What did you do to the engine to "mod" it, and what brand and size of bearings are you using? Are you shimming your clutchbell?
Not shimming a clutchbell properly will cause bearings to fail quickly. Not oiling them every so often will cause them to fail quicker.
I run a modified Picco .28 in a 1/8 monster truck and I get about a half gallon to a set (3 - $1 AvidRC bearings) of clutch bearings if running hard. Engine break-in took about 1.25-1.5 gallons of fuel and the first set of bearings went poof at the 1gal mark, and I just changed them a few weeks ago at the 1.5gal mark. I oil the bearings every 2-3 tanks of fuel. I shim the bell for about .001-.0015" endplay and because my bell has two pinion gears on it, I can run a 3rd bearing with a special Ofna shim kit.