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I have a 2.5 powered Monza. My car did not run when I bought it (technically it did but 2-cylinders in my book doesn't count). This last weekend I started to disassemble the engine for removal and discovered a Quarter size hole burned through #2 piston. Now to the part where I ask questions... I have a 2.5 from a 1987 S-10 that is in really good shape. How much of this engine can I swap over to my Monza block? I know the crank is a no go since the flywheel bolt pattern is different. Can I use the rods, pistons, and roller valve train?

You would be better off swapping the entire engine, I am not aware of much that swaps over between the non cross flow (77-78) engines and the later cross flow (79 and later) engines. They had to redesign the Block to change the angle of the distributor when they put the intake manifold on the passenger side of the engine. I don't know if the oil pan would work with the later engine, or if there is any difference between the oil pan on the different types of Iron Dukes. Sorry.

I am not sure about the Rods and Pistons, Maybe but you would have to compare them. The valve train I would suspect is different due to the block differences. Pistons are probably differrnt due to valve placement and combustion chamber size differences. I would suspect only the rods would possibly swap,

My engine is a 1980 crossflow so I would assume the cam lobes would be in the same place. I could be wrong.
The main concern is the rods and pistons but swapping in a roller cam would be nice.

Sorry I missed that you had a cross flow engine. I think it would be easier to bolt your Intake and Carb to the 87 engine (and Possibly Oil pan and Pick up tube from the 1980 engine) that try and swap all the internal parts. I do not know if any modifications were made to the 2.5 engine blocks to install the roller cam. I will do some checking though.

after a bunch of research I can report that the rods and pistons will interchange but no information is available about the roller camshaft. I will report back when I've torn both engines down.

I finally got a chance to tear into my S10 engine. there are some interesting parts in there. the crank is identical to the fabled 88 heavy (still not an SD but the best production unit out there), the camshaft is a roller hydraulic unit with a retro fit lifter retainer system ( it looks like it will fit the earlier 80 block because it uses the side cover bolts to retain the lifter guides), rods and pistons will directly change over the upside is these will increase the compression on the earlier engine quite a bit probably get to 9.5:1 or more. I'm actually excited about building this engine even though it will probably be the least powerful I've ever built. I will post photos of the parts and verify the fitment of the roller lifters once I get the engine out of the monza. If these parts work it could spell a budget way of making a little more power out of a cross flow 2.5.

Interesting, looking forward to seeing the pictures.

The one issue with this swap is there is a gear where the fuel pump lobe should be so an electric pump will be required to run this setup.

Nobody mentioned it yet, but the Monza 2.5 shares it's bellhousing pattern with a normal Chevrolet V8, while the S-10 version of the block has the smaller "Metric" bellhousing pattern that it shares with the newer FWD V6's. So the path you're on to swap the internals into your block is correct.

I originally bought the Monza to be a commuter, I have a Shelby Dodge I was using for my autocross habit. Unfortunately No aftermarket struts are available for the Shelby and it destroyed its struts on its first outing SOOOOO... it looks like my commuter car is getting turned into a racecar. The reason I prefaced this post this way is I am considering higher power alternatives to the Duke. I have considered v8 and the OMC 3.0 options but they have inherent drawbacks. The v8 adds too much nose weight to the car making traction a challenge, the OMC is a good choice but the parts are not the easiest to find and repairs could get expensive. I am looking at V6 engines as my alternative. The shorter length will help the balance and a mildly built v6 will be reliable while providing good power. I plan to keep the same t200m transmission (rebuilt and beefed up of course). what I have in mind is a 90degree chevrolet v6. ive had a couple of 4.3 v6s and they are torquay engines they don't like to rev very high or quickly so I'm looking at the odd fire 229. the stronger crank means it can handle more rpm and the odd fire throws center the rods in the pistons.

I will build the roller high comp 2.5 and run it this season but I am looking to the future and I know more power is needed to take on CAM class.

Odd(and even fire) 229(3.8 L) and 196(3.2 L) V6's are Buick/Olds/Pontiac bellhousing. There are a few 3.8 V6's out there that are Chevrolet Bellhousing pattern; just a smaller version of the 4.3. i had an 80 Malibu with one in it. Slooooow.
Does you 200m have both bolt patterns? I know most 2004R's do.

The engine I'm looking at is a Chevy 229 not the Buick 231. Although I do know where there is a 1974 Vega with the 231 Buick that I could probably pick up for a decent price. My t200 is Chevy pattern only. I may go with a 4.3 if I have to.

My bad. You obviously know the difference between the V6's. Kind of confused me by referring to the Chevy 229 as an "odd-fire". I guess technicaly it is that, but that term is usually only applied to Buick ones.

Both offered odd fire cranks. The 4.3 was the first even fire ( pre-broken ) cranks. The biggest thing about the 229 is it uses
SBC connecting rods and 305 pistons. That combo should make adding compression easy.

I had no idea the 3.8 and 4.3 Chevy V-6's had different style crankshafts. According to Wikipedia, the 3.8 was "semi-even fire" with an 18 degree split between the pairs of crank pins, and the 4.3 was fully even fire with them split 30 degrees.
I guess you're never too old to learn something new every day. lol
I will admit that the 3.8 Malibu wagon I drove as a winter beater for a few years did sound strange; especially with dual exhaust. And it wasn't as smooth as the 3.8/T-50 powered 79 Sunfire wagon that came before it.
P.S.- My apologies for hijacking this thread; I forgot that it started out comparing the two generations of 2.5's.