Will a 4L60E bolt to a small block 350?

The bellhousing bolts up, but which 4L60E you have decides whether the converter fits.

There are effectively three 4L60E versions:

1. Early small-block Chevy version (1990s): built for one-piece rear-main-seal small blocks; GM trucks used them from 1993. This is the straightforward choice for a 350.
2. Early LS version.
3. Later LS version (about 2002 on): a deeper bellhousing for a larger converter.

Why LS transmissions don't fit a 350 directly: most LS engines put the crank flange about 0.400 in closer to the front than a small block Chevy (the 1999 iron 6.0 truck engine is the exception). An LS-style 4L60E on a 350 pushes the converter into the front pump. In Jeff Smith's example the trans bolted up, but the converter wouldn't turn — driving it would ruin the transmission and the engine's thrust bearing.

The fix for an LS-style 4L60E: a 0.400 in adapter plate between the block and bellhousing with longer dowels and bolts. Chevrolet Performance sells a kit that includes a one-piece-seal flexplate. Lakewood sells a similar spacer kit without a flexplate, for two-piece-seal engines or if you already have a flexplate.

Check before driving:

• Converter bolt pattern: older TH350/TH400 converters use a large or small pattern, and the LS pattern falls between them. A large-pattern flexplate can be opened up about 0.020 in to fit.
• Converter clearance: with the converter pushed fully into the pump, there should be about 1/8 in gap to the flexplate. Too little: machine the converter pads. Too much: shim with thin AN washers.
• Controller: the 4L60E needs a standalone controller when the engine isn't running a GM computer.
• Speedometer: it has no mechanical speedo cable drive. Options are a mechanical-drive tailhousing, a VSS-driven converter box, or converting the speedometer to electric.
• Driveshaft: the 4L60E is about 2 in longer than a 200-4R, and the torque converters don't interchange.