Finally got my oil filler tube screwed back into the rocker box after the darn thing unscrewed and refused to screw back in on Friday when I topped up the oil .
Had to make a special tool for the job to engage in the two lugs at the bottom to get it in.
Usually simple hand turn'n tighten job when its happened before but this time it decided to go in a bit under a turn before jamming. Even under hand tightening pressure it needed a strap wrench to release it. Not good. At £70 a pop I wasn't going to risk breaking the tube by forcing plastic inevitably embrittled by years and heat.
After scraping the thread in the rocker box made made no difference I measured the thread on the tube so I could make a chaser. Thread is 1.640" diameter, 11 tpi with a 60° thread angle. 1 - 1/4 BSP except for the wrong angle. Actually got as far as setting up to make a 60° chaser before realising that a 60° male thread going into a 55° female would usefully tighten the grip on the plastic. So I modded a brass plumbing fitting to make a chaser and screwed that in. Didn't go as free as I'd like so clearly something in the thread. In and out a few times got it clear with some crud collected by the chaser. Felt right for a 55° thread in the box matching the chaser.
Tube still wouldn't co-operate so I set to and made the tool.
If anyone else has to make one the included angle on the lugs is 32°, slots need to be more than 9 mm wide at the outside end, mine are 12 mm, and tool OD no more than 33 mm so it drops down the hole. Other details to taste. I made my slots the full 10 mm depth of the lugs and relieved the centre. My body is 40 mm long, plenty to keep it stable in the tube, with a 65 mm long 12 mm Ø shaft to reach up the tube having a 13 mm hex (Screwfix M8 joining nut) on the top end for a spanner.
When fitting the tube needed what felt like maybe 4 or 5 ft/lb torque to pass the tight spot then screwed in stiffly but freely with maybe 2 and bit ft/lb force. More than easily done by hand for sure. Frankly it was right on the top end of the torque I felt safe when it jumped the tight spot.
If it comes out again I can see a spiffy custom billet alloy replacement happening.
Clive