How to determine TDC after bottom end rebuild?

don't do what you said to do you could smash valves or possibly cause fires ... bad things might / will happen. Get your cams and crank lined up first before spinning anything much less firing the engine.

1) use a gauge (not sure the name but typically they are a dial gauge that measures in hundredths or thousandths of an inch.) mounted to the deck of the block and spin the crank till the proper piston (whichever should be up depending on your engines shop manual) is as high as it will go. (this could conceivably be done through a sparkplug hole)

2) look through the spark plug hole and eyeball it till the proper piston is nearly tdc (you may be off a few degrees but at this point some marking on the crank should be pointing up, typically a dot on the timing cog)

3) set the crank so piston one is full compression stroke (or between exhaust and intake strokes) set the cam to this position as well. You will need to visualize the cam turning and opening and closing the valves to make sure the rest of the firing order works correctly. if you spin the engine with a ratchet you run the risk of striking a valve. also check the piston that is between the exhaust and intake strokes has the proper valve overlap then belt it. You may need some other measuring equipment to make sure the cam is opening and closing valves at the optimal times though.

Just to reiterate: do not try cranking the engine or firing it till you get this all sorted out. and if you spin the crank with the head on that can damage your valves if it is an interference engine or a non interference engine with aftermarket flat top pistons, bigger cam profiles, etc.

---

to address your comment. If the cam is 180 degrees off (360 deg for the crank) it should not make a difference. because each 360 the crank does the cam does 180 meaning piston 1 will be tdc when the cam lobes are pointing up and again when they are down.

but you still need to make sure your cams are at exactly their 180 degree spot relative to the cranks 360 degree spot. You can't set the crank to 0 deg and the cams to 45 deg this could end very badly, possibly even while you're torquing down the head.

There is no concept of a compression stroke if you remove the cam from the equation. The bottom end simply goes up and down. When piston number 1 is at the top of it's travel, your bottom end is at TDC.

I think the question you are actually asking is how to time the distributor / ignition to the camshaft. To answer that, it would be vital to know the specific engine you are working on because they are all designed differently.

All TDC's are not equal. TDC occurs at the end of compression stroke and end of the exhaust stroke. The valves are in completely opposite postion, both closed at end of the compression stroke and the exhaust open and typically the intake closed although or give or take 5-10 or so degrees depending on engine design. So you need to be TDC at the compression stroke to set your cam gears & crank in synch to have the valves in proper position. Just believe if your are not properly aligned your valves will crash to piston heads unless certified non interference engine.

So how do you tell you are TDC on compression stroke with the valve covers on? You can get a rubber hose with a spark plug interface used for various diagnostics , screw it in to spark plug and put a balloon at the top of the tube, when the balloon expands and holds the air, you are on the compression stroke, if it was the exhaust stroke, the exhaust valve would be open and while some air may initially go into the balloon it will deflate. Thats a general method to say you are on compression stroke somewhere near TDC, when the balloon is full doesn't mean TDC exactly as valve overlap occurs, but generally you are there.

You need to watch the balloon start to expand and hold it well and not deflate, then remove the balloon and insert some sort of measurement device that can slide up as the piston continues to top of the compression stroke. Once that device reaches its peak, that's TDC and you are on the compression stroke. But you should have that set and locked with crank and cam gears aligned per their marks before you button up the top.

But this is NOT a perfect method unless you have a really good device that can accurately measure the travel of the piston without sliding around.