Valve guide seat shrinkiage? - PeachParts Mercedes-Benz Forum

Lexxani · #1 07-31-2008, 10:43 AM

Excuse my lack of knowledge on this, but I cant for the life of me think of an acceptable reason why the valve seat on the guides on my M117.985 would shrink.

Id think brass would not shrink to this degree. I expected to have to order oversize guides as I expected the head ports for the guides to have expanded (as they are aluminum) but once I got the digital caliper out, this is what I found. Any ideas, Im really stumped as to why this would have happened.

Chas H · #2 07-31-2008, 11:42 AM

What are you attempting to do. Replace the valve guides? Brass and aluminum only shrink or expand according to temperature. Is it possible that the valve guides have been changed sometime in the past?

pmckechnie · #3 07-31-2008, 12:20 PM

My guess is they got loose and then started to wear away. I am not sure what alum alloy MB uses but it doesn't seem to wear out but the brass will.
Paul

Lexxani · #4 07-31-2008, 12:21 PM

Im getting ready to replace the guides given these two are loose and allowed oil to enter the cylinders (MB's look so bad blowing blue smoke).

The consumption issue began around 2 1/2 years after I bought the car, so im going with wear. If they were always loose, say from a shoddy head job, the car would have always smoked.

As far as having the heads redone, Im thinking they are original. I say this because the car was a weekend car for an older couple (they were the 1st owners, im the 2nd) and had relatively low wear on the car when I purchased it. I put it into service as a daily driver after 27 years. I can not say with 100% certainty that they are original, just a hunch.

I got those numbers in my origional post after taking a caliper to them and I was surprised at the outcome, I fully expected the ID of the intake ports on the head to be 14.10 mm or so and the guides to be 14.00 mm (the wear occouring on the softer metal), this has me a little perplexed.

Lexxani · #5 07-31-2008, 12:25 PM

Quote:

Originally Posted by pmckechnie

My guess is they got loose and then started to wear away. I am not sure what alum alloy MB uses but it doesn't seem to wear out but the brass will.
Paul

That sounds plausible, I didnt find any reminents of the guides in the ports, maybe it ground into a fine powder like state and entered the cylinders and was expelled?

all 8 cyl's have perfect angular honing marks on the cyl walls, it was really encouraging to see this as now I know I dont have any abnormal wear on the bottom end. That rules out pieces of brass scoring the cyl walls.