there is no casting number that wasonly used on 4-bolt-main blocks. If you want to be sure, you'll have to pull the pan. But you may not need a 4-bolt block. And the 4-bolt blocks tend to have cracks in the main webbing area. If you're okay with under 450 HP and under 6000 rpm, a 2-bolt block is a good choice.

 

I'm not an expert in these matters but; there should be casting numbers in two places on the block:
on the passenger side - front and the driver's side - rear.
These will be "machined" surfaces, on the engine block, with some combination of letters and numbers stamped there. Also get the intake (located on the driver's side rear) and pop off a valve cover and get a head casting number, (both are around 8 digits long).
If you have a friend at your local GM parts dealer, (or if you don't - shame on you - make one) - give that person these numbers and ask them to look them up in their "Motors" book manual. The years of use, specifics (4 bolt main etc) and applications for that block will be found there.

Side Note
There are places online where you can look-up your numbers but unintentional deletions and mistakes could have been made when transferring the casting number info found in the original manuals to online websites. This happened to me with my 350 - 4 bolt 1993-94 4WD Suburban; the online info stated that my engine was a 1979 305 2 bolt main. I am the second owner - purchased in 1999 with 40,000 miles on the odometer, (now at 263,000 btw).

 

those numbers cannot tell you if it's a 4-bolt. Even if it can tell you it came from a 1-ton, there's still no guarantee. Plus popping the pan is easier, faster, cheaper and foolproof.

 

Thanks for all of your input. I was always under the impression that a 4 bolt main block was the better version. i figured that pulling the pan was the best way to tell. can anyone tell me why the 4 bolt main blocks have cracks in the web area? thanks again

 

Just found this while surfing

"Some of the Engine Suffix codes were used a number of times. For example suffix code "CMJ" was used in 1974 on a 350 and in 1980 on a 305.
You need to look at the block casting date and casting number to narrow it down. Do not just rely on the suffix code to identify the original use of the block because it can be misleading in some cases. You should always look at ALL of the casting identifiers to determine exactly what engine you have."

 

350 dude. if you get the engine, pickup a 1 piece pan seal. no sealant needed. reusable neoprene

 

I actually prefer the 2 piece rear main, you can change it with the engine in the car... No stock roller lifters though.

 

There are a few commonly held possibilities, and I hold that it's usually a combination of at least 2. So, in no particular order, they are: because the extra holes weaken the webbing (which is true); because the 4-bolt blocks get rebuilt to higher power levels more often (also true); because the vehicles they came in were more prone to detonation (also true).

 

ok thanks for the info. ill probibly get the engine it was recently overhauled back to stock so its not been built into a gas drinkin fool yet so i think its pretty safe. thanks again