I'm not talking "a couple of tenths"; I've seen +/- variations well beyond that. And, yes, the bit does need to be SLIGHTLY smaller than the desired hole size.

How closely do you want to track your tune?

my large set is in a wood case. nice set that i bought from a friend. The small set i found on ebay. Pin Gauge your Holley main jets jets. You will be surprised at how different they can be in the same size.

calipers can be "tuned" by how they are used. some have hands that can cause variations just because of how the calipers are held. also, remember this. measuring tools are their most accurate when used at the temperature they were calibrated at. this also includes humidity. so now we need to get into the lab with your part and gauges/measuring devices, wait until both have "normalized", then measure. suuure we will....
my point is, you are getting reference information to allow you to make the best decision possible to use in your particular endeavor. this also apples to engine building as well as spacecraft work, although the work conditions are vastly different between the two.

Legit Holley jets are stamped after determining where they flow in comparison to a specified high/low range for a given jet #. As such, jets that both fall within the same #'s range can have a different hole size, yet still be rated the same.

Holley used to offer some "close limit" jets that were refined in their ratings even more. If you see a #602, that's a #60 that flowed in the middle 3rd of the acceptable range for that size. The BLP and MaxJets are rated by .001"-incremental size and are even more refined.

I bought some Jeg's house-brand jets recently to fill in a couple of sizes I was low on. One size measures slightly smaller compared to the Holley of the same #, but the Jeg's jets have an obvious bevel on the entrance to the hole that the Holley doesn't, so they very well could flow the same. On the other hand, one of the three sets of Jeg's jets isn't even sized consistently between the pair. The smaller one doesn't appear to have been machined all the way thru the hole, so I have zero faith they flow the same, much less that they are rated correctly as a Holley replacement. Don't think that I will be buying any more of those...

I need to fine tune and while tying to get my bleeds in order I have bleeds where you can't see the number or it has none.So I tried to use the pin drill set ti figure outfor instance the 20 #23 air bleeds and nothing I had fit properly in the orifice.Very frustrating to say the least!

A quick update on additional jets I purchased: I was disappointed w/ the Jeg's jets I mentioned buying above, so I bought some additional jets from eBarf which were advertised as Holley jets and used the Holley part # in the description:

"2 PACK HOLLEY CARBURETOR MAIN JETS SIZE 81 122-81 1/4"-32 4150 4160 4500" (Bolding is mine to emphasize their use of Holley name and Holley part #)

The jets that arrived are NOT Holley jets; they appear to be the same QFT/Proform/Jeg's jets that haven't measured consistently, which I wanted to replace with "real" Holley jets.

The full order hasn't arrived, but if -- as I suspect -- they're all "non-Holley" jets, I'm going to be in touch w/ the vendor about it.

Boldin the word size and the discription changes

plus if they fit your Holley carb they are Holley carb jets.I aint sayin its rite but it can be interpreted several ways.either way it still sucks.

Yep, and a customer almost hurt his 950+ BB Chevy dynoing one of my 1150's and when he needed just a bit more jet, he used the Jeg's and LOST power and went stupid lean only to find out the 100 Jeg's jet was WAY smaller that the Holley..............BLP to the front FCK the chinese copy cat crap..........I don't play craps..........

These days, that's a problem with parts for anything you can imagine. They use the words "for" "replacement" "compatible" "fits" etc. Look at the brand name in the description, pictures, and read the ad carefully. Price is the big give away. You can usually pick out scumbags selling this counterfeit trash. If it was sold as genuine Holley, that's fraud. Proceed as you see fit.

Yep...time to ditch all that other crap and convert to BLP jets. I did and I'd never go back to using the other stuff. In fact, I gave all the other jets away.