617 cylinder bores

[Brian Carlton]

As most of you know, I've got the head off the 617 and need to replace it due to cracking on three cylinders, directly above the combustion chambers.

The question that I have is related to the condition of the cylinders.

All cylinders appear to have a slightly smaller and rougher surface for the first 1/2" or so, measured down from the top of the block. It appears that there is a parting line at this 1/2" point that is very slightly smaller than the remaining liner. Above this line is a noticeable difference in the surface. I've polished this surface with 1400 paper and have improved it significantly, but it almost seems like some corrosion has occured here. It does not seem random, however. All cylinders show this to a greater or lesser degree (the rear cylinders are the worst) and the surface deterioration suddenly stops at this 1/2" point down from the top of the block.

Anyone familiar with such a phenomenon and can provide some advice?

I may be able to get a photo of same, but, it's very difficult to see after polishing. It can be felt, however.

[t walgamuth]

the top 1/2" is the part of the cylinder that the rings never touch because they sit 1/2" below the top of the piston. anything above the top reach of the rings is simply there, and the only thing it does is contain the combustion and hold the head to the rest of the block. what you have done already is about all that you can do. the important thing is the condition of the bores below.

did you ever say how the head got cracked?

tom w

[Brian Carlton]

Thanks Tom. That makes perfect sense. I'm polishing the corrosion for nothing.

I don't think that anyone can say why it cracked, other than to conclude that somebody overheated it at one time in the past.

There is a very fine crack, directly in the combustion chamber on three of the cylinders. The cracks are so fine, they almost look like casting flaws. Apparently there is a coolant jacket above the face of the head. Cooling was passing through the cracks as evidenced by corrosion leading from the tip of the crack.

I'll see if I can post some photos of it today. I'd be interested in your thoughts.

[engatwork]

If you ever do remove the pistons from the cylinders you have to borrow (or buy) the proper tool to remove that off of the cylinder walls prior to shoving the pistons through.
Can you see any cross hatching on the section of the cylinder below or does it look like a mirror?

[Brian Carlton]

Yes, I understand from the gassers that this is the "ridge" and a "ridge reamer" is necessary to get rid of it. It just looked awful strange on a diesel. No real "ridge" to speak of. Just a very slight reduction in diameter that can be felt but not easily seen. Makes perfect sense once Tom posted it.

The bores still have cross-hatching. There is no mirror finish that I can see, but, my lighting is not all that great.

[leathermang]

Originally Posted by Brian Carlton

The bores still have cross-hatching.

Another one of those deals where MB is different from all other cars which I have seen....They intend and expect that cross hatching to remain for oil supply to the rings or something....

If your bores were worn mirror smooth as you would expect of other engines MB would consider them worn OUT.

I have never heard of " choke " being called for in boring or honing a cylinder bore.... and MB certainly does not call for that.

I have heard of " choke " being produced by the differential pressure against the walls of the cylinder due to crank throw. But that is something measured and if too much is used as " needs reboring" measurement.