Readers’ Tech Q&A
I have a 1978 Cutlass Supreme with a 260 V8. It was rebuilt a few years back, but I don’t think it was done very well. It has a rod knock now. I don’t know which way to go: rebuild this one again (they want $2,800 to rebuild it), or upgrade to something with a little more get up and go. If I replace it with a used engine, what newer-year Oldsmobile would bolt directly in? What size would work best?
There’s no replacement for displacement. Always get the largest engine you can afford. That’s my motto, so coming from a HOT ROD performance perspective, I wouldn’t waste time or money on a stock 260 rebuild when complete Olds 1980–1889 307, 1964–1967 330, 1968–1980 350, and even 1977–1979 403 small-blocks are direct bolt-ins for a 260 (they all have the same external
dimensions). Searching online, I see you can get an inexpensive stock rebuilt Olds 350 long-block for $1,695 plus a refundable $400 core-charge from outfits like Rebuilt Crate Engines. A baseline mild 350 performance version goes for $3,299.95.
If searching the wrecking yard, the most common larger-displacement Olds engines of similar or later vintage to yours would be a 307 or a 350 (be sure it’s an Olds 350 gas engine “VIN-R”—there were scads of Olds diesel 350s made during this period). The 403s were only made for three years, so they’re relatively rare, but you never know, it might be your lucky day.
Whether used or rebuilt, Olds experts like Mondello and Dick Miller say you need to pay attention to the following items when swapping Olds motors:
• Olds engines are externally balanced. Flexplates were individually balanced at the factory, so try to grab the original flexplate with your new engine. The larger the displacement, the greater the amount of flexplate unbalance, so your original 260 flexplate may cause a slight vibration if used on the larger engines. Many replacement flexplates are universal, meaning not ideally balanced for the individual motor, regardless of displacement.
• Mondello’s experts say 260 and 307 harmonic balancers interchange. Ditto for 350 and 403 balancers. But don’t use a 350/403 balancer on a 260/307—or vice versa.
• Accessory drives, pulleys, and water-pump snout lengths may vary on different models. If necessary, transfer these items as a complete, matched set, from the old engine to the new engine. Block and head accessorymount bolt holes never changed.
• Depending on the model, you may need to
transfer the oil pan and oil-pump pickup.
• A 260 intake manifold only fits a 260
because of port-size differences. There
Norm Brandes 02] The numbers or letters on the front of the block near the oil fill tube are one way to ID an Olds. If the large digit following the smaller casting number is a letter, you have a big-block; a large number or number/letter is for a small block. Casting No. 390925, ID E, tells you this block is a 1966–1967 big-block 400. Mondello’s Oldsmobile V8 Technical Reference Manual, Vol. 3 ($26) is full of hardcore data and engine-building tips like this.
01] A big-block Olds V8 like this hot rodded 455 also bolts in place of small-block Olds engine, but they’re slightly taller and wider, so check engine-bay clearances. Big-block Olds engine-swap headers for a GM 1977-and-later A/G-bodied intermediate chassis are available from Mondello and Dick Miller.