Tap your holes out, chase the threads, or use a tap & die set to figure out what the thread is. The holes really should be 1/4-20. Handy is a set of socket type tap drivers, a lifesaver in many cases.
Not too many mystery threads on American cars besides some odd large threads like seat belt bolts.
Hex head "bolts" starts at 1/4-20, 5/16-18, 3/8-16, rarely 7/16-14 but I've seen it on exhaust flanges, moving on to 1/2-13. Some use of fine threads, such as 1/2-20 on oil drain plug.
Some bigger bolts underneath, K member bolts must be at least 1" thread size.
A torque chart.
Torque specifications are for stock factory type bolts with light engine oil applied to the threads and the underside of the bolt head. Other lubrications offer reduced friction and increased bolt tension, and will affect the torque figure. If using aftermarket performance bolts, like ARP, you should follow the recommended torque specifications of the aftermarket bolts. |
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Fastener | Torque | Thread Size | Camshaft Lockbolt | 35 | 7/16-14 | Carburetor to Manifold Nut | 7 | 5/16-24 | Clutch Housing Bolt | 30 | 3/8-16 | Connecting Rod Nut – Plain | 45 | 3/8-24 | Crankshaft Bolt (Vibration Damper) | 135 | 3/4-16 | Cylinder Head Bolt 273-318 | 85 | 1/2-13 | Cylinder Head Bolt 340-360 | 95 | 1/2-13 | Flywheel Cover Bolt | 100 in/lbs | 1/4-20 | Flywheel Housing to Block Bolt | 50 | | Flywheel to Crankshaft Bolt | 55 | 7/16-20 | Fuel Pump to Block | 30 | 3/8-16 | Intake Manifold Bolt | 35 | 3/8-16 | Main Bearing Cap Bolt | 85 | 1/2-13 | Oil Pan Drain Plug | 20 | 1/2-20 | Oil Pan Bolt | 15 | 5/16-20 | Oil Pump Cover Bolt | 95 in/lbs | 1/4-20 | Oil Pump to Block Bolt | 35 | 3/8-16 | Rocker Shaft Bolt | 30 | 3/8-16 | Spark Plug | 30 | 14mm | Timing Cover Bolt (cast) | 30 | 3/8-16 | Torque Converter to Flexplate Bolt | 270 in/lbs | 5/16-24 | Vibration Damper Pulley Bolt | 200 in/lbs | 5/16-20 | Water Pump to Block Bolt | 30 | 3/8-16 |
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