gear on the crankshaft - when both of the #1 and #4 pistons are at TDC, the mark on the crank pulley will align with the mark on the timing cover - when it is at TDC on compression on #1, the "UP" on the cam gear will be generally up - when it is at TDC on compression on #4, the "UP" on the cam gear will be 180 degrees off, or generally down - you don't need to worry about the distributor as the camshaft has the slot that the distributor shaft fits into offset so that the distributor will only go on one way - when you put it on it will automatically be in the correct position - do not use the manual that was linked to in one of the posts above if you have a USDM car with a D16A6 engine - that manual is for the cars in the UK - in the US, the 88-91 D15 engines camshafts are not lined up the same as the 88-91 D16A6 engines - with the D15 engine, you align the marks on the cam gear with the top of the head - with the D16A6, you align the third mark on the cam gear, which is at about 7 o'clock, with the pointer on the inside timing gear cover