I'm about to finish putting my motor together after a blown head gasket and I'm getting paranoid about bent connecting rods. I have the head of and have brought the pistons to TDC and compared how high they go, by eye and touch. I can't say I notice a difference between any of them. I would measure them with some kind of instrument, but that's where I run into some uncertainty. The clearances in the rod bearing and wrist pin probably add up to .001 - .002 and there is no guarantee that the total clearances are all that close between piston assemblies. It seems like a significant bend, less than ten degrees?, could produce a difference in height that is well under .002 in, since the rods are so long. In a nut shell- how much of a height difference in the pistons would suggest a bent rod rather than just variation in clearances?

Is the motor in the car still? If not, it's a perfect opportunity to rebuild the engine!

Still in the car, I just rebuilt the engine and put 70 miles on the car before the gasket blew out...

I think the tolerances on the crankshaft are not that good.

I'd be more worried about why the head gasket(s) failed so soon after the rebuild.

I am concerned about why it failed so early. I couldn't find a definite answer but my best guess is I dented the gasket while trying to align the head on the guide pins and that weakened it. I can't find anything suggesting that the a new gasket will fail prematurely. Does anyone have experience with trying to measure the piston height? I would really hate the idea of just putting this thing together assuming the rod isn't bent.

Im pretty sure a bent rod, even moderately bent wouldnt really have a measurable amount as far as piston height. If you suspect any are bent, now is the time to pull them out and have them checked. Id want them perfect. Ive got some pretty nasty bent ones here, and they dont look that much, if any shorter than a good one. One is out of my boats 454 that swallowed a valve and stopped immediately when the valve head wedged between the head and piston at 5000 rpm.

You would have to know what the piston height was before the failure for the measurment to be of any use at all. I can't see any reason you would be worried about a bent rod, why it (the head gasket) failed early should be what you are working on fixing, just put it back to gether and be happy. Larry

[This message has been edited by trotterlg (edited 01-04-2014).]

So uhh... what makes you think it bent a rod? Did the engine hydro-lock, or drop a valve, or something?

perhaps you didn't torque down the head properly...

Unless you have a specific reason for suspecting a bent rod that you haven't mentioned, it is unlikely. Many things could contribute to a failed head gasket, and they usually are a result of incorrect installation if they fail shortly after installation. Check the new gasket carefully for imperfections, and make sure you torque it correctly, in the correct sequence.

What engine? where was it rebuilt?
It takes something pretty detrimental to happen to bend a rod. Pistons take the hit worse. Its hard to tell from the top unless you see piston damage. If you see piston damage your taking it apart anyway. You are better pulling off the oil pan and inspecting.

I bought a 3.4 about 2 years ago with 3 RODS So Bent, it was a joke. Was told it was a Low Mile rebuild. Better to "Check". at least put a straight edge over the block and measure the clearance compared to all the other cylinders. Water doesn't compress. Did you use NEW Bolts when you rebuilt ?

I really don't have anything pointing to a bent rod, other than I heard a mysterious quiet popping or knocking noise after the gasket blew. There was hardly any coolant in the cylinder that blew so I don't expect it hydro locked. I'm just being cautious. I had used new bolts, head surface was freshly cut, block surface was in spec, double checked the torque on each bolt, etc. I guess I'm quadruple checking everything when I replace the head gasket this time.

[This message has been edited by smmilke (edited 01-04-2014).]

Mine was very obvious with the head off. The piston was broke into 5 pieces and the valve was shoved 1/2 way in the top. Id guess yours is prob fine. Do you see any marks on the piston top or the head chamber, Are the valves seating straight ? Use a quality head gasket...dont cut corners.

quote Originally posted by smmilke: Still in the car, I just rebuilt the engine and put 70 miles on the car before the gasket blew out...

Something smells fishy here. You didn't say what engine you put in or what you consider a rebuild. If I rebuilt an engine and 70 miles later the head gasket fails, it seems like I did something wrong, Or the engine has issues and time to put a 3800 in it.

it's a 2.5.

quote Originally posted by Gall757: it's a 2.5.

What year is his engine a 2.5? If it's an 84, he might just be better off getting a new motor anyway... one of the 85.5+ motors with the roller cam. They run a little bit less noisy... don't know if I would call it smoother... but less noisy. Heh.

it's an 84, but keep in mind it is in Fairbanks, Alaska.

Here is an earlier thread.

//www.fiero.nl/forum/Forum2/HTML/130838.html

Could have been hydrolock, I picked up a nice 87 base coup for $300 years ago with a fresh remanufactured engine that wasn't running. Turned out to be the CPS, once I got it started it was making a terrible clacking sound indicating the motor was bad. On disassembly I discovered the original intake that was used on the new motor was leaking internally and had leaked water into the cylinder eventually ruining the rod, piston and bore. It was doomed from the start since no one tried to figure out what caused the previous motor to fail.