Finding TDC

zimmy · #1 (**permalink**) 05-26-2002, 09:26 AM

Here's a question for you engine guys. Does anyone know an accurate and easy method for determining the exact TDC of an engine when it is fully assembled in the car. I'm guessing that the only way is to determine the travel through the #1 spark plug hole. Any suggestions and gadget help would be appreciated.

Here's the reason for the question. Inglese has been telling me to increase my timing for the Webers. They said that they need as much as the engine can stand. I was told the best way to determine this is to increase the timing a bit at a time until the car is fighting the starter. Then back it off a degree or two. Well according to my timing marks on the balancer, I am at 30 Degrees initial and it starts easily and quickly!!! The car actually runs better and I don't think there is any pinging with 18 degrees of mechanical (48 degrees total!!). This can't be right. My guess is that the timing marks are wrong. The other thing that I haven't checked yet is the fact that I am timing it at about 900 RPM. I have the weakest MSD springs in the MSD distributor. I am going to check to see if I am actually getting a bit of mechanical thrown in at that rpm. I'll try it down around 750 or so.

Any thoughts???

Zimmy

ron427 · #2 (**permalink**) 05-26-2002, 10:23 AM

Zimmy

Make a stop from an old spark plug by removing the center electrode and outer shell from the upper section. Install and
SECURE a short bolt through the center hole. Be sure it is tight
so you won't loose anything into the cylinder bore. Have the
bolt extend about an inch beyond the threads. All you are looking for is a rigid stop for piston travel. With this installed in
number one, rotate the engine carefully BY HAND untill the piston
comes to rest. Mark the c/s pully and reverse direction and do the same thing. The center of the two marks is TDC. Do this slowly and carefully and you won't damage anything.
Ron

427sharpe · #3 (**permalink**) 05-26-2002, 05:05 PM

t's just as easy to just pull a plug and look in the cylinder using a flashlight--you can see when the piston reaches tdc.

mr0077 · #4 (**permalink**) 05-26-2002, 06:51 PM

Zimmy, use Ron427's method to make a piston stop, but do two things...1) tap the inside of the sparkplug shell (I think I used a 3/8" tap), and 2) make the end of the bolt going toward the piston ROUND and SMOOTH. Now you have an adjustable piston stop. As you turn the engine over, you can screw the bolt in or out as needed for the engine, so the piston will contact the stop (end of the bolt) maybe 15-20 degrees from top dead center. Contact the stop going both ways, mark each location. The middle of the marks is TDC.

HighPlainsDrifter · #5 (**permalink**) 05-27-2002, 12:45 PM

Zimmy,
After you determin that you balancer is ' out '.
The balancer should be replaced if it has slipped and is coming apart,it can be an explosive device if revving and comes apart.Also if it has slipped your engine will no longer be in balance and put undue stress on your bearings. Then again it may be something as simple as you clipping the timing light onto the wrong spark plug lead,it's easy to do,I'd check that 1st.
Good Luck,
Perry.

Mr.Fixit · #6 (**permalink**) 05-27-2002, 07:23 PM

I've fired up motors where the timing was "close enough" to get it fired, and aafter putting a timing light of it, finding out it's got 50 degrees when at 2500rpm. Nothing bad at all noticable, but running it without a load. When corrected to about 34-36 it did sound better however. I recommerd and also have andjustabe piston stop like mr0077 that I made myself a few years ago.