I have a 1320, but looking at the parts diagram for the 1720 on Messicks, everything looks the same. It is actually very simple.
1. Disconnect the electrical connector from the bar that goes around the glow plugs.
2. Loosen the small nuts on top of each glow plug. They don't have to be removed completely.
3. Slide the bar off of the glow plugs. It may be a challenge to get it out from all the injector lines and other tubing.
4. Use a deep well socket and remove each glow plug. Hopefully they are not siezed fast to the block. They should come out just like a spark plug.
Check each new glow plug before installing. To do this, you can use a set of jumper cables. Attach one end to the ground of the battery and the other to the lower threads of the glow plug. Hook the positive set of the jumper cable to the positive side of the battery and touch the top of the glow plug (the small threaded end) and make sure the glow plug heats up. It should start to get hot instantly. Do not hold smooth tip of the glow plug in your hand. When I did this, mine started smoking immediately. About one or two seconds is all you need to check if they work.
To install the plugs, just reverse the order above. Remember to put a small amount of anti-sieze on the threads.
Good luck.
