I am looking to build an "Iron Duke" for my "Baja" and I of course am going to beat the snot outa it. There is alot of negativity out there about the lower end and I am asking for "confirmation". I am not talking about rods, just main caps breaking or the crank actually breaking. "Iron Duke" rods suck, period. I am asking for crank or mains failures here. No hearsay please, either it happened to you or you SAW it firsthand. If you have any pics that would be great also, a pic is worth a thousand words...

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carpe diem

My brothers friend ran his '88 out of oil and put a rod through the side of the block, knocking the balance assembly/oil pump loose and putting burrs on the gears on the crank that drive the pump. As far as I saw in tearing down the motor (I also was interested in the durability) the crank could be reused if the drive gear was deburred. It rotated perfectly in the main journals (suprise, the block was still straight even though holed). But remember this, the engine was never overreved, and the rod broke and holed the block at low speed.

My take is it will live well if not over stressed by RPM. Also remember this was an '88 motor with the extra mass in the crank that is inherent to the '87-88. My suggestion if you're running a balance assembly engine is to install a rev limiter set at 5K rpm (a driving instructor overrevved my '88 and took out the balance assembly in 7 miles on our local road course) deburr, shot peen, and cryo the stock rods and everything else will take care of itself.

One more note. In the yearly grandprix races we have at this same course there is a dad and son team (from Portland OR area?) that run fieros in a stock class and all they did was change to the '87-88 crank used in conjunction with a conventional oil pump, stovebolt 6 rods (I think) and they spin 6k+ rpm every race with no problems other than a little oil consumption in the corners. FYI

I was thinking of using the 88 crank as a matter of fact. I like the added mass of it. So the score so far is rods 1, crank zero.

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carpe diem

I dont see the point of adding the extra mass over a non-weighted crank. You are still going to get a crazy vibaration between journals either way. The weight really isnt a substitute for piston counterweights. Thus said, I cant see an advantage of using the 88 over, lets say, and 85 crank, except you are effectively adding more rotational mass and your acceleration will suffer slightly. I'm not sure about the later dukes, but the 84 loses lots of torque above 4000 RPM even without a catalytic converter.

Then again, that is not to say the metal between the old and new designs are the same, which they very well could be different enough.

Here is a good racing balancer: http://www.fluidampr.com/PONTIAC.htm

If you want a bad duke, get a 70's Duke. At least they have piston counterweights.

[This message has been edited by AP2k (edited 05-14-2007).]

4 cylinder engines don't need counterweights...

At least not nearly as much as a V8 does.

[This message has been edited by Will (edited 05-14-2007).]

They at least need to be neuthrally balanced with as little distance between balance points. The Duke crank is roughly neutrally balanced, but its a very bad design to have counterweights on the ends and middle only; performance-wise that is.

Can that harmonic balance be used on a 85 duke crank? It doesn't give much info on the site, but that would definitely be a plus! Any more info about an aftermarket balancer would be great!

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carpe diem

I would think about the merc cruiser duke or one from a 90s grand am or other gm product ,gm used these untill 1993,they have a better induction system and have 15 more H.P. than a fiero

I am just putting my 85 Duke back together with an 87-88 crank as we speak. The 88 balance shaft stuff always blows up, abused or not.
The toughest parts seem to be the 87.

If you put the 87-88 crank next to the 82-86 crank you will clearly see why the extra mass is justified. The rod throws are hollow and therefore subject to more deflection when hit with the torsional load imposed due to firing events and bending moment from spinning the partially counterweighted crank at high RPM. When the crank deforms, the journals go oblong and bearings spin and rods chuck out of the block.

I have yet to see someone who has fit a mercruiser crank to the FWD block, btw. Mrossum is the only one I can find to have made the claim and I haven't seen any pictures or specs or any of that. I know that a lot of those cranks are for a two piece rear seal block and that all of them have the SBC pattern on the back of the crank.

Good luck!
Kurt

Edit to say I'm pretty sure that the Grand Am and other VIN U cranks don't fit.

[This message has been edited by KurtAKX (edited 05-14-2007).]

ATI makes a much better balancer for the Iron Duke.

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My Web page | The Turbo Super Duty Build.
You know that little voice that says it can't be done? I duct-taped its mouth shut and pushed it down a flight of stairs.

And its a rubber one?

quote Originally posted by KurtAKX: I am just putting my 85 Duke back together with an 87-88 crank as we speak. The 88 balance shaft stuff always blows up, abused or not. The toughest parts seem to be the 87. If you put the 87-88 crank next to the 82-86 crank you will clearly see why the extra mass is justified. The rod throws are hollow and therefore subject to more deflection when hit with the torsional load imposed due to firing events and bending moment from spinning the partially counterweighted crank at high RPM. When the crank deforms, the journals go oblong and bearings spin and rods chuck out of the block. I have yet to see someone who has fit a mercruiser crank to the FWD block, btw. Mrossum is the only one I can find to have made the claim and I haven't seen any pictures or specs or any of that. I know that a lot of those cranks are for a two piece rear seal block and that all of them have the SBC pattern on the back of the crank. Good luck! Kurt Edit to say I'm pretty sure that the Grand Am and other VIN U cranks don't fit.

