As far as boring the B18 to the B20 sized pistons, not a swell idea. Refer to Phil S' article on Vclassics.

Can be done, but you really have to be sure you have yourself a really good block. Sonic testing is in order beforehand. Also it'd be best to get a B20 cylinder head to top it with, a B or on F would work.

The IPD B20 big bore kit allows you to utilize B21 pistons. A friend had that on his '73 ES; with an F head, isky VV-71 camshaft, and D-jet. Got a steady 19.5 MPG, even through the automatic transmission, no matter how he drove it.

I don't know what kind of head work is in order, but making use of the F head is a good start.

If you can get a search going there's a lot of good information on this site on that particular upgrade.

Seeing though as it's a lot to bore out, you're looking at finding a block, a rebuild, the IPD kit, machine work for block & head, as well as a new exhaust as per your previous post. My opinion is a rebuild of a B20 would be adequate & cheaper, and good for a decade or two of solid, dependable service.

B20s are plenty peppy out of the box. What kind of carb(s) are you using?

-Sean
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'66 122s, '70 142s, '74 142e... Blue is beautiful.