I have located a GMC engine.The only ID I could find was a L86/NA5 number and it is of the GAR family.The engine was in a 1981 school bus. It also indicated that it is a big block 366 cubic inch and to use 91 octane or higher. I would like to know if this engine has a steel crank and if I could convert it to 427 cubic inches by changing the rod lengths. I would like to use this engine for a marine application or 1/4 mile
drag car. Any info in on this engine would be greatly appreciated, since it is running and almost a freebee.

Thanks,

a 366 is a raised big block truck engine It has very small valves and tiny pistons and cannot be bored to 427 or 396 it is a hi torque low rpm slug and belongs back in that schoolbus, but may have a steel crank

Thanks for the information.How can I identify the steel crank and could you please expand on the term raised deck? If this crank is steel could it be installed in a another big block such as a 454 and change the cubic inches by rod lengths?

First the steel crank is the same as a 396/427 it will have a wide parting line where the journal centers are lenthwise if its cast the mold line will be thin and have the letters NF cast into the crank second a hi block or tall block has about 3/8 taller deck height than a car block the intake manifold is wider the reason is the pistons were 4 ring design

sugesstion trash it and buy a good motor youll appreciate it in the long run

The tall decks are .400 taller and the rods(and pushrods)are accordingly .400 longer. The intake is wider and the disributor is longer. You won't change cubic dispacement by altering rod length. Only piston diameter and stroke length affect it.