Original block still has an oil leak at the rear (bottom drain hole, trans bell house cover). Leak 1: shop built long block used 4-piece type oil pan gasket. Replacement oil pan in 1986. Leaked not long after first start Leak 2: Installed Fel-Pro 1-piece and new rear seal (don't know if front seal is thick or thin). Tightened with inch-lbs torque wrench. Dry for a while then started leaking Leak 3: Dropped pan, checked, new rear seal, reinstall. (Removed rear seal had a small slice but could have happened during removal). Leak began shortly after starting. Can not see or feel any oil on the pan rails forward of the rear. No oil coming off the oil filter area. No oil from the valve cover No oil from oil pressure sending unit Should the front seal be thick or thin? What is the measurement of the needed thickness? What can cause the above problem?