Just purchased a 99 353 with 502's with factory installed stellings boxes. A few people have said there of no importance on this application. I've never owned a boat with boxes so any input would be appreciated. Is there any addintional maintence that is required ? I was also told that they put more stress on the drives is this true ? Thanks for your assistence.

Extention boxes will add 3-4 mph on a 353. They need to be greased at the universal and carrier bearings every 50 hours. I don't think they add stress to the drives.

You may find a tendency to overtrim with extension boxes. They need less trim input than you are used to. If you over-trim, you will actually see your speed drop. you will find the sweet spot with just a little time at the wheel.

I agree with reddog on the stress factor but here at LOTO the extra speed is not there. I think you need rougher water to so a constant peed increase. We sold a 382 w/500 and boxes, I ran a copy of the boat along side it with NO boxes and it was a dead heat. The other driver was a Formula salesmen also.

RedDog has it correct with the 50 hr maintenance and the 3-4 mph. The carrier bearings are expensive if you have to replace one, don't ask me how I know

Boxes make a bigger difference on the 353. Possibly 1-2 mph on a 382.

If you want to add boxes to 353 that came without , use the imcos with 3`rise

seems the boat runs the best with about 2-2.5 clicks of trim which is very minimal