Frequently Asked Questions
Distributors
Are 348 & 409 distributors interchangeable with other Chevrolets?
No. They are shorter; the gear is interchangeable as well as the top pieces, but the shaft and housing are different. 348s have an oil tube on the side of the housing. The 409s have a short oiler or no oiler at all.
Overheating
My car runs hot. What do I need to look for?
- The most common problem is the use of a small block fan shroud, or no fan shroud at all. The 349/409 fan shroud is shorter. To get the best results, it should cover half the fan blade.
- Over painting the radiator—too much paint on the radiator acts as insulation and won't disperse the heat properly.
- If you have converted your car from a small block or 6 cylinder, make sure you are using a 349/409 radiator.
- Fan Clutch/Water Pump. If your car overheats at an idle, but cools down at highway speeds, your fan clutch may not be operating properly, or your water pump may not be circulating enough coolant at low rpm's.
- Piston to cylinder wall clearance my be too tight.
- Lean burn, poor timing, and bad gas can contribute to overheating.
- Moly rings will run cooler than cast rings.
- Thermostat--make sure your thermostat is opening and closing properly. We recommend a high-flow thermostat. Running without a thermostat will actually make the problem worse as the coolant moves on to the radiator before the heat is absorbed from the engine.
Ring Compressors
The right ring compressor will work on standard, +.020 & +.030 overbore. If your block is bored +.040 or more, the clearing at the top of the chamber is lost. You then must use your fingers to gently squeeze the rings together while you tap the piston into the block. We recommend reboring the top of the chamber .060 larger than the cylinder bore.
Are the 348 & 409 truck blocks a tall-deck version?
A: No, all 348 & 409 engines had the same deck height, 9.600.
How can I tell the difference between a 348 & 409?
A: The surest way to tell the difference between a 348 and a 409 are the block casting numbers. Use our number pages to determine the correct year. Another way to differentiate between these two engines is to note the location of the dipstick. A 348's dipstick sits on the driver's side, while a 409's dipstick is on the passenger side. Beware, however, that oil pans are interchangeable between the two engines, so this method does not always yield correct results. Finally, a 348 has a round rear crank bolt flange, while a 409 has a bell-shaped rear crank bolt flange.