I have a TKO600, with an aluminum driveshaft. I basically have NO part of the yoke showing. Should I have the driveshaft reworked,or just cut the yoke?
Kinda the wrong way to look at it... How much is sticking out in itself doesn't matter.
The way the basic rule of thumb works is that you should push the yoke all the way until it bottoms inside of the trans, and then pull it back out from there ~3/4". I learned this because even though I have ~1.25" out of mine, my yoke splines bottom out on the output shaft splines while ~1/2 - 5/8" is still sticking out. If I only had 3/4" sticking out, It would probably be very badly hammering away on the trans output shaft with the suspension moving around.
If you want to get technical about it, I did some research while installing my T56 and racers will use, for the amount of spline engagement b/w the yoke & output shaft, 1.5 times the diameter of the yoke (or maybe they meant times the output shaft, which = yoke ID), and I assume that's for pretty high hp applications. Mine falls short of that, but I'm only making 270 rwhp, give or take, and I'm not that worried about it twisting the splines. Maybe after I install the nitrous, I'll check the splines occasionally.
So summing up, if your yoke doesn't bottom out until it hits the housing, then 3/4" is good. If it bottoms before that, then you want ~3/4" more.
For my car, it turns out that I have somewhere between 9/16" minimum to 3/4" maximum play. That seems to be alright in my application.
FastDriver is spot on. It’s been a few years since I had the DS for the T-Bird built and I do remember them telling me to install the yolk fully seated and measure from the u-joint centerline to the flat face of the pinion flange. They then subtracted the 3/4” and built the DS.