Homemade Valve Spring Compressor.

33IMP said
I made an adapter for putting air in the cylinder, out of a gutted spark plug. Only problem, I tried it on my tired old 283 in my 55....... and the air turned the motor over, and pushed the piston to the bottom of the cylinder.

I read that post. Did a ton of research. Can't know how the pressure is any different at TDC than BDC. I think RRR was alluding to that.