I let my HPI RS4 setup for years, and the motor froze up; this is what I did. First take the glow plug out, and then squirt some after run oil or something similiar such as Marvel Mystery oil or any good light penetrating oil. Grab ahold of the shaft or the starter wheel if it has one, and then try to move the engine, put a little back bone in it if you have to but not too much. You just want to free it up a little to work the oil in. Then let it sit for awhile, about an hour or so. Go back and try to work the motor a little more. You might be able to free it up to turn it over. After that I put about a teaspoon of fuel in it, which is also a good solvant, and work the motor over and over. Eventually you will be able to turn it over just fine , and put some more fuel in it. Be sure to leave the glow plug out, this will keep it from binding, or trying to blow all the crud out of the exhaust. I was able to get mine working fine and ran the motor for a couple of years more after this. This motor was a OS MAX .12 and held up great even after all that. My opinion is that the fuel (especially castor oil blended) eventually dries up and makes a kinda of glue and actually glues the parts together because it was real smooth after I did this. I never saw any rust. The important thing is not to force the engine because you could break something internal.
