Frame tube dimension

See below..........

It should be 1" or 25mm on the later Italian made850frames. Guage I would guess is around 16swg. Could be 14 but might be overkill. Someone will advise the correct dimension. On the head steady I would go for the Dave Taylor model available from RGM. I made my own version of the Mike Taglieri job which is similar but have reverted to std (to try and ID a rattle). It will go back on next time the tank is off.

I gave up on the Isolastic type of head steady as they don't tend to fit with adequate clearance under the tank. (Fastback in my case) There are some very slim isolastic headsteadies out there now but nothing is cheap. I too made my own version of Mike Taglieri's rod end head steady and it works well. That was cheap, with a couple of bits of angle iron and a rose jointed link from a Bell 412 tail rotor mech. I'm getting round to fitting the Mk3 spring kit to offload the Isolastics but I'm expecting more vibration with this mod

Also, you can expect the spine to bend when you weld in the supplementary tube by anything up to 1/4"