New Rodbender Theory
 

Ok, so feel free to call BS on this but I have a new theory on why the .970 rods like to bend. Since the bores are larger in the .970/971 this allows the aluminum cylinder head thermal expansion to apply more strain on the iron block (since it is less stiff) making the top of the block strained at temperature and the bottom of the block and crankshaft not strained. This could result in a slight offset of the rod allowing it to buckle. This could explain why #1 and #6 like to buckle out of plane more than other cylinders. This also explains why the updated rods sort of help. They are stiffer and resist this more, but can still fail. Any comments?....
Thanks,
Ashley

The originals were not strong enough to resist the forces in the combustion chamber.
LOL

So why are #1 and #6 most likely to bend and why do some of the remans fail? I'm mostly asking because the concencus seems to be that even the updated rods aren't enough.

That sounds good to me... that even the updated rods are not up to snuff.... looks like the evidence points exactly there...

You need an old cast iron block and head 617 so you can sleep better at night...

It is always tricky mixing metals like that... it can be done.. but if you find an engine series has a reputation for problems.... get a different engine stye....

While I do appreciate the old school engines, I just have a thing for 124's and 0-60 in under 10 seconds. Besides, as long as the updated head issue is taken care of om603.960's are great motors. But back to the original point, the 5 cylinders with the same geometry as a 3.5 liter don't fail.

You know, there are BDSM groups that are always looking for Masochists to join the party...

Does your theory account for why the rods tend to fail by bending forward? I think the bending is due to interference between the piston crown and exhaust valve. Some crud accumulates on the hotter exhaust valves.

Sixto
87 300D

Those must be some kind of strong valves and piston tops to be able to transfer that force down to the rod...

So why would 1 and 6 exhaust valves build up more than the other ones?

-J

They were only put on the far ends of the engine because they were antisocial in the first place...so it make perfect sense that they would attract more carbon..

Which engines were those? Europe only? Or how the 617 is about that displacement?

-J

Not sure I can explain the bending forward but it explains the bending out of plane, which would cause the rod to twist when the piston started rubbing on the walls unevenly. Need to think on that one a bit, but heck it beats chaotic combustion theory, right?;) I am rather fond of the rock theory though.

I was refering to the sprinter 2.9L I5 but yeah, a 617 has similar geometry too.

I don't think it's a single impact that bends the rods, rather the consistent pounding over thousands of miles. Maybe the crank flexes more at the ends than the middle. I haven't see wear on the main bearings to corroborate crank flexing, though.

Sorry, didn't mean to rain on your theory :)

Sixto
87 300D

Its not BS... but it has been accounted for by the headgasket and bolts that are smaller than their aluminum bores.

The head is allowed to expand without putting a shear force on the top of the block.

Along similar lines, this is the reason I think early versions of the 603 head are more prone to cracking. As the bolts and headgasket age, the head is allowed to distort with uneven forces applied. You will find that the loosest bolts are in the back and front of the engine and that these bolts most often have some form of contamination due to a failed headgasket. The center area bolts will be just fine.

Assuming the ends of the head become looser before the rest, the uneven heating that happens on a head causes it to warp concave up if given the space to do so. Loose bolts, or a failing headgasket could allow this to happen.

Going back to rodbenders: did they fail after something like this happened, or did it just take 150k miles for the rings to finally wear away enough at the walls that the smoke was noticeable by the owner?