Photo tach and cranking RPM
 

Harbor Freight has one for $39. I didn't feel like spending that much and found one on ebay for $12. It works by measuring the reflection of a laser beam off of a reflective tape that you glue onto whatever you are measuring. I put a square of the tape on the harmonic balancer and my idle was measured at 690 RPM. I also tried to measure the cranking RPM by disconnecting the glow plugs and cranking on a cold engine. It was hard to get a reading but I was able to get it just one time and it was 250 RPM. Is that too slow? There's a tip in the instructions for measuring low RPMs by putting 2 squares of the reflective tape 180 degree apart and dividing the result by 2. I am going to try that next.

Would you mind posting a link of what you purchased? I would like to know the idle speed on my NA 617.

Anyone know the spec for starter cranking RPM?

I actually paid $0.01 for the photo tach . The rest was shipping.

https://m.ebay.com/Member/G/ViewItem.aspx?aid=150493661192&emvcc=0