d4rt
Active Member
Hey there FABO, I've gotten myself into a bit of a pickle and I'm looking for some feedback.
I have a 360 that I acquired with a crankshaft that requires 0.030 in. undersize bearings. This is going in a 1970 Dart. The car is mostly street but I will also be racing it a bit too. I don't plan on spinning this engine past 5900-6000 RPM, maybe not even that high. I checked the main journals with a 2-3 in. outside micrometer and they measured between 2.7813 - 2.7816 in. Based on guidance from Mahle/Clevite, this is 0.0008 - 0.0011 in. out of spec (2.7795 - 2.7805 in. is the target range) for a 0.030 in. bearing. Using the specific measurement for each journal, I zeroed my dial bore gauge and checked bearing clearances. They're all tighter than I'd like by quite a bit. See below for a list of measurements:
1. Grind the crank to get the additional clearance. I've read that it is extremely difficult to grind 0.001 of a crank and keep everything true. My machinist didn't sound very excited about this proposition.
2. Polish the crank to get the additional clearance. This apparently can be done but it's hard to do without adding a bunch of egg and taper.
3. Get a different crank that hasn't been cut/cut less. Seems like a reman "crank kit" is the ticket here.
4. Run it and pray to the Mopar gods that I don't spin a main bearing ~1500 miles from my house
Option #3 is probably the best choice ugh. Not looking forward to another $400 - $500 sunk into this build. Option #1 was my initial thought, however that might cost nearly as much as just getting a crank kit and then I still have a 0.030 in. crankshaft. It sounds like a PITA to setup a crank grinder to take off only a 0.001 - 0.002. Plus, based on what I've read, it'll require tooling and talent that might not be present in a standard machine shop. Not a fan of option #2, although I guess some egg and taper, as long as it's within a reasonable spec, is a better reality than super tight bearing clearances.
What does everyone else think?
I have a 360 that I acquired with a crankshaft that requires 0.030 in. undersize bearings. This is going in a 1970 Dart. The car is mostly street but I will also be racing it a bit too. I don't plan on spinning this engine past 5900-6000 RPM, maybe not even that high. I checked the main journals with a 2-3 in. outside micrometer and they measured between 2.7813 - 2.7816 in. Based on guidance from Mahle/Clevite, this is 0.0008 - 0.0011 in. out of spec (2.7795 - 2.7805 in. is the target range) for a 0.030 in. bearing. Using the specific measurement for each journal, I zeroed my dial bore gauge and checked bearing clearances. They're all tighter than I'd like by quite a bit. See below for a list of measurements:
- Main #1: 3.0055" Main Bore Diameter, 2.7814" (Minor) - 2.7816" (Major) Journal Diameter, 0.0016" - 0.0018" Bearing Clearance, 0.0002" Egg, 0.0000" Taper
- Main #2: 3.0054" Main Bore Diameter, 2.7813" (Minor) - 2.7815" (Major) Journal Diameter, 0.0017" - 0.0019" Bearing Clearance, 0.0002" Egg, 0.0000" Taper
- Main #3: 3.0054" Main Bore Diameter, 2.7813" (Minor) - 2.7815" (Major) Journal Diameter, 0.0017" - 0.0019" Bearing Clearance, 0.0002" Egg, 0.0000" Taper
- Main #4: 3.0053" Main Bore Diameter, 2.7813" (Minor) - 2.7813" (Major) Journal Diameter, 0.0019" - 0.0019" Bearing Clearance, 0.0000" Egg, 0.0000" Taper
- Main #5: 3.0050" Main Bore Diameter, 2.7813" (Minor) - 2.7816" (Major) Journal Diameter, 0.0009" - 0.0012" Bearing Clearance, 0.0003" Egg, 0.0000" Taper
1. Grind the crank to get the additional clearance. I've read that it is extremely difficult to grind 0.001 of a crank and keep everything true. My machinist didn't sound very excited about this proposition.
2. Polish the crank to get the additional clearance. This apparently can be done but it's hard to do without adding a bunch of egg and taper.
3. Get a different crank that hasn't been cut/cut less. Seems like a reman "crank kit" is the ticket here.
4. Run it and pray to the Mopar gods that I don't spin a main bearing ~1500 miles from my house
Option #3 is probably the best choice ugh. Not looking forward to another $400 - $500 sunk into this build. Option #1 was my initial thought, however that might cost nearly as much as just getting a crank kit and then I still have a 0.030 in. crankshaft. It sounds like a PITA to setup a crank grinder to take off only a 0.001 - 0.002. Plus, based on what I've read, it'll require tooling and talent that might not be present in a standard machine shop. Not a fan of option #2, although I guess some egg and taper, as long as it's within a reasonable spec, is a better reality than super tight bearing clearances.
What does everyone else think?
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