Refreshing a really old thread, specific to the NH 16LA bucket cylinders.
The bucket cylinders on my 2001 16LA use threaded end caps, 2-1/2" diameter heads with 2 small flats at 2-3/8" across. That doesn't provide much bite for traditional wrenches, they'll just round-off the corners. I had to cook-up a clamped-on arrangement, using a 4-foot 2" angle iron, and thick steel brackets from my engine stand.
Edit: Looks like the attachments loaded out-of-order.
Attachment 2 - Typical thick steel engine stand brackets.
Attachments 1, 3, 4 - The 4-foot angle iron, thick brackets, 1/2" grade 8 bolts/nuts.
Get everything lined-up on the end cap's flats, torque the bolts down 'till your elbows crack.
At about 80 lbs force on the end of the 4-foot angle iron, the end caps *snapped* loose.
That's about 300 ft-lbs of torque to loosen those things...