Q&A / Tech

Mailbag: Adjusting Bearing Clearances and Crank Alignment on an Engine That Won’t Turn

This week in our Mailbag, the Summit Racing tech department is troubleshooting bearing-clearance measurement and engine-cranking issues in a small block Chevy.

Q: I’m building a 350 Chevy engine and have a problem. When I torque the main bearings down, the crank is very hard to turn over. When I install the connecting rods, the starter won’t turn the engine over. But when I loosen up the main caps, the engine turns over easily. I have miked the crank and it’s right on. What is causing this problem?

A: You said that you miked the crank, but did you plastigage it and the connecting rods with the bearings installed and torqued down in the block? This would have given you the bearing clearances for the crank’s rod and main journals. You might find your clearances are way too tight, making it hard to spin the engine.

You might also want to check the following items:

  • Crank alignment
  • Shape of rod big ends (rods may need resized)
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  1. I had a similar experience once. Turned out that the torque wrench I was using was adjusted wrong and the nuts on the rod bolts were overtorqued by 40 ft.lbs. Readjusted the torque wrench, retorqued the rod bolts and everything was fine.

  2. michael a pyles says:

    Make sure your main and rod caps are in the correct location and direction.

  3. John love says:

    I had one to do that and i didnt have my new pistons there to match bore each cylinder to each piston

  4. My crank journals have large radiis and the bearing changers were too small. Created larger chamfers. My problem was cured.

  5. John miller says:

    Check crank For run outAs it may not be straightAnd remember To stay clean

  6. Duncan Wright says:

    You’ve forgotten to mention that the rod and cap go together one way and one way only. In addition, the rod and piston assemble one way and one way only. (I learned this the hard way rebuilding a SBC over 45 years ago). This is why it is suggested to number the rods and caps before disassembly.

  7. I agree with the other posts I read but also check crankshaft end play

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