I just got some 661 Shinpads , went out for a ride and they rubbed the tendon on the back inside of my knee raw. I tried messing around with the strap adjustment to no avail. Will this go away after a break-in period? Does anyone have any suggestions (short of tossing them or dealing with the pain)?
Thanks 
You could try wearing some cycling knee warmers like these from Col d’Lizard
I like to wear the knee warmers under my Roach pads to prevent possible chafing. I don’t get a lot of chafing without the knee warmers, but I like to wear the knee warmers anyways.
Make sure that you aren’t making the straps too tight. I leave the straps up top around the knee loose.
An alternative to cycling knee warmers could be some old socks. Cut the socks off at the heel and put them up around your knee.
I take one of the top straps and lead it down to the velcro patch on the next level down. It doesn’t really do anything in that position. The other top strap I keep with only light tension.
In that position I have very little chafing and they are quite comfortable, and never come out of place.
I have tried John’s knee warmers but they made my knees… well… warm. 
But on a really really long trip they could be quite helpful to keep overall chafing down.
Re: 661 Shin-KneePad Pain Suggestions?
The only responses so far are from folks having a similar problem (and
suggesting solutions). Do all have this issue? I have the 661 4 X 4
(that’s the type in the original post right?) and I have no shafing
problem at all, even after hours of semi-continuous MUni. Am I the
only one?
“the helmet is to protect the brain, not replace it. - iunicycle”
I had that problem and was getting really frustrated but then realised that they were on the wrong legs. The straps should attach on the inside, for example on the left hand side of the one that’s on your right leg. This completely stopped the rubbing for me.
Andrew
I haven’t had a problem with chafing using my 661 4x4’s.
I wear them with the ‘buckes’ to the outside
Fair enough.
I don’t remember where the straps are. I wear them with the large 661 lettering to the outside which (to me) makes sense in itself.
Klaas Bil (off on holiday in 1 hour)
I wear mine with the logos/buckles outside and the two top straps as loose as can be. No chaffing here.
On strap got snagged on a bush, stretched, let go of the bush and slapped me in the thigh. It gave me enough of a suprise to UPD.
I found my Roach leg armour really uncomfortable around the knee area for the first few rides. I rode with the top straps undone completely. After a few rides they softened up a bit and were more comfortable to ride in.