Engine Making a 5.0 piston stop questions

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
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Kearney, NE
I need to check TDC in my 90, 5.0 Mustang with very ported E-7 heads. So I took an old plug, broke out the porcelain, and threaded it for the closest fit bolt like I have for other motors. But it is not working.
So - 1. How far beyond the plug body end should the bolt stick? (How tall is the stock combustion chamber right under the plug?) 2. Should I turn or grind down the end of the inner bolt to a smaller diameter?
I have used a piece of thin dowel rod, marked for the top edge of the plug hole, but I think an actual stop would be more accurate.
Thanks, picture to follow.
 
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TDC location process courtesy of Michael Yount
If you're trying to find ACTUAL tdc for cam installation/degreeing, follow the steps below:

Positive stop – obtain an old spark plug for the Ford engine – most of us have at least one laying around. Put it in your vice, and with a tool of destruction you deem appropriate – cut off the ground tip and bust up the ceramic around the center electrode. Keep pounding to get the ceramic and center electrode removed so you’ve got a good ¾” or so of hollow space inside what used to be the ‘business’ end of the spark plug. If you happen to break the ceramic on the other end of the spark plug, don’t worry about it. Once you’ve got a nice hollow space in the tip of the plug, take a 5/16” tap, and cut threads inside that end of the plug. I found that diameter to go right in without any drilling required. Once you’ve cut threads, screw in a 5/16" bolt about 1.5” to 2” long. Then take your hack saw and cut the head off the bolt. Wouldn’t hurt to take your file/grinder and dress up the freshly cut surface so no sharp edges or shrapnel enter the combustion chamber. Also clean up the threads on the plug so that it easily screws into and out of the plug hole.

Determining TDC – remove the passenger side valve cover. Rotate the engine (socket on crank bolt) until the #1 piston is down the cylinder on the intake stroke. Unscrew the spark plug in that cylinder, and replace it with the positive stop you just made. Only screw the positive stop in hand tight. Install your cam degree wheel and pointer (no easy task in some cases as harmonic balancer and other accessories will have to be removed). You may want to remove all spark plugs at this point because it will make it easier to smoothly turn the engine over in small increments. GENTLY rotate the engine clockwise until the piston comes to rest against the stop. Note the degree wheel reading. GENTLY rotate the engine in the opposite direction until the piston comes to rest against the stop. Note the degree wheel reading. TDC is exactly half way between these readings. Calculate where that position is on the degree wheel, remove the piston stop from the cylinder, and rotate the engine to TDC as you calculated it.
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That’s what I did, minus cutting the bolt head off. I do NOT want it to fall into the combustion chamber. But it should be long enough, right? There is nothing else for it to hit but the piston like in a flathead or Chevy W motor. I will try again and get a cheap borescope (colonoscopy anyone?) if it still hits something besides the piston.