How long do engines in monster trucks normally last?

that varies alot. things like fuel, plug, conditions, driving style, motor it's self. their is no real average, but if you run them a little rich and take care of them, they can last a long time. the losi 427 is known to get over 10 gallons, when cared for. but expect 5 or more, if you're careful.

Generally dosent matter if its in a mt, or not. Generally offroad engines are subjected to more harsh environments than on road engines. Usually stock engines dont last as long as the real high end engines but there are its exceptions, mgt, is known for 10+ gallons on its engines and same as the mach.

some will go 20 gallons... some won't make it to 10. Depends on who made the motor and how well it's tuned, and maintained.

Most higher quality racing engines (picco......) will generally last longer than others. Funny you say the mach can make it to 10+ I have an LRP 28 and it ran for about 2 gallons with severe clutch/diff issues (slipping) in my truggy and 6 gallons later brass is showing through the sleeve but its still goin pretty strong [X(]

That's a factory defect. The chromium plating on the sleeve should never wear, only the much softer piston. If you can see brass, the plating has probably flaked off due to improper adhesion to the sleeve. SH engines aren't exactly superb quality, so it wouldn't surprise me too much.

x2... aluminum (piston) will wear 10x faster then chromium (sleeve).