What causes excess crank endfloat?

[FSJNut]

My 360 in my GW has nearly 1/4" of crank endfloat, yet still has good oil pressure. When you put it in gear you can see the crank pulleys move forward, and when you mash on it it moves forward enough to put the flex plate bolts into the engine. When you put it back in neutral it shifts back to the normal position. What is causing this forward movement under load? I'm worried it will happen again after I fix the engine if I don't know what caused it the first time.

[andy d]

end play is caused by worn shoulders on a thrust mainbearing.1/4" is huge! how do you keep oil in it? try one thing,try tightening the bolt that holds the crank pulley/vibration damper on. maybe the pulley is loose and sliding on the key way. my 81 and 84 both with over 150k on them would rap until oil pressure built up. the rap was that thrust bearing. both leaked oil past the front and rear seals does yours?

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'88 gwag,pure stock

[FSJNut]

It is leaking very slightly at the front and rear main, but not as bad as you might think http://www.ifsja.org/ubb/smile.gif. I wish it were just the pulleys loose, but the flex plate/torque converter are moving also. I know the engine has to be rebuilt, but I have heard of 2 situations where the endfloat returned after 10-20K miles. One on a FSJ and one on a Chevy. The Chevy went thru 2 rebuilds and another complete engine in short order, each with excess crank endfloat. In the process they had also replaced the torque converter twice.
My dad was wondering if the splines on the input shaft of the transmission was twisted if it could create a coarse thread of sorts, creating a forward thrust when under power.