Depending on how big the parts are (block or something more manageable?) Evaporust works great and no worry about scratching or marring the surface like with wire brushes, etc.

(I use it on vintage tools, and works great and no worry about hurting the fine stamped in writing, etc on them.)


For example...(and these were only soaked and then rinsed off, nothing else) WWII issue wrench handle after a soak;




And a 1927 extension after a few hours soaking...