I'll stress the need for examining the submerged GM Atlas 4, 5 & 6 Cylinder Engines Deep COP Cavity using a 7-9 mm USB Bore-Scope with the feature of being able to visualize the positions of ALL FOUR VALVES as the Piston slowly rising upwards, Before Top Dead Center (BTDC) when ALL of those Valves
SHOULD BE CLOSED.
Also... While achieving this condition, the Piston Head should be
WELL BELOW the reach of the TIME-SERT Tools as they will need to enter into the Upper Combustion Chamber to first, properly cut in New Threads and then to ensure that the NEW TIME-SERT Spark Plug Insert is SECURELY LOCKED IN PLACE using the Lubricated Insert Delivery Tool.
Having the Piston located at least 1" to 2" below that position on its BTDC Rise is very important, as the Upper Cylinder will eventually need to be Cleaned Out with Compressed Air and then Checked that No Aluminum Dross is left inside of the Cylinder or can Escape Around any Open Valves.
You can accomplish this by first establishing TDC on the #1 Piston using the linked Extra Long TDC Tool and after Reading the Engine Firing Order right off of the Plainly Embossed-Imprinted Valve Cover, gradually Rotate the Engine By Hand,
ONLY in a Clockwise Direction using the TTY Crankshaft-Harmonic Balancer Bolt with a Socket and a Ratcheting Breaker Bar very gradually until the Involved Cylinder being worked on achieves the Correct "Closed Valves - Piston Rising BTDC" condition BEFORE beginning this Repair.
Innova used to sell the Longer Version of this Tool, but nonetheless, this is still the Most Accurate Method of confirming TDC of the #1 Piston on its Compression Stroke: