The Stovebolt Site Forums The Shop Area Engine Shop 2-bolt vs. 4-bolt 350

Hi guys,
Pardon my ignorance, but I'm going to be shopping around for a rebuilt 350 for my '71 GMC 3/4T 4wd. How can I tell if it is a 2-bolt vs. a 4-bolt main? It isn't original, has 6 studs holding exhaust manifold, dipstick on drivers side (if that makes any difference???) Thanks,
Ray

The only difinitive way to tell is to pull the oil pan and visually confirm. The engine code stamped on the land in front of the right side cylinder bank supposedly can be used to tell but there are many blocks that have the same codes or even casting numbers that can go both ways.

Go find out what the numbers/letters are stamped on that land and also get the casting number which is on the back left side of the block. Let us know what those are and we may be able to pin it down....maybe.

Since it is the Christmas Season someone will probably take mercy on you and give you an answer but for the record we are pretty much six banger guys on this board. Soooo many places to get V8 info.

Merry Christmas.

Hy guys, I thought we were a pre 73 Chev. and G.M.C. sight since V8 engines have been around since the 55B series I have no problem answering a question concerning them at any time of the year.

I'll try to get the engine code/casting # in the daylight tomorrow. Is a 4-bolt better than 2-bolt? If they are basically the same, should I care which one I get? Or, are there differences in manifolds/pan bolt patterns? Thanks,
Ray

Incidentally, I like the 6's too, but this truck came with the v8. I started my car career with a '40 Chevy sedan w/216, then got a beat '42 also w/216, then a '61 Biscayne w/235, a 66 1/2T GMC w/305 v6. Only 6 I still have is a '52 GMC 2-1/2T w/302. I'd love to get it going again, but the $ for restoring the big bolt and insurance just about kills my interest. The '71 is my regular wood/garbage/heavy object hauler. It came from the factory w/350 and it seems silly to consider changing it to anything else. Just my $0.02. :>)
Thanks, Ray

hey no reason to defend your choice. If it is like that now, I wouldnt change it either. Sorry I cant help you with the # bolt main part.

Unless you are going for serious horse power, it wouldn't matter if you went with a 2 or 4 bolt main. Generally, if it came from the factory with a 4 barrel carb, chances are it has a 4 bolt, when dealing with V-8's from the 60's and 70's. That's not always the case but it's a good way to weed out the 2 bolts. And feel free to ask about V-8's whenever you need to. I'll answer any questions that I can,
Scott

I'm with Scott, the 350 was sturdy in either configuration. They all had good forged con rods and lots of meterial in the main bearing caps. In my junkyard days in the 70's, I never saw a factory 2bbl 350 with 4bolt mains, but 4bbl 350s came in both 2 & 4 bolt flavors.
Mike

it isn't too hard to pop off th oil pan... and then you can know for sure. other than that you are just guessing...but if you are gonna go to all the trouble of the swap then it isn't much work to look at the bearing caps. i agree about the v-8 thing...they came both ways and both have their charm and strong points. good luck.

By the way,there's a one piece oil pan gasket available for the smallblock,and it's the ONLY way to go when you drop the pan in the truck.(or anyplace else,IMHO) Just a little more costly than a brand name multi piece set,but a ton easier to install while working inverted,and it WORKS-no leaks!! Follow the directions to the letter and you can forget about that dribble pan in the driveway.
You're welcome. Merry Christmas!
Speed

A lot of ways to tell.If there is a visible 1/8 in.pipe plug behind the water pump.it is a 4 bolt main.pretty easy to spot while looking in from the front of the vehicle or engine.To the best of my knowledge all truck engines were 4 bolt mains.A lot of people think the 4 bolts are stronger,and this is not nescisarilly true.The 4 bolts use four small bolts in each main,while the 2 bolts use 2 larger bolts.So there is really not much difference in strength.

rpg 52.There is a book that is very informative on the small block Chevy engines.The name of the book is Chevrolet Small-Block V/8 ID Guide,by Pierre Lafontaine.List price is $19.95.You can probably get it from Amazon .com a little cheaper.It is a very valuable tool when trying to figure out the small blocks.I use mine frequently.

Thanks so much for all the info guys, I'll put it all to good use. Have a great Christmas.
Ray

If the boss at the top of the rear of the engine where the bell housing mounts is drilled & tapped it usually is a 4 bolt main engine.
Early V8s had stove bolts too.

Go to www.mortec.com and www.nastyz28.com/sbchevy/sblock.html for engine numbers. Pickups got both 2 and 4 bolt mains. Early on the 4 bolts went in bigger trucks, where they were needed due to the high loads incurred at low rpm.

There is actually a growing group of rodders using the 2 bolt engines for performance. I haven't seen the stats, but the 2 bolt is supposed to be superior for a high performance street engine. My buddy is building a 2 bolt 400 inch with around 500 ponies.

I personally have never broken a 2-bolt small block, but have a 4-bolt.

I know it goes against the conventional wisdom, but I'd be inclined to choose a 2-bolt block unless I was really going to ultimate horsepower.

As for telling them apart, I'd drop the pan.

check the number on the block in front of right head it will tell you what you want to know.If it is the original engine it is a 4 bolt as all 4x4s with 350 4 bbl carb were 4 bolt blocks from the factory

I know some 4bbl 350s weren't 4-bolt blocks. Most L48 Corvette engines are 2-bolt. L82s were 4-bolt.

All factory 4x4 350s were 4 bolt blocks.they are no better than 2 bolt for everyday use if your going all out racing use the 4 bolt otherwise not worth the extra $$$$$$$$$$$