Finding TDC
There are occasions when you need to accurately determine Top Dead Center. Here's how I do it.
Assemble the following:
- Degree Wheel
- appropriate crank adapter (right) - I built this one based on the 5/8x20 inner thread of the rotor nut.
- Piston Stop (left) - I had to modify this one so the intake valve wouldn't hit it.
- You'll also have to rig up a pointer that won't move on you. I attached some bent wire to the inner cover central stud.
1) Pull the primary outer cover
2) Mount a suitable degree wheel and pointer on the crankshaft
3) Pull both plugs
Standing on the left side...
4) Put your finger over the left spark plug hole and put the tranny in high
5) Rotate the rear wheel in the forward direction (CCW) until your finger is blown off the hole
6) You are now close to TDC on the left
7) Screw the piston stop into spark plug hole until it contacts the piston
8) Set the Degree wheel to something around TDC (This is just to get oriented.) And note this angle.
9) Rotate the rear wheel the other direction (CW) - gently - until the piston hits the stop again.
10) Note the angle
11) Do this math: Calculate the spread by (Angle1 + Angle2) If you get something less than 30, I would screw in the piston stop a few more turns and repeat 7 -11. You want to get at least 15-20 degrees on each side of TDC because as the piston gets closer to TDC it moves less vertically. This will reduce the accuracy of the measurement.
Let's say your first number you set the wheel to was 21 degrees and the second you found after hitting the stop was 15, The spread is 21+15 = 36. Now set the degree wheel to half that - in this case 18 BTDC. This "centers" up the degree wheel between the two stop points. (In theory, TDC is half way between the stop points.)
12) Double check your work - Don't skip this step - Rotate the rear wheel the forward direction (CCW) again until you hit the stop again. If you had everything perfect, the wheel would read the same - i.e. 18 ATDC. If not however, you can correct as follows - Find the difference between the two numbers and move the wheel half of that. i.e. if you set the wheel at 18, then rotate back and get 20 - you are 2 degrees off. You need to compensate by 1 degree, so set the wheel back to 19. At this point I would recheck again until you get the same number on each side of the stop.
13) The degree wheel is now timed to the engine and will properly indicate TDC.
To verify the timing marks
1) Do the above and then set the wheel and engine to 30 BTDC (Make sure you go the right direction).
2) Carefully remove the wheel and indicator in such a way as to not disturb the crank position, so you can mount the outer cover.
3) Remove the timing inspection cover from the outer case.
4) Mate the outer cover to the inner cover and read the timing scale. Note the difference, if any, of the the scribed line on the rotor and the 30 degree mark on the scale.
In my case, I was about 4 degrees off!