The third hole works BEST when the transmission is on. When I do it with the transmission off, I usually have to use a small ratchet strap hooked onto the front engine pulley to pull the engine more horizontal.
I've not had a sparkplug/lifting eye pull out. Never heard of one pulling out.
The engine/transmission together weighs 475 lbs.
Lets say the annular area of the spark plug in shear is 2-1/2" long - and the length of the threaded section is 1/2 inch. That is an area in shear of about 1-1/4in2
Shear modulus for cast iron is perhaps 30Ksi*1.3 (See
http://en.wikipedia.org/wiki/Shear_strength) or perhaps 40ksi. Cast iron in shear is actually stronger than cast iron in tensile - perhaps all that carbon getting in the way instead of acting like fracture points?
Given the 1-1/4in2 area, perhaps 50klbs total pull.
Ok - maybe half of 50klbs with a side-pull on the spark plug and "cantilevering"
I think I can hear the engine laughing - even at this distance.
I can see reasons for using two spark plugs - it allows one to use a "lifting bar" or other arrangement to "level" the engine - but it's not necessary.
Joe K