I bought the IPD counterhold tool to hold a B230F harmonic balancer while loosening the bolt. However, the B21F harmonic balancer lacks holes to insert the counterhold tool. Using a B230F harmonic balancer in a B21F engine would make life easier next time I need to change a timing belt, but are they interchangeable?

The B21/23 doesn't have a Harmonic Balancer. It's just a pulley, and can't be replaced with the B230 HB. The IPD ad for the tool says it's just for B230 engines.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

As Matt points out, the shimming setup of the B21/23 front pulley was a rather primitive arrangement, but that (crankshaft to AC compressor) belt never needed adjustment after the initial fitting, in my experience.

The advantage is that you can remove that front pulley from the crank by removing the 6-10mm nuts and bolts - no need for a crank holding tool or the use of significant musclepower when changing the T-belt.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

I don't think they are interchangable.
As I recall, the older style B23 (or B21) crank pulley uses shims between the 2 pulley pieces in order to tighten or loosen the A/C belt. The newer style B230 crank pulley does not have this. Thank god, because that was a shitty way to tighten a belt.


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Matt L. -- Cary, NC -- '91 740 wagon