i have a fiero with a 2.5 in it and i have a extra engine and i heard that you can bore them out and put 350 pistons in it instead of 305 i was wondering if this was true if the sleeve was thick enough to bore that far any responses would be great thanks

With a stock duke block prob not I can see it cracking.

[This message has been edited by linuxpowered88 (edited 07-05-2008).]

quote Originally posted by linuxpowered88: With a stock duke block prob not I can see it cracking.

Ummm, not likely to crack from a 0.000" overbore:

Stock 2.5 Pontiac bore = 4.0"
Stock 305 bore = 3.746"
Stock 350 bore = 4.0"

350 Chevy and 2.5 pontiac both use the same diameter wrist pins and have near identical compression heights (350 pistons will sit about .030" farther down in the bore) but they will usually have less dish so you can get slightly more compression.

oh sorry didnt look up the bore size thats useful to know thanks

Most people don't know it, but the 2.5 has a lot in common with the stupid '77-'81 Pontiac 301 V8. But getting +.030" oversize pistons for a 2.5 shouldn't be any issue at all. The real concerns are (1) the block, as they all have that weak spot where the specs say don't torque that one head bolt as much as all the rest, and (2) the head. Some mid-'80s-on heads used a restrictive high-swirl intake port, and if you want performance, and you're using a manual trans, get the non-swirl head and get it ported, with larger intake valves. My '84 2.5 Fiero failed the block and head when the belt let go at 4000 rpm, and overheated it all. I'm driving an '89 S-10 with the same engine, and I feel the only reason it's still alive is the serpentine belt.

The Duke block has thin cylinder walls and should not be bored. I believe that some type of small block piston fits the Duke but you can try and get ahold of the stickman and ask him. He did a performance Duke build a while back and has alot of experience with these engines and with the SD4.

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87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by Dennis LaGrua: I believe that some type of small block piston fits the Duke

You can use either the small block 301 Pontiac pistons (I've got two +.030 sets in my garage right now actually) or SBC pistons have been used.
The one thing about doing this is that you should always check valve clearances when eliminating piston dish and putting a bigger cam in.

The problem with SBC pistons is (1) the valve reliefs don't match any 2.5 head, and (2) the measurement from the center of the wrist pin to the combustion surface is different. Also, the wristpin dia. is probably different, thus you end up using aftermarket connecting rods. Or maybe bushing the weak 2.5 rods. I don't have any useful numbers handy. You can safely bore the 2.5 to 4.030", the thinness is elsewhere. If you want serious power, you could tip the bare block up and put block filler such as Moroso's Hard Blok in the water jacket, near the head bolt hole that gets less torque. But do this before boring the cylinders, Which you will have to do. You can do it yourself IF you properly torque the main caps before starting, IF the block is cleaned enough, and IF you immediately torque the head on correctly, with a new gasket, as soon as the filler is in. Even so, after 24 hours you'll still need to get the cylinders bored. I've done 3 blocks so far, with good results. Nevermind that I work as an automotive machinist.

quote Originally posted by Isolde: The problem with SBC pistons is (1) the valve reliefs don't match any 2.5 head

quote Originally posted by KurtAKX: The one thing about doing this is that you should always check valve clearances when eliminating piston dish and putting a bigger cam in.

People have successfully used 350 pistons before, so I'm sure the valve relief thing isn't insurmountable.

quote Originally posted by Isolde: (2) the measurement from the center of the wrist pin to the combustion surface is different.

Yes, but its only about .030" different and the 350 piston sits lower in the block. This partially offsets the lack of piston dish in the 350 pistons as far as compression.

quote Originally posted by Isolde:Also, the wristpin dia. is probably different, thus you end up using aftermarket connecting rods.

2.5 piston wristpins are .927", just like a SBC.

quote Originally posted by Isolde: If you want serious power, you could tip the bare block up and put block filler such as Moroso's Hard Blok in the water jacket, near the head bolt hole that gets less torque. But do this before boring the cylinders, Which you will have to do. You can do it yourself IF you properly torque the main caps before starting, IF the block is cleaned enough, and IF you immediately torque the head on correctly, with a new gasket, as soon as the filler is in. Even so, after 24 hours you'll still need to get the cylinders bored. I've done 3 blocks so far, with good results. Nevermind that I work as an automotive machinist.

I'm glad you clarified the order of operations on that process. I'm filling my block (the first time I've ever filled a block) and this will certainly help.

The block on my duke is bored and it has no problems at all, running, reving, or cooling....

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Tristan
84 Fiero SE

quote Originally posted by Isolde: Most people don't know it, but the 2.5 has a lot in common with the stupid '77-'81 Pontiac 301 V8. But getting +.030" oversize pistons for a 2.5 shouldn't be any issue at all. The real concerns are (1) the block, as they all have that weak spot where the specs say don't torque that one head bolt as much as all the rest, and (2) the head. Some mid-'80s-on heads used a restrictive high-swirl intake port, and if you want performance, and you're using a manual trans, get the non-swirl head and get it ported, with larger intake valves. My '84 2.5 Fiero failed the block and head when the belt let go at 4000 rpm, and overheated it all. I'm driving an '89 S-10 with the same engine, and I feel the only reason it's still alive is the serpentine belt.

You seem to be alot more knowledgible, so do correct me if I'm wrong, but in 84 when GM went with the swirl-port head the compression ration was able to be raised from 8.2:1 to 9:1 which increased power. Restriction of the head is also a nonissue anyway since the cam and intake manifold are the restrictions, the engine is cammed for low RPM, and the exhaust ports for our engines are poorly designed.

When it comes to things like original factory compression ratios and HP ratings, I don't give them any thought, because I know what I want and how to get it. Or at least who would know what I don't. First, the pistons. Unlike a 301 pontiac or a 2.5, getting forged pistons for a 350 is a non-issue. Most 350 pistons are 1.56", but beware, as some are as short as 1.54", which would have them .050"in the hole. You ideally want your pistons to be around .040" from the head, including the gasket. This would mean decking the block, and possibly custom pushrods. And there's plenty of styles od 350 pistons, including dish, flattop and dome. You can run 9:1 with the non-swirl head if you want. No problem. And unless you want to run a stock cam, that won't be a limiting factor. You also mentioned the restrictive exhause ports. With a bit of porting, which is hard for anyone to goof up on the exhaust side of most GM heads, that issue goes away as well. If you get a good port job with larger valves for both the intake and exhaust, get a decent cam, have your intake manifold extrude-honed, and run 9:1, assuming an adequate exhaust system, you should see 150 hp at 4800-5200 RPM. Or run 8.2:1, add turbocharging, and see 200. Or, for less money and no more hassle, convert to an Ecotec. With a cold air setup and a good muffler, you're over 150 HP, and you gain a world of potential.

Actually what I was refering to on the exhaust ports is not that they are restrictive, per se, its that they were poorly designed. Check it: https://www.fiero.nl/forum/Forum2/HTML/074087.html

Still not as bad as say a mopar 440, and even those respond to proper porting.