3929039 Camshaft fits Cummins 4B 4BT 4BTA 3.9 3914638 3925582One New Aftermarket Replacement Camshaft for Cummins 4B 4BT 4BTA 3. 9L Diesel Engines This aftermarket replacement camshaft fits the Cummins B Series 3. 9L four cylinder diesel engine family, including the naturally aspirated 4B, turbocharged 4BT, and turbocharged aftercooled 4BTA variants. The camshaft is a core valvetrain component that controls intake and exhaust valve timing throughout the engine's operating cycle. The Cummins 4BT 3. 9L (CDC 4.