New Rodbender Theory - Page 2 - PeachParts Mercedes-Benz Forum

ajnorris · #16 06-19-2011, 05:02 AM

Sixto, what about the rod bearings, seems like they (not the mains) would take the brunt of the wear if my theory holds any water, what do they usually look like? (for the record I have only seen one of these apart and it didn't have much wear so I'm basing this mostly on what i've read here) By the way I'm not really suggesting the crank is flexing, just the block at the top. It seems plausible that the cylinder/pistons wear slightly first, then bends the rod once they can rock slightly in the bore. Also by rough calculation (delta CTE x delta T) the head grows about .025" more that the block assuming they are at the same temperature, which may, or may not be the case. Head gasket slip and bolt hole clearances don't seem enough to account for that much expansion without warping significantly. Isn't the spec only a few .001" for warpage to the head? If the updated head has more stiffness than the block it would not warp, but pull on the block. By the way I would love to be wrong, as I still have a 3.5 with updated rods I would like to build.....rain away

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jt20 · #17 06-19-2011, 05:01 PM

Even if those numbers are correct (I get something very close, depends on Al type and length of head), they are the total change for the entire length of the head. Each section (from bolt to bolt) only sees about 1/13th of that.

Accumulating at each end, there is 1/2 of the total expansion on the very last set of bolts (in a perfect world). This where the headgaskets fail most often.... no surprise there.

That still leaves plenty of room in the bolt bores for expansion.

sixto · #18 06-19-2011, 05:27 PM

1/13th of the expansion per bolt is small but I expect there's about no expansion between the #3 and #4 cylinders and a heck of a lot at #1 and #6. That correlates with the tendency of #1 and #6 to bend.

I don't remember - are there locating or alignment pins between a 603 block and head?

Sixto
87 300D

ajnorris · #19 06-19-2011, 05:44 PM

Well either the top of the block is strained and changes by the same length as the head, or the head slips and transfers no strain and the bolts near #1 and #6 would be off by ~.013" from their original position at temp, or the head warps. My guess is that the strain on both is the same. The head does have positioning pins, but they are not solid. By the way the thrust bearing is between #4 and #5 I think, have to check.....
I assumed, A356 or similar for the head, and cast iron for the block (~13 ppm/degree F vs 5.8 for the block). Anyone know what the crank is? forged 4340?

jt20 · #20 06-19-2011, 06:45 PM

Quote:

Originally Posted by sixto

1/13th of the expansion per bolt is small but I expect there's about no expansion between the #3 and #4 cylinders and a heck of a lot at #1 and #6. That correlates with the tendency of #1 and #6 to bend.

The expansion between each section is the same (theoretically) but the total displacement or 'movement' of the head in relation to the block is cumulative as you approach each end.

So each end sees 1/2 of the total change in length. Like you said, the center sees very little distortion.

ajnorris · #21 06-20-2011, 06:55 PM

I just have a hard time with the head being able to slip like that. It is a function of the head bolt torque and the friction coeficient of the gasket. The few finite element models of aluminum diesel heads that I have seen showed the greatest stresses between the valves and not much near the bolts suggesting the clamping force keeps the head from moving too much. The om603.970 block also has strain relief features between the bores on the deck allowing more expansion of the block at the top, which seems to be unique to mercedes. I'll take a look at the protrusion of the head over the block cold and at temp to compare on my 3.0. The other thing I've got to bring up is torsional crankshaft flex. The timing between #1 and #6 is going to be different because of it, by how much I don't know but a straight 6 with 7 main bearings is about as bad as you can get. That might also have something to do with it.

leathermang · #22 06-20-2011, 06:58 PM

What do you mean a 6 inline with 7 main bearings is bad ?
How many more do you think can be stuffed into there ? LOL

ajnorris · #23 06-20-2011, 07:14 PM

7 Main bearings flex the crank (torsionally) more than 4 main bearings like beamer straight 6 has. Better for diesel combustion loads though. It's all a trade off.

leathermang · #24 06-20-2011, 07:39 PM

I do not understand that concept at all....
If you have a main bearing between EACH inline bore.. that is the best of all possible worlds... that supports the crank the most number of times possible with 6 inline bores... X+1 is the formula.....

jt20 · #25 06-20-2011, 07:41 PM

Quote:

Originally Posted by ajnorris

I just have a hard time with the head being able to slip like that. It is a function of the head bolt torque and the friction coeficient of the gasket.

The head does not "slip".

The HG flexes. Consider it a malleable membrane. Thats what it is designed for.

When you mill a head, you have to end with a certain stone so that you have the right 'bite' or 'grip' on the headgasket. It is not designed to slip at all.

The numbers we are talking about are so small. I think you may be underestimating the capabilities of the headgasket.