the 425CI, V8. yeah, it has a few more cudes than most small blocks, but i was reading that its based on the olds 350, just bored out, with a much taller crank shaft, and a higher deck. it also has valve covers that are the size of a chevy 350's, just with more bolts. so i would think small block, but i don't know if it has big or small crank journals. (what determinans weather or not its a big block)

EDIT: it has been determined that i got bad info, please forgive the dumb. (me) thanks though.

 

The 425 came out before the 350. The 425 and 350 have a diffrent cam bank angle . Olds valve covers are not the same as sbc 350's and sbo's and bbo's valve covers are same size. The 425 is considered a big block . If you where to place an sbo and a bbo to the untrained eye they would look almost identical. Olds engines arent like chevys where the diffrence between big blocks and small blocks can be told by the pig nose on the heads or valve cover sizes. The 425 has a 3.975 troke and i belive a 7 in. rod. The 350 has a 3.385 stroke and a 6 in. rod

 

Yes, the 425 came out in 1965 as a replacement for the 394 in the large Oldsmobiles while the 330 lasted through 1967 as the V8 offered in the Cutlass and low-end Jetstar 88 and then Delmont 88 lines. The 425, like the 455 that replaced it, is most definitely a big-block engine. It was offered through 1967. Both the 350 and the 455 were first offered in 1968.


http://en.wikipedia.org/wiki/Oldsmobile_V8_engine

 

So, it came out earlier, has a different bore, stroke, main journal and rod journal diameter, different deck height, different head volume, it is "Based" on a 350?? Makes zero sense.

 

I think his info resource may have been full of bad info considering the fact the 350 came out after the 425. Which is why we provided info to educated and not question the confused.

 

425 is based on the 330 engine architecture that came out in 64, just adapted for the larger displacement, and the 350 is based on the 330. That may be where the confusion is coming from.

Rest assured the 425 in your Ninety Eight is a BBO, and arguably one of the best BB engines there is. Big displacement, short stroke. Tweak it for its inherent oiling problems and you have a badass bulletproof engine. Even the oiling problems are no biggie unless you wanna consistently turn it up past 6k RPM or so.

Rule of thumb for 64-later engines:

260, 307, 330, 350, 403 Olds engines are small-blocks.

400, 425, 455 are big blocks.

How is a 403 a SB and a 400 a BB? The 403 is based on the windowed web 350 block and has large siamesed cylinder bores to get the displacement. Unfortunately the siamesed bores make it prone to overheating if overbored appreciably.

 

JEEZ, you know your stuff on engines... lol. well thanks for clearing this up, yeah, info anywhere other then forums is hard to find on some of these engines. so, dumb question, you said "425 is based on the 330 engine architecture" but the 330 is a small block, and 425 is a big block, can you just clear that up for me? i know, dumb question, but still.

 

What that means is they designed a taller deck (big) block that could use the same machine tooling as the short deck (small) block so it would be cheaper to develop.

 

I do have a 425 SB, but I really had to work for that one........LOL.

 

Fwiw the 425 shares the same pin height as the small blocks. So you can stick 425 pistons in a 350 . I think the 400 may as well since I have heard guys sticking 350 pistons in the 400.

 

I wonder if the guy got the 425 and the 403 mixed up. Isn't the 403 a punched out 350? I'm not sure just checking.

Larry

 

The reality is that ALL 1964-1990 Olds motors (except the 394) share a common architecture. For convenience, the short deck motors (260, 307, 330, 350, 403) are defined as small blocks and the tall deck motors (400 short stroke, 400 long stroke, 425, 455) are defined as big blocks. In reality this is more like the difference between a Chrysler 383 (B-block) and 440 (RB-block), but the BBO/SBO definitions have been pretty much in place since the 1960s.