I have made some new adjustable tie bars following what has been done in the past by other Marlin owners. They comprise the front part of the old tie bar, a turnbuckle and some rose joints. The idea is not so much to make the suspension adjustable but to be able to dial out any inaccuracies in the chassis geometry. From what I have read this is not uncommon. I am going to remake the U shaped chassis brackets as I don't think my first attempt looks substantial enough.
tie_bar1 by
Robin Martin, on Flickr
tie_bar2 by
Robin Martin, on Flickr
Cutting the 1/2" UNF thread on the tie bar proved tricky. Ideally a lathe should be used for larger external threads but I don't have one. I do however have a decent pillar drill - so I used this to drill a perpendicular 1/2" hole through some substantial box section. I then attached this to the die holder positioning it with a 1/2" UNF bolt. The hole through the box section acts as a guide resulting in a perfect thread on the item to be threaded.
modified_die_holder by
Robin Martin, on Flickr
die_holder_use by
Robin Martin, on Flickr
I should say the guide wasn't my idea. I found the basics on t'internet after a failed attempt at cutting a thread.