Home Depot (and I'm sure other places) sell cut-off risers that do exactly what you want. They are around 6 inches long and have grooves where you can cut them as short or as long as you need. They come in 3/4" and 1/2" diameters. The smaller size is most common on shrub and "small lawn" heads, the larger for "big lawn" heads.

Also, you don't need a Rainbird brand cut-off riser - any brand will work so long as you get the right diameter.

Thanks for answering Steve.

I have seen those risers but they need to have male and female threads so tt goes on the existing riser that pops up and then but the sprinkler head on top of that.

I bought a tool at Home Depot or Lowes that I use to extract the riser. It looks like a "T". You put your hand on the top of the "T". The bottom of the "T" is flared out. You put it in the inside of the riser, push down while turning to unscrew the riser. It is nice because it doesn't mess up the threads and you can use the riser again. I really like mine. The link below is a picture of one type.

Here is a link that might be useful: Riser extractor

Sprinkler Supply houses will carry exactly what your are looking for.

Female threads x Male cutoff threads. You can adjust length, we use them all the time, saves having to dig all the way down to the pipe.

Another trick, if you can't find them, is to use 2 threaded "Street" El's. Thread one on to the riser, and then the other to the male threads. gives you about an inch lift.

I would like to make new threads by using "dies" on a broke off riser poly pipe, does anyone know the "dies" spec on both the 1/2" and 3/4" poly pipes?

I can't imagine that you would be able to make new threads..I'd just cut out the section that broke, install a coupler and call it a day.

It's totally possible Jeremy, a Die is commonly use for making threads on blank steel bolt not to mention on plastic material such as riser pipe , you can acquire them at some hardware stores. The idea is to customize your own length, whether to resume a used broken pipe or start from a blank riser, a coupler is also an option but lesser the joint the better the performance, my pipe breaks often by accident from garden service and I am tire of running to stores for parts.

Photo

Houzz Photo

I've done it with metal before, just never knew you could with plastic..learn something new every day..you're right though about having to run for the store for parts..I learned my lesson and keep a dozen couplers on hand along with flex tubing..