I have Synchro alu cranks that I really like, and that I'd like to install on my recumbent. However, they're 170mm and I need shorter cranks on the bent. I'm thinking of converting the cranks to 150mm by drilling and tapping the new hole to 9/16" x 20 tpi. That would leave about 5.7mm of alu between the edges of the 170mm hole and the 150mm hole. Since I wouldn't honk on the recumbent, I reckon that'd be enough metal left. What do you think? Has anybody done something like that, or should I get new cranks instead?

5.7mm is probably enough, but I think new cranks are a much better idea. How do you know you'll like them as 150mm?

I know because I always ride 150mm cranks on recumbents The reason I need a new crankset is because I cannibalized my bent to fit the 150mm cranks on my retrodirect bike. Since I have these very nice 170mm Syncro cranks that never failed me from my last mountain bike, I figured I could reuse them instead of having to hunt down a 150mm crankset and spend extra cash on yet another bike part.

I finally found time to make shortened cranks and install them on the bent, readjust the boom, lengthen the chain and change the front derailleur cable. I ended up reusing an old pair of wide 175mm Suntour MTB cranks: I drilled 13mm holes (9/16x20tpi == 13.02mm inner bore. How lucky ), threaded them with standard R and L pedal hole taps from Park Tool, then faced the outer side of the holes with a 19mm milling bit and filed the inner side flat.

The new holes are 13.5mm deep, as opposed to 15mm for the 175mm original holes, due to the fancy triangular section I had to mill through to create a flat spot, but the pedals don't screw all the way through anyway, so I reckon they'll be just fine.

Here's what the cranks look like:

Looks like a good job!
Could you explain what is the proablem when you use a 170mm crank length?

Knee pains essentially. I run 150mm cranks on all my bikes because my knees are trashed. For recumbents specifically, it's quite well known that short cranks are almost always a good idea: they give better performances (you spin much more easily) and they save your knees even more than on an upright. Many riders who have no trouble on an upright but experience knee pains on a bent solve their problem with shorter cranks. Many recumbentists without problems who switch to shorter cranks go faster and climb better as well. Personally, beside saving my knees, short cranks give me 2kph on the flat, and allow me to continue spinning comfortably above 50kph going downhill with a 44/11 combo.

This time I went with the "luxury" option (i.e. homebrew 150mm cranks with a spider that can take my favorite chainring sizes) instead of a crummy child crankset with riveted steel rings. I think I'll do that with the other bikes too. Too bad it snows today, I'm dying to go ride

Bummer on the knees.
I have a tired looking Dura Ace crankset 7400 series with the flat arms would be perfect for this, I might even have the taps. Just another winter project.

Actually you might not want to do this: IIRC, Dura-Ace 74xx cranks are flared around the pedal hole and narrow along the arm, which means there won't be enough metal around the new hole. You want wide, as well as thick crankarms. Flat isn't really a problem as long as the thickness you get after spot-facing the hole with a 19mm miller is sufficient. In my case, the cranks have a V profile, and after facing the area around the new hole completely, it ends up 1.5mm under the original one.

You also want to avoid modifying those new hollowtech arms, as you might end up drilling into the hollow core.