Marathon's are a division of Rheem. Is this an electric unit you are replacing? They are excellent units, a little higher on the cost scale but well worth it since they are warrantied for a life time. You won't be able to buy replacement elements anywhere other than through Marathon or an authorized Marathon/Rheem dealer. The threaded base is much larger than most heating elements. I believe they are 1-1/4 inch, maybe1-1/2 inch, I don't have the specs. in front of me. The larger opening allows one to use a shop vac. or other methods for getting inside for cleaning should it be required. The heating elements run about 50-60 bucks each. If you have acidic water, they have a high end element that runs 60-70 bucks each and last a long time. You can get a better feel for how they are constructed if you visit their web site at marathon.com. There are many utilities that offer incentives if you install one of these. Do a search on the web for "Marathon Hot Water Heater" and you will get a better feel for what others think of them, how they are constructed and if incentives are offered in your area.