"Also I was thinking about digging a little 6" circle around each sprinkler head "

Grass keeps dirt out of the heads ... but in time will cover heads and cause problems so you should cut back grass each spring but six inches is a bit excessive and will lead to more mud around the head and dirt problems. it also looks quite silly like you have a disease on the lawn !!

Keep in mind no grass around the head also provides a great place for weeds to start growing. When I worked on the east coast ( Long Island ) a 1/4 inch or so around each head was all that was needed for the each growing season.

Now heads to deep is most likely the cause of "dirt in your head" syndrome. I don't know what you have in the ground but buying a set of nipples ( 1/2 inch most likely )of different sizes and some couplings threaded on both ends is the standard parts you will need. It may be possible to unscrew the head without digging a hole but this requires a bit of practice and some luck. A spade and small trenching shovel along with some Channel lock wrenches are useful tools. The old brass heads use to have a special wrench to move the head if you can find one available.

The head needs to be flush with the top of root zone of the turf so don't overraise the heads and lets face it heads get old and should be replaced at times. Perhaps thats the cause of your problems ??

Good Day ...

Another technique is to use "swing joints" on each head. This allows you to dig up a head at anytime and raise or lower it without adding parts.
One way to build a swing joint is with three better yet four polly elbows threaded on both ends. Although you will need a bigger hole to work with swing joints.

Good Day ...

Thanks for your response...I'm not sure that the heads are in need of replacement quite yet..although...I will certainly look at them. What I am noticing is that some of the heads are leaking around the bases....thus...there will be a little puddle, whilst its still working and doing its thing. I will ensure that all the heads are "flush with the top of the root zone"

Good recommendation on the weeds thing. Thanks!

Another problem, which is actually more of a pressing thing is that one of the zones is stuck on...yes the valve is stuck open and I can't seem to locate the valve itself. Its been recommended that I take a metal detector to locate the valve. For reference, I found that I have 40 total heads in the whole system with 9 zones.

I realize this is an older post...

Swing joints are a good option. To combat the "dirt in the head" problem, I build some cheap sprinkler guards out of some stacked ABS couplers, then backfilled with pea gravel:

I did this for all my in-lawn Rainbird 5000 rotors and it has worked out well. Also helps prevent sprinkler lead "lean" over time on slopes.

Yes, it requires digging out each head but you hafta do that anyway to add swing joints. Step-by-step install guide here:

http://havingfunalongtheway.com/save-the-drowning-sprinkler-heads/