i did a search on these blocks but did not find what i was looking for. 1st: what is the differance between these two blocks? and which one is better for building a 496 or 502?? 2nd:is this block better than a gen 4 block? 3rd: and last question, which would be the best move buy the gen 5 or 6 and build it__________OR have a 2 bolt main 454 converted over to a 4 bolt.? the price for the gen 5 and 6 is $400. thanks for your answers in advance...

Internal parts for the old Mark 4 block are fairly plentiful and inexpensive so i'd stick with a used "MK4" over the newer block. All the late model blocks have different cooling passages that aren't compatable with the older stuff. A two bolt block block is stronger than a four bolt except for the main caps. It can easily stand up to all the abuse you can throw at it and shouldn't need any more than the two bolts it came with, it's a waste of money. We had one that ran low 10's. in a '70 Chevelle at 3550 lbs. and never has had any block problems.

Hi Harold.
How is a 2 bolt block stronger than a 4; they are the same block are they not?

I can appreciate the caps being stronger in a 4 bolt but also wnder about the mass that was removed to put the second bolts in.

Hi Harold.
How is a 2 bolt block stronger than a 4; they are the same block are they not?

I can appreciate the caps being stronger in a 4 bolt but also wnder about the mass that was removed to put the second bolts in. When the blocks are drilled they become a little weaker in the bulkheads where the holes for the extra bolts go. I've never seen any rat motor have bottom end trouble. The valvetrain though is a completely different story. The factory "bowtie blocks" have the main caps splayed and the main bearing oiling is feed from up near the camshaft tunnel. In the production block it's down along the pan rail and is near the outer main cap bolts on a four bolt block.

lemonice,
I built a Gen VI 496 from the ground up and it was a piece of cake. Unfortunatley people hear this and that about a certain block, head, etc and believe whatever they hear to be true WITHOUT researching it. I also have a book that deals strictly with the Gen V and VI blocks.

While it is true that the Gen 5 and 6 blocks have different cooling passages than the mark 4, only the Gen 5 is limited in the heads it can use. The gen VI (also known as the Mark "fix" at GM), improved many of the design flaws in the Gen V. The Gen VI will take ANY cylinder head (mark 4, gen V, Gen VI, and most if not all aftermarket heads that a Mark 4 will). I have Brodix Race Rites on mine without any problems. There are only a few differences in the Gen VI and the Mark 4 and I think they are all for the better. First is they are all 4 bolt blocks. Second, they already have taller lifter bosses to use roller lifters (factory or aftermarket), and come with a cam retaining plate to prevent cam walk instead of a nylon button to keep it in place. Third, the blocks come with oil cooler bosses/passages already machined. I don't use an external cooler, so I just plugged mine. Fourth, the Gen VI has a better oiling system internally than the Mark 4. Unlike the Gen V (which I would avoid), the Gen VI also has a mechanical fuel pump boss and bosses for manual clutch Z-bar. The Gen VI uses a 6 bolt timing cover instaed of the traditional 10 bolt one. Also uses a one piece rear main seal and a one piece oil pan gasket-I've never had a leak from my oil pan! Thats about all the differences from the mark 4. there is a book titled "How to rebuild the Gen V and Gen VI engines to stock specs" by Mike Mavrigian that is very helpful with these motors. There is also some sections on performance as well. Like I said before, this was my first complete build from the bare block up and I found it to be a lot easier than I thought. If you already have a 2 bolt 454 and all the parts, then it might be easier and less money just to convert to a 4 bolt block. If you are starting from scratch, then I would go with a Gen VI. If you have specific questions feel free to PM me. Good luck

The on-piece seal is a neat deal - you can change the seal w/o dropping the oil pan (if you had to change it). Just remove flywheel, remove old and tap a new one in.

ok. so its run my 2-bolt with a good steel crank, good h-beam rods,and forged pistons. looking to make 700 hp with this combo.

My guy tells me that the Gen stuff is a bit thinner on the cylinder walls and decks. That makes it tougher to sleeve but as long as you're not going over about .060 over, they're fine. I'd use the 2-bolt, install main studs and have it align honed. That should do it.
Get in touch with Mike Lewis (Wolfplace). He's given a lot of members smokin' deals and runs an honest business. He can take care of the balancing as well and seems to be very picky about the way he does things. Good luck, start buying stock in your favorite tire company - you'll need it!

Keep in mind, the GenVI timing set (c/w hardened retainer plate) can be used in a Mrk IV, all you do is rotate the retainer 90* [I have a GenVI cam in my '88 MK IV block right now]. Main oil passage is definitely better in the GenVI. Cylinders are thinner though. For GenVI, only the 454 production block has a fuel pump boss. One piece real main seal on the GenVI is a little more foolproof, although can't really say I ever had a 2 pc. main leak.

A gen 6 block for $400 seems like a great deal - here is pricing for a new one from GM (Scoggins Dickey)

12561353GM Performance PartsBB Chevrolet 454 Bare Block, Gen VI $1,099.95
10237292GM Performance PartsBB Chevrolet 502 Bare Block, Gen VI $1,629.95