Finding True TDC

Finding top dead center can be done numerous ways.  You can use the stock markings on the engine block and the advance mechanism, you can use a dead stop, you can watch a screw driver move up and down, you can even yank the head and use a dial indicator on the piston.  All of these are valid methods... To a degree (or several).  But when you're setting things up, why not do it the best way possible?  Especially when this precise method doesnt take much longer than simply lining up the stock marks.

The stock marks on the CB750 can be off by a few degrees.  Ever notice that the little pin on the back side of the advance mechanism doesnt fit very snug into the detent on the end of the crankshaft?  This adds one error.  Another error is the size of the arrow casted into the top case that you line up the T-dash mark with.  That casting mark imay not be held true to the line boring of the crankshaft over the years.  Then there's the distance between the dash mark and the casted arrow versus the angle of your eye through the viewing window of the points plate.  Etc. etc.  Propagation of error.

The errors in using the stock timing system can add up.  Whether it's significant or not is up to you.  For me, and since the dead stop method is so simple, it is significant.  So heres how to get true top dead center using a degree wheel and a dead stop.

The above picture shows you all the tools you need to find true Top Dead Center (TDC) using a dead stop.  A degree wheel and a dead stop.  The dead stop I'm using is made from a gutted spark plug and an M8-1.00 fine thread bolt.  A dead stop is a tool that is placed into the spark plug bore and is used to stop the pistons travel in the cylinder.  The bolt can be adjusted until it is in the path of the piston.

First mount your degree wheel on top of the advance mechanism using the long M6 stud.  Remove the large nut and mount the plate against the teeth protruding from the tip of the advance mechanism.  You dont need to tighten it down very much, but make sure that it doesnt move easily and stays mounted solid to the advance assembly.  Next take a small piece of wire and secure it to somewhere on the engine.  Adjust it so that it acts as a pointer on the degree wheel.  Again, make sure that the pointer is very secure and will not move.  Set the degree wheel so that the pointer is at 0.

 

Next place the dead stop into cylinder 1 or 4 (shown above, between the fins).  I use cylinder 4 since it's on the side of the engine where I'm working.  Run the bolt down a bit and turn the engine by hand until it stops rotating (i.e. hits the dead stop).  DO NOT USE THE STARTER TO ROTATE THE ENGINE.  If the engine rotates without hitting the dead stop, adjust the bolt down further.  Again, rotate until the engine hits the dead stop.  Once you've hit the dead stop, look at the degree value on the wheel.  Mine is reading 51* ATDC in the picture below.  Since we've set the degree wheel to zero initially, we simply divide the number indicated by the pointer by 2.  51 divided by 2 is 25.5. 

 

Move the degree wheel so that the pointer is now at 25.5*.  Moving towards 0* of course.  Following along, my movement can be seen in the picture below.  Only move the degree wheel during this process, not the crank.

 

Now, rotate the engine in the opposite direction until you hit the dead stop from the other direction.  When the piston hits the dead stop, look at the value on the degree wheel.  It should be opposite of your other reading, but identical in value.  My reading, shown below, is now 25.5* BTDC.  If everything was done correctly, the value you adjusted the wheel to previously and this new value should be the same. 

Now we know where true TDC is.  Remove the dead stop and rotate the crank until the degree wheel is at 0*.  This is true TDC.  See the below picture... and my knee in the clutch cover.

Now, just to show you how much my plate was off, here is a picture of the alignment of the T-dash mark on my advance mechanism lined up at true TDC.  It's a little bit to the left, which means that if I used this to mark my TDC, I would have been a little retarded (~2*).  No pun intended.

 

Again, the significance of the difference between true TDC and the TDC determined using the stock marks is up to you.  When installing a new cam, making ignition adjustments, etc, etc.  precision is key.  Especially considering you're already working with a few degrees in either direction.