If you want to go old school, the old ProGear 251 is about the same size as the Calcutta. Guts are Jigmaster so parts will be available forever. I replaced the stock 5:1 gears with Jigmaster 4:1. I think the Jigmaster 506HS 5:1 gears also fit. Replace the stock drags with a Jigmaster HT-100 drag stack. No anti-reverse bearing which always seems to fail in my Calcutta. It also has a spring loaded dog which is more reliable than the ambassador style in the 400.
If you want to drop some serious coin, the Penn TRQ-15 is also about the same size. The Fathom 15 is a less expensive alternative. They are 6:1 which is not the best for cranking power. You can change the TRQ-15 to 4.8:1 gears but that will set you back another $50. There are aftermarket lower speed gears for the Fathom.
I'm really surprised how long my 400 has lasted, given the number of times is has been underwater
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