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1963 Corvette: Service Bulletin: Gear Block-Out and High Shifting Effort

Subject: Gear Block-Out and High Shifting Effort
Model and Year: 1963 1000 Series and Corvette with Muncie 4-Speed Transmission
Source: Chevrolet Technical Service Bulletin
Number: DR #611, Section VII
Date: August 7, 1963


Gear block-out and high shifting effort on Chevrolet Muncie built transmissions may be due to one or more of the following problems:

  • Rough surface finish on the 1st, 2nd or 3rd gear synchronizer ring contact surfaces.  Rough finish at this location will cause the bronze synchronizer ring to "hang-up," resulting in high shift effort or total block-out.
  • Synchronizer sleeve spline wear
  • High clutch key spring tension

If difficult shifting is encountered in transmissions built prior to May 31, 1963, transmission number PO-531, the problem may be corrected by replacing the synchronizers and sleeves with Part No. 3850930 Synchronizer Unit, and inspecting the gear cones for bronze metal pick-up.  if metal pickup exists, the parts involved should also be replaced.

Beginning with transmission number PO-531, the new synchronizer units and polished gear cones entered production.


Part No: 3850930
No. Req: 1
Description: Synchoizer Unit

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