Actually, generally speaking, the cast forward drum in the AOD is plenty strong enough for most applications, so many will tell you not to even bother with this. The only benefit will be that the later stamped steel drum is slightly lighter.
However, since I had a complete 4R70W donor, I figured I might as well use everything I can.
So first off, the forward drum is where the outer input shaft of the AOD connects. On the 4R70W it's where the single input shaft connects.
So because the AOD requires the dual shaft system (unless you're replacing it with a single aftermarket unit anyway), the 4R shaft will need to be pressed out of the drum so that the AOD shaft can be pressed in.
Before I removed the AOD shaft from the original drum, I measured from the gear on the back of the drum to the end of the shaft. It measured 213mm (approx).
After pressing the AOD shaft into the 4R70W drum, it measured 217mm. This would mean the shaft was around 4mm further into the torque converter. This may not be an issue, but I did not want to have the converter balloon and press into this shaft and cause issues.
After inspecting the two drums, there is a concave section to the AOD drum where the shaft spine is, and it's lower compared to the 4R70W drum. You can see the concave section in the pic below:
So it was off to dads place to make use of the trusty lathe again.
We machined approximately 0.160" (or 4mm) from the top of the splined area so that the AOD shaft could fit further down.
Now with the shaft pressed back in, it measures 213mm to the gear face, and also sits nice and flush at the back too.
