Originally Posted by TN6vols
I recently purchased an Chevy 355 that is in need of a rebuild. I took it to the machine shop to get the block checked to make sure it didn't have any cracks. I also took the crankshaft to get it check out. The guy at the machine shop told me that he doesn't recommend using the crankshaft because the mains have been turned down 30 and the rods were turned down 20. Can I use the crank or is it safer to purchase a new one?
This being a forged crank, the turndown of the journals is in a structural sense not an issue; see your comment ……"The crank is forged steel. So, it needs to be replaced. I’ve been thinking about a 383. Is a cast crank ok for a 383 build? " The forged crank comes with more than enough strength to sustain a lot of journal diameter reduction, in fact it's often done on high RPM race motors to reduce the surface speeds between the journal to bearing insert, especially on the rod.
Not all Chevy forged cranks received surface hardening so if it was originally it's now gone; if it didn't have it there isn't any issue. Depending on year of the crank they were made from slightly different materials 1046 or 1053 steel. The older lower carbon 1046 was nitrided to harden the journal surface to improve wear, the later crankshaft forging was made using harder, higher carbon 1053 steel so these shafts usually weren't nitrided as increased surface hardness wasn't needed to achieve the wear qualities they were looking for in a 100,000 mile engine life. There should be casting/forging numbers on the crank that will lead you to which it is.
A 400 horse power level is pretty easy to attain from a 350 with modern heads and pistons at very moderate RPM (under 6000) so it isn't the problem keeping the rotating parts together that it once was. Today this is done on cast cranks with no ill effect; pieces like SCAT's 9000 series cranks are very good if you decide to go that way. However, I'm more concerned with rods, pistons, balance and harmonic dampers these days. The original GM rods, even pink rods, are getting a lot mileage on them which translates to loosing (actually using up) designed fatigue life; so I lack any enthusiasm for reused or rebuilt rods when there are so many inexpensive high quality 4340 replacements out there. For good power the pistons need to work with the head. These days if you want 400 horses without the trouble long duration cams bring, the answer is the L31 Vortec or similar aftermarket clones. Those combined with the timing of something like a Comp XE268H flat tappet, a 750 CFM carb on an Edlebrock Performer RPM, a set of long tube headers it's easy to build a slightly better than 400 horse engine that doesn't have to turn more than 5600-5800 RPMs to do it. This of course means good piston selection which with the need for tight squish/quench clearance of .040 inch means the use of a D dish piston to keep the squish/quench tight on the far side with an adequate D shape dish under the valve pocket to keep the compression around 9.2 to maybe 9.5 depending on vehicle weight and gearing.
The end game I'm getting to is that if your current crank measures correctly for journal diameters, concentricity, and taper; I put my money in better rods, pistons, balancing, a darn good damper and heads.