premium or cheap drill bit needed

I am going to drill my oil passages from crank to oil gallery. I will be very careful as to not break off a drill bit & trash the block & I am wondering if I bought a premium drill bit that I assume would be harder or a cheaper one that might flex a bit before snapping if that would increase my odds. thank you for your time.

My vote is for the best bit you can find. It will stay sharp longer and it will be designed better and make a cleaner hole
Gus

I like good drill bits, but in that application, you really aren't doing that much, I would probably buy the cheapest one I could find. You aren't drilling a hole you are enlarging it. And cast iron cuts like butter.

Use a reamer. They are not that expensive in the smaller sizes and they do a better job, leaving a smoother surface inside the hole.

I have used this one: https://www.amazon.com/gp/product/B00DMAUAPA/ref=ppx_yo_dt_b_asin_title_o07_s02?ie=UTF8&psc=1
But it is a bit on the short side, and I held it in a drill extension, but worked good for $12

If you can get a longer one it would make the job really easy.

Use the reamer at low speed and not a lot of pressure and pull it out often to let it clean, don't get in a hurry !

I was able to get aircraft bits (partial flute, long reach) at my local hardware store. They only carried 1/4 and 5/16" They required a lot of "pecking" to keep the chips from clogging the flutes.
Cleveland twist drill carries the 9/32' that McCandless suggested in the MP engine book and they have fully fluted bits.Even with the full flute don't try to get it all in one stroke.
https://www.cutting-tool-supply.com/ClevelandTwistDrill/
Be sure to use the correct speed, it's a pretty splindly bit. The calculations work out at approx 1000 rpm.

this pristine fresh block which looks like gold was machined ten ways from sunday & with a $1700 machine shop bill I cannot afford a fatal mistake at this stage of the game. Thanks guys.

use a reamer only ! it won't grab and snap like a drill will.
i don't know what size hole you have to start with, but a reamer is designed to only enlarge around .010 [ten thousandths] above the drilled hole size. you will more than likely, have to buy a couple of them to arrive at your desired hole size. use plenty of lube, relieve the hole often to clear the chips, and take your time.
reamers come in extended reach in virtually all sizes, so you don't need to settle for a shorty.
try mcmaster-carr for your needs, or possibly the bag.
the only thing to watch for before you start, use something like a coat hanger or welding rod, and run through the hole to make sure there is nothing obstructing the hole. i have see all kinds of "stuff" [especially in cast holes] hiding in long holes that still allowed the hole to function as designed, although not at 100%, such as chunks of broken tooling and casting boogers. both of which can shatter quality tooling upon contact. you shouldn't find anything, but you never know.

Use a reamer as mentioned. A 6” chucking reamer will barely break thru, but it will work fine. Just chuck it at the end of the chuck mostly. Also, use an air drill. It gives you better control, and is less torquey than electric or even a cordless. You’ll also appreciate the smaller drill, when you team #5.You can also help by just chamfering the edge of the hole, either with a file or bit to help reamer engagement. Ream with steady pressure. Don’t horse it, but keep it steady, and keep pulling the reamer back to clean the flutes. You don’t need lube, the cast iron doesn’t need it. When done, put a 1/2” cartridge roll on an extended shank, and go in and touch all the inside of the galleys. Reaming WILL leave little shards of metal, and you must clean them off.

This!
When you clean the reamer , DO NOT use a rag in your bare hand, the reamer will slice through it like it is not even there. Ask me how I know, DOH!
use a stiff bristle brush to clear the chips. Paul

Sometimes I think guys overthink some of these jobs. I will be grabbing my long drill bit out of my engine building box and doing two blocks this week. Like I’ve done many times before. Spray drill, spray drill, repeat. Job done.

When it comes to buying an extra long bit like the one required to do the job, you're not going to have much choice between "cheap" or "premium". FWIW, the one I bought back in 1974 has served me well and is still in fine shape to do many more blocks. But I should note that I have never used that bit for anything else.

The 6" reamer works fine on the #1 through #4 mains, but the drill chuck gets in the way of the block at the rear seal retainer area, so that is where a drill bit extension comes in handy to clear the block wall at the #5 main.
If you are a decent welder, you can also weld a rod to the reamer to make it longer and not have to spend $50+ on an extended length reamer.

Drilling works too, done it several times and only broke one drill bit, but after using the reamer, it just seems to do the job easier and quicker, with a cleaner finish, less burrs and such where it breaks through, and smoother walls inside the reamed hole.

By an extra long reamer and keep it in the original case in your tool box and it should last a long time. You can get them for around $30 from a tool supply house or order one on line.

Alright, it sounds like reaming is the way to go, I just hope I dont have to buy several depending on how much material is needed to get to 9/32 but I will if need be. I just dont feel lucky on this operation so I am leery of a drill bit tho I have done it that way in the past. thanks guys for the excellent feedback.