They normally don't need shims.
On the Housing opposite of the Solenoid, it will say "NO SHIM".
Some early ones did require shims, 2mm according to the shop I just bought mine from.
I had to put 0.5mm of shim in my new one because it would hang or not fully engage.
I was measuring the gap between the ring gear and the starter shaft, but failed to assure the ring gear tooth lined up with the shaft. On gassers, this dimension is 1/8". I have not crawled back under to remeasure the gap on the 6.5.
With the initial measurement I made before shimming and after shimming, I added 1/64" of gap. I blame the need to add some shims to the rebuilt housing and sand blasting or bushing alignment during a replacement.
If the solenoid is not pulling in, shim until it does. I believe the 1/8" gap is correct but need to verify it on our motor. Grab your drill index and check it out.
This was my post that I was working on when I realized my measurement error:
We learned about 6.5 Starters today
Symptoms:
Slow Crank
when hot, cranking was slower
Troubleshooting:
Check cables, condition good
Check Terminal, clean no signs of any corrosion
Tested batteries, tested good on electronic battery tester
Stopped in to talk with a local auto electric shop. He suspected brushes or bushings were worn out. Either could cause the starter to slow down.
Had him build one up, he was cheaper than the auto parts stores.
After installation, Crank speed was great. But I noticed the bendix did not seem to retract properly. It didn't sound right. Like it was hanging in the ring gear.
Went back to the shop with the core and for him to listen to the noise. Later in the day, the starter did not fully enage until the key was hit twice.
We learned there are two nose covers for our starters.
The early version required 2mm of shim.
The later version had the words "No Shims" cast into the nose piece. (On the solenoid mount, trans side) The dimension from the mounting surface to the shaft center is 49mm on the later.
The nose housing was verified to be a "No Shim" piece.
We also learned that the clearance between the ring gear tooth and the starter shaft is different that the 1/8" clearance for chevy gas motor starters. I measured at least 7/32".
After adding 2 1/4mm shims, the clearance was 15/64", but less than 1/4".
The starter seems quieter. I will monitor.
So I'll crawl under the truck soon and check it out. Its rained the last couple times I had time to do it.
Hope this helps.