IR ADV 4051 2nd side registration

[MFPTech]

Hi
I know that there's no adjustments for the 2nd side on this machine but even for the leading edge registration on the 1st side I had to go all the way down to value 1 and still is not very good. The range is 1-100. I am using the glass. Printing is ok.
All the blanks are perfect and so is the magnification ratio. It's very frustrating with the limitations of adjustments on this model.
Any suggestions will be much appreciated!

Thanks in advance!

[northernlife]

Copier > Adjust > Feed-adj > adj-refe adjusts the write start position on horizontal scan 2nd side. Applicable for all trays.

Check for a slipping reg/vert feed shaft or bad reg sensor flag if you have to play with reg that much without much change. May warrant a dcon clear as they should be relatively bang on at factory settings.

[MFPTech]

Copier > Adjust > Feed-adj > adj-refe adjusts the write start position on horizontal scan 2nd side. Applicable for all trays.

Check for a slipping reg/vert feed shaft or bad reg sensor flag if you have to play with reg that much without much change. May warrant a dcon clear as they should be relatively bang on at factory settings.

Tried DCON clear, firmware update, confirmed/matched the adjustments settings with the ones on the front door lable, tried ADJ-REFE different settings and rebooted the machine each time - no change.
Tested the same original 1-2 from the glass and the lead edge of the second side is shifted forward by .5 inch and almost .5 inch from the trail edge there is a 2mm thick line.
Tested from ADF and there is a bigger shift (more than 1 inch) but no thick line at all.
The machine is fairly new and the customer says that they had this issue from the very beginning.

Basically it looks like the 1st page is scanned in inch (as it should ) and the second side is scanned in metric (A4)

Any ideas will be appreciated!

Thanks in advance !