To summarise recent discussions on the Autox Forum.
My BN4 has a problem in that the tight spot of the steering worm is about 3/4 of a turn off centre to the right. That means I have a lot of slop at the steering wheel in the straight ahead position and noticeable resistance as I pass through the tight spot turning to the right.
During my investigations I found that the drop shaft splines at the bottom of the shaft have a slight twist in them which causes the Pitman arm to be set a few degrees further to the right than it should be.
This is a photo montage of the twisted shaft and a straight one inserted into a Pitman arm held in the vice.
The straight shaft is the one on the left.
The shaft splines look like this.
The distorted shaft is on the left.
This damage was probably the result of a crash in the past in which some serious impact torque was put into the drop shaft. That would certainly result in the worm being off centre when the wheels are straight ahead.
I really should have found the distortion prior to installation, however the box was a rebuilt exchange unit I bought some years ago to replace the LHD version that came with my project car. I assumed it would be good and didn't strip it to check.
My DWR column/worm assembly arrived today which has to be a good upgrade anyway. I will also be using the replacement (straight) drop arm in the illustration so that I get equal left/right split of the available travel. I haven't driven the car far enough to notice, but if the tight spot in the centre of the worm is way off to the right that probably restricts the amount of left lock remaining.
I will be re-installing the steering box this weekend. and will update this posting with my findings.