What is Bottom Dead Center (BDC)?


#1

I have a 05 camry with 2.4 engine. Engine was recently swapped with a parts car engine. The upper oil pans needed swapped which means the balance shafts were swapped. I originally retimed them via timing marks and TDC. There is a bad vibration and after research on the toyota forums it needed to be at BDC.

This got me thinking. What is BDC? TDC is top on compression stroke of the #1. The crank is at the bottom each revolution but twice each combustion cycle.

So What is BDC?

this brings me to another question. Where did TDC come from. Top Dead Center. My thought is it has to do at the time when the crank stroke is aligned in the center of the rotational circle of the engine? Top Dead Center is when it is aligned on the top side with the piston closest to the head and Bottom Dead Center is when it is aligned on the bottom and the piston is furthest from the head.

Anytime i think of TDC I assume it is always on the compression stroke. Maybe I have just conditioned myself to make that assumption and it is actually refereed to TDC on the compression stroke.


#2

It’s exactly what it says; on a clock it would be the 6 o’clock position.


#3

BDC and TDC each occur once each revolution, when the piston is either at the Top or Bottom of its stroke and the connecting rod big end is Centered in the bore of that cylinder.


#4

The DEAD CENTER designation is to make it clear that the piston will be at the top for a significant sweep of the crankshaft. Relative bore/stroke and rod length result in varying the dwell. TDC is the mid point of crankshaft sweep while the piston remains at the top.


#5

Even if the piston isn’t moving, of it’s at its uppermost point of travel it’s at top dead center.
If it’s at the bottommost point of its travel it’s at the bottom dead center.
If it’s in the middle, it’s just in the middle.


#6

OP writes …

Top Dead Center is when it is aligned on the top side with the piston closest to the head and Bottom Dead Center is when it is aligned on the bottom and the piston is furthest from the head.

That’s correct. Each piston reaches TDC at different times. When the manual says align the crankshaft engine to TDC, that usually refers to TDC for cylinder number 1. On most cars when cylinder number 1 is at TDC, there’s a marker on the crankshaft pulley that aligns with some kind of pointer. Of course you could just remove the spark plug, insert a wooden dowel, and watch it go up and down as you manually rotated the engine to confirm what’s happening. TDC for cylinder number 1 occurs twice in the valve sequence used for 4 cycle internal combustion engines, which your Camry has. There’s a TDC at the end of the compressions stroke, and another TDC at the end of the exhaust stroke. In both cases the prior mentioned marker would appear to align and look exactly the same. For engines using distributors you can look at where that’s pointing (on the inside), it will be pointing at the number one spark plug wire at the TDC at the top of the compression stroke. It won’t be pointing there at the top of the exhaust stroke.

And BDC is just as you say above. Likewise BDC can either occur at the end of the power stroke or the end of the intake stroke.


#7

If you have your shaft out of phase…it is now moving IN TANDEM…with the pistons. No wonder she vibes.

Like everyone said…TDC…is Top Dead Center…BDC…Bottom Dead Center…basic basic


#8

I got it all figured out on the car. The fact is the balance shafts have a 2:1 turn ratio and it will not matter if set at TDC or BDC. When I assembled the engine, I must have had the engine 90* out (required to time the oil pump) when I set the balance shafts on. Simple fix of dropping the pan and retimeing the shafts. Camry is finally back on the road.


#9

SWEET… You basically had a handle on it the entire time tho…