Max Bar Size For H-51?

[BWS29128]

Does anyone have any idea what the max "recommended" bar length is for a Husqvarna 51 and/or 61? I'm currently running two H-51's; one with a 16" bar and one with an 18" bar, both Oregon, and both running Oregon .050 X 3/8" chain. My H-61 has an 18" Oregon bar with the same chain. I'm considering another 51 or possibly buying a 55 from a friend of mine and putting a 20" bar on it...any suggestions or ideas?
Thanks.

 

[2FatGuys]

I have 16", 18" and 20" bars for my H-50. It will also pull a 24" if using skip chain (not as well as a bigger saw, but better than doing without). I use Stihl RS chain on the 16-20" bars.