A one piece rear seal mercruiser crank/rods/pistons assembly is sitting in the back of my blazer right now waiting for me to machine the rear seal area to 60 deg. specs. I also have a way figured to drill the smaller flywheel bolt pattern without having to add a big chunk of iron on the end of the crank. I am not putting it in a FWD block though. An S10 block modified to take the side saddle water pump will be my focus. The heavier main web attracted me. Now I just need access to a machine shop to let me get to work... The exciting part is SD head/manifold mated with the 3.0L crank. Should have some small increase in power.

I would still like to know about the original question though. Verification of a broken crank in a 2.5....

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carpe diem

I have broken an 84 crank just behind the #1 main. It happened at about 4200RPM while accelerating about 10 minutes after some 5000-5500 abuse. I have also chucked #2 and #3 rods on seperate occasions. I recently inherited an 84 parts engine that was in good shape save for a wrecked #3 main.

It is an inevitability for cracking any given reciprocating part in any engine. I take that back, with the rarity of those Duesenbergs, I would suspect that no cranks have broke in them.

On that 84 crank, did it have alot of miles on it? was the engine abused all of it's life? Any other particulars you can provide would be handy. I think that ALL of the 84 engine parts were pretty shoddy anyway....

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carpe diem

Bump for broken...

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carpe diem

Come on y'all, everytime anyone brings up a duke here, it gets flamed for having a weak bottom end. All I have gotten so far is one actual broken crank. Somebody? Anybody?

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carpe diem

I too have heard how weak the cranks are and yet have never actually met anyone who has said that and personally broken one. could be this is a fiero rumor that has just been passed around as gospal by everyone. You can abuse any engine past its normal operating RPM's and something is going to break. I took a ride in Fiero owners 4cyl which had forged pistons and beefier rods. It had a small Turbo and I was amazed at how much pull the little duke had. When he told me the crank was stock I was shocked that he had not broken it. He told me that he did not over rev the engine, ever, past what the crank and block was engineered for. He said he had rolled more than 10k miles on the motor with this set-up and had no problems. I forget what his boost was set to.

certainly there are more powerfull motors to turbo in a fiero, but i thought it was important to show how reliable these cranks are when they are not over revved. So far the only one broken in this thread was one which was admittedly over revved.

The 80's economy-style motors have LONG since stopped being produced. Hell, there are owners who are younger than their Fieros.

Not only are you asking the wrong demographic (not all that many Dukers here, much less performance-minded Dukers) but in the wrong time. If you asked this 10 or 20 years ago, people would remember freshly their engine breaking or a friend's breaking in their non-Fiero econobox.

I doubt another person will come forward and announce their broken engine because of those two issues. The same reason you dont have many reports of overworked Dukes is the same reason you dont see much genocide: people realized it was a bad idea because of people that did it beforehand.

th mercury duke is the same as the 79-80 chevy monza crossflow duke. The 77-78 non crossflow duke has a beafier crank and mine has been 6k on several occasions and ALL that I have had to do is replace the head gasket twice in 30k miles. If you get a merc duke or 77-80duke you can put a SBC head on it or if you have the cash to throw at it a 2.0 cosworth DOHC head. I have only seen it done once. Personally I am unaware of the 70's dukes breaking cranks as bad as the 81-current.

Yea, the older dukes can be built pretty powerful. Any info on that Cosworth head?

The cosworth head might be nice, but what about that crank? Will the 70's non-crossflow crank fit the duke? As far as the wrong demographic, look around, most people here have had a love for the fiero since it's inception. Also, many here ARE original owners of thier cars, bought off the showroom floor. I would like to know more about that turbo setup I just read about, I'd love to pick that dudes brains about his setup. As far as "the urban legend" about the cranks, I'm leaning heavily towards that point. As this thread is showing, it seems to be just that, just a nasty rumor. Rods poking through the block is what killed the fiero, not the crank. I believe when they did the recalls, all they changed WAS the rods. I will not let this thread die until I have at least 10 confirmed kills, we're not even close yet.....

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carpe diem

How far are you away from building your motor?

From the info that I've been reading, it wasnt so much that the the crank often broke, it was that it would flex at high RPMs due to poor counterweighting, and cause dmage to the bearings. Based on that, I'd be wary of winding it up for long periods or at high power. When I had my duke powered '86, I'd fairly regularly spin it up to 5k, but never above, and never for extended periods of time. She ran great when I sold her.

All that being said, I venture that you could probably get a decent about of power out of the duke (exactly how much I don't know) without it breaking, provided you kept the revs down, most likely to a limit below the stock redline. That is pure conjecture though, considering I've never done it.

