Simple question (I hope)

Peter J

Peter J

Well-known member
I've heard that a bent or twisted rod could be the cause of engine noise. How in the world does a rod get bent or twisted without some sort of catastrophic engine failure and how often does it happen?
Thanks,
Pete
 
A connecting rod can get bent in any number of ways from fuel-lock, over stress or mishandling during a rebuild.. It probably doesn't happen very often, but, it could more than some realize..
 
most rods will get bent when there is a head gasket leaking water into the cylinder, as the engine is cranked the water in the cylinder will cause a hydralic lock as the cylinder runs out of space to handle all the water in there, the piston comes up and is stopped by the water, if there is not to much water in the cylinder the starter can apply enough pressure to bend the rod and the engine will keep turning, and if your just repacing the head gasket you might never notice it
 
Thanks. I was sort of thinking of a hydralic lock would do it as far as bending but how about the twist that some folks check for? It was suggested that the clicking in my engine could be from the rod working back and forth against the wrist pin??
 
Pete, the twist can also be caused by a hydralic lock, instead of bending to clear the lock the rod can also twist or colapse in a twisting fashion and shorten the rod enough to get past the lock , when a rod is twisted it does not run true with the wrist pin causing mis alighnment with the piston, it runs crooked in the cylinder
 
A better question might be, why are Model A rods routinely checked for straightness as a matter of course during a rebuild when only a tiny fraction would ever experience something like hydraulic lockup? My guess is that a freshly poured big end may not be perfectly parallel to the wrist pin bore, and that rod straightening takes care of that. Perhaps one of the engine builders/babbit pourers on this site could chime in?
 
I would bet that the reason for checking the rods for straightness is that they could get bent in lots of different ways....and putting a new engine together with bent rods is just asking for trouble.
 
gilitos said:
A better question might be, why are Model A rods routinely checked for straightness as a matter of course during a rebuild when only a tiny fraction would ever experience something like hydraulic lockup? My guess is that a freshly poured big end may not be perfectly parallel to the wrist pin bore, and that rod straightening takes care of that. Perhaps one of the engine builders/babbit pourers on this site could chime in?
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Unfortunately it's NOT common with many builders and certainly not to the extent that Ford explained was necessary. There are two pages dedicated to it in the January 1929 Service Bulletin. I've seen (and "heard") the results.
 
i always check rods for alignment and this forum isnt big enough to list the ways or reasons they get bent. if youve ever bored out a well worn model a cylinder they often have deep eyebrows worn in by the rings due misaligned rods.
 
Mr. Gilitos has got it right, for the Number one reason for Rods not to be in alignment. There is not a Rod lathe, Milling machine Ect. that will that will bore a Perfect aligned Rod! It should be up to the Rod rebuilder, to check Rod Alignment and after that, it would be up to the Engine Builder to recheck, before assembly, but many don't because they don't know they should.

There was a comment about New Rods a while back saying, the New Rods don't have to be checked, as they are Newly made, and they never checked them, and have put them in many engines with out doing that, lots of luck to the owners of the engines they did!

We check all Rods, and out of 30,000 Model A rods, we found ONE we didn't have to Tweak, and that is a big deal, as it is rare!

Even with the checked aligment on a Model A Rod, when you hone the New Bushing for a wrist pin, they should be checked for twist, and bend. The OFF SET should not change if it was done already.

When we talk alignment, we are talking .001 to say .010 thousandths, out but we have had reports from other shops, they have had rebuilt rods as much as 1/4 inch. You would think that a little far fetched, but after there rod lathe was seen, and there was a 3/8th's belley in the Ways, all became clear!

Many Rods that were straight, have been bent in vice Jaws, taking off the caps, and putting on the pistons, by some owners doing there own work.

Thanks Herm.
 
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Thanks Herm,
It sounds like if a person doesn't have the proper tool(s) to straighten a tweaked rod, it would be best to replace it rather than try a farmer fix. Is their a way to check if a rod is the noise maker before tearing the thing down??
Pete
 
Peter J said:
Thanks Herm,
It sounds like if a person doesn't have the proper tool(s) to straighten a tweaked rod, it would be best to replace it rather than try a farmer fix. Is their a way to check if a rod is the noise maker before tearing the thing down??
Pete
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Other then sound Peter, none that I know of.

If the rods are all right for wear, they can be checked for alignment, but you would also have to check crank Pin, and Cylinders.

Thanks Herm
 

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Sounds like a winter project eh? Since we're talking about very little tolerances, how would eyeball the rod? Also, when I got the engine, it had some minor scoring on the #1 cylinder which might be a clue of some prior damage. I did not rebore it before I put it back together.
 


Hi Pete,,



The next time you have the oil pan off I'ed have somebody turn the engine over by hand as you look at the rod's...


They should be "centered" on the pin's..I had a motor way back that "Knocked" & upound looking at the rod's I saw a few that were off..I don't know how much they were bent,,But I could see they were off just looking at them...



This is just something to look at the next time you have the pan off..



Greg out West












Greg out West
 
Just for kicks, when you have nothing else to do, put an old rod in your vise and try bending it. A rods need to be forced way out of plane in order to actually bend or twist them. It would take a builder with a BIG hammer to bend one while assembling the engine! However, it is always best to check them before installing them.
 
I bought the engine (B type) out of the car so I never heard it running. I guess I didn't ask the seller the right questions. Still a good deal though. I understand their is a whole section in the original manuals on straightening rods so it seems to be sop to check em.
 
Peter J said:
I've heard that a bent or twisted rod could be the cause of engine noise. How in the world does a rod get bent or twisted without some sort of catastrophic engine failure and how often does it happen?
Thanks,
Pete
Click to expand...

I have seen rods that were bent, rods that had a twist in them. I have seen them push on the wrist pin so hard that it knocked wrist pin keepers out, and gouge the cylinder wall. Model A rods are longer than most and twist real easy. Another thing that I have found is some re bored engines have there cylinders in the wrong location, from sloppy work years ago. You should check every rod with the piston on it before assembly. They are off for a lot of reasons.
 
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