Neil L;
I am pretty bored today as I am currently on extended medical leave from work so I googled Howard to obtain some data. Howard attachments appear very solid from direct observation, but I have not yet owned a Howard product. I checked on the PTO HP requirements and tiller weight. The HR-16 series "requires 25 - 65 PTO HP. The
"60" is 70" wide and weighs 703#'s, the "65" is 74" wide and weighs 736#'s, the "70" is 79"wide and weighs in at 1029#'s. THe HR-20 series "requires" 50 - 90 PTO HP. The "60" is 69" wide and weighs 879#'s and the " is 79" and weighs in at 1029#'s.
To begin with I think the HR-20 series is a little too much for a "29", but I think your tractor could handle the HR-16 series. I do not put a lot of stock in PTO HP number requirements as they are one of the manufacturer's ways of protecting his product from liability claims. I do believe the ranges give an estimate from which to determine appropriate size attachment. The "29"'s PTO HP is reported to be 23+ HP. I believe that you could "drive" any size HR-16 tiller in the right soil conditions, but you mentioned heavy and I am assuming a heavy, clay based soil so a HR-16-60 and 70 would be pushing it IMHO (I have a rocky, heavy, claybased soil.). It appears that the HR-16-60 tills a "60" patch within a
70" tiller box which makes a 70" path. I do not think the 703#'s would bother your 3PH, but I would keep the FEL on and/or add some front end weight for balancing out the rig.
Breaking up virgin sod and finding immovable objects

puts the most stress on the tiller. As you suggested breaking up sod by tilling shallow would be the best method of starting flower and garden beds. The
second and/or third pass should get you to an 8" depth.
As an aside I really liked the gear driven KKII tiller when I saw one. They appear reasonable in cost, but that is not saying much as it relates to tractor attachments

. Good luck!
Jay