I gotta say though, I kinda liked the Duke too, and maybe someday I'll get the opportunity to try to build one, at least a little beefier anyhow

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'87 GT, 5 Speed, Loaded. Resto Project

'04 Impulse Blue GTO Auto (Daily Driver)

I'm gathering parts right now for my engine. S-10 block and head, 350 flat top pistons (don't know how much over they are, they are out of an old racecar motor, haven't measured yet), working on finding I-beam rods, still searching for the right water pump, might have to re-plumb though. Why do you ask how far along I am? Do you have an idea for something? As far as the RPM's, I am not looking for them. The "Baja" fiero needs more mid-range peak. Plus I decided to go with a auto instead of a stick, of course it will have a shift kit and a slap shifter (still working on the design for that one). Yes, RPM's can kill anything, engine load at mid-range is what I seek, HP@3500-4000. Still after all of these posts though, only one broken crank.....

65 year olds here, listen up the rods in some dukes are probably the originals,the crank is outstanding for a fiero driven normallyi remember many fieros breaking cranks in the years when they were still late models,broken cranks are NOT a rumor many young people purchased fieros especially after they were a few years old and the price came down,,one reason there were more broken rods was the rods broke before the crank!!.I personaly saw some of these fieros at the junk yard...back in the early 90s there were many turbo dukes,some put hi strength rods in this is where the broken crank rumor started,I never personaly saw one of the trashed cranks from the turbos soon there were many fieros in the junk yard!! "I had a G.T. duke " I now have a 88 notch duke, the duke will deliver most excellent M P G and is fast enough in day to day driving, gas may go to $4.oo a gallon, there is a site for duke performancewith crank info but cannot remember the address,anyone know

So what I am gathering is that turbo's+duke=bad. That's cool, turbo's are really only good for rpm's over my limit anyway. Like I said, I want MAX HP@3500-4000 RPM, and all the torque I can get. Another thing I am realizing from this post is that the block webbing or main caps have never come into play as being weak, yet the whole bottom end has REPEATEDLY been touted as being weak. I have had a 6.5L (diesel), 5.7 and a 4.3 in the shop with broken cranks in the last year and a half. Do they ALL have weak cranks too? Bottom line is, I want to build a bulletproof 4-cyl, duke preferably. I was going to have main caps made with splayed bolts, and search for a full race crank, because of all of the rumors. If I cannot confirm these rumors though, how true are they and how much could I save? I am all about research, it can make or break a project, so any information I can get on this topic will end up in the final design of "Baja" fiero. Thank you all for all of your input thus far, this is a great resource with alot of good people. If I offend, I apologize, but alas, I am only after the "truth".

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carpe diem

Bueller....Bueller....

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carpe diem

Here's what many have found after attempting to build up a Duke and learning the hard way:

They can be built up to 120 HP and still be reliable. Go past that and you're a short drive away from a broken engine.

It's been proven time and time again...

Whether the Duke crank can be broken is solely an issue of how much compression is involved. Dukes or versions of them have been a backbone of GM econo cars all over the world. Some of these little 4-bangers go unbelievable distance and yes, if you push up the compression you can break them. I think the question can be answered 2 ways.

1. If you are building a reliable commuter or get-about, the Duke will take a licking and keep on ticking
2. If you want to apply 18 lbs of boost - forget it. You'll grenade the engine.

Hope this helps your decision.

Arn

Thank you for your replies, but I am taking the "Mythbusters" role on this. I realize NO engine is indesructable, but come on people, only ONE confirmed KILL!!! I am all about data here. This thread has already been refered to in another post, I can't give up now. All I am asking for is confirmation. KILLS, pics, eyewitness accounts, engineering data, SOMETHING! Or is this like "Smoky Yunicks" unfriggin believeable engine! I should just believe it because y'all "Said so". ONE crank out of how many replies, and that one WAS abused, and no other info given. I quest for the TRUTH, not heresay...

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carpe diem

i let my friend drive my 86 duke one time, he stalled it 5 times at a green turn light. fed up with his sucky driveing i told him to rev to 3 grand and dump the cluch. know not good but he never shifted into 2nd and all i seen was the tach needle bouncein off the red. i will say its a tuff little 4banger trip broke at 140 sumthin and still goin

OK, so now crank 0 (one abused crank broken with no further info, and one crank taken into the redline with no ill results), that is the score. The bottom end of the "Duke" or "Iron duck" (I kinda like that one), is looking more and more like pretty beefy unit to me. Opportunity is yours, I am dispelling this myth with each post. The 2.5L is flamed everyday and for lack of proof. Either provide proof, or flame not...

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carpe diem

Remember, all you've proven is that the crank itself is stronger than the rest of the bottom end, which isn't saying much. Ask how many people have spun bearings, broken rods, or burned pistons and you'll get a lot more answers. The crank may not be the weakest link in a Duke, but there's a reason the SD4 crank is a lot tougher - the rest of the engine is too.

I thought I might like to add that I've bought several Duke short blocks where the oil pump was in perfect operating condition, yet the crank was welded to the main bearings. I beleive this to be due to crank flex causing the clearances to go out of spec.

frankenfiero, the Duke crankshaft is not a fine one by any means. The casting quality is poor, as is the balance weight location. Remember that at higher levels of power the crankshaft becomes a more heavily fatigued item. Crank flex CAN lead to fatigue failure in the long run.

I've destroyed a rod but never the crank. But then again, stock duke rods suck.