Konstan's 1982 300D Engine Out Thread - Page 9 - PeachParts Mercedes-Benz Forum

konstan · #**121** 04-01-2010, 11:06 AM

thanks for clarifying about the wrist pin... yes, the oil ring is what was stuck, only in one spot on the piston. I actually ended up breaking it in order to remove it. It was stuck real good in there.

jt20 · #**122** 04-01-2010, 01:13 PM

that is what is referred to as 'stuck rings'. All the rings should have free play in their respective grooves and move quite easily.

as a reference point, old/ removed rings are used to clean the grooves from whence they came before installing new ones.

KCM · #**123** 04-01-2010, 02:03 PM

The oil ring being stuck would definitely cause oil consumption in that cylinder (but not knocking). If the groove is too narrow, or pinched, this is not good and would require replacing the piston. The ring needs to be able to rotate freely around the piston but should have very little play from top to bottom. It is possible the narrow spot is due to carbon build-up, which can be removed using an old ring as jt20 mentioned.

Sounds like if you had enough of a ridge to use a ridge reamer, then the engine needs to be bored out for oversized pistons to do the job right.

One measurement you can also check is the clearance between the piston skirt and sleeve with the piston inserted into the sleeve (without the rings). A feeler gauge is used for this. For a new or rebuilt engine, the gap should only be 0.0015". Not sure off-hand what the maximum limit is which would require rebuild. Excess slop here could also cause knocking.

konstan · #**124** 04-01-2010, 07:10 PM

Took #2 out. The piston is, compared to #1, is in better shape. #1 has scoff marks on its sides, #2 piston's sides are smooth and it is cleaning up nicely... I might consider replacing piston #1.

layback40 · #**125** 04-01-2010, 07:48 PM

Pics of pistons especially no. 1# please.
An easy way to measure bore taper is with an old ring. If you place it in the bore & measure the ring gap at various locations, say every 1/2" from top to bottom, (you can use the piston to push it down square). Calculate the difference between the largest & smallest gaps, divide the answer by 3.14, this gives you the measure of the taper (bore diameter variation) without needing a bore micrometer.

I use paint stripper to clean pistons (not the caustic based type), that way you dont damage the ring lands. Maybe biodiesel would work.

Dont get too worried about a few scuff marks on a piston, slop in the bore is more important.
Are the compression rings stuck ?

Happy hunting

konstan · #**126** 04-02-2010, 10:49 AM

layback40 yer a genius with the ring gap thing.

Took all five pistons out. #1 is the worst and is the only one with the marks on the side and a stuck oil ring. All the others were in decent shape, just some black (from blowby, I guess). Will measure and post the pics tonight. Last night, after all the reaming and stuff, I had to clean the floor around the engine, vacuum, replace all the newspapers on the floor.

The reason that cleaning was so important is that reaming leaves these metal specks and they were everywhere, on my hands, on the floor, and I was worried about them getting into the oil passages.

layback40 · #**127** 04-02-2010, 11:06 AM

You are absolutely correct about the need for cleanliness!! Any of those metal fines in your oil galleries & you could end up with bearing damage real quick!! I normally place plenty of grease around the bore. The metal fines tend to stick to it rather than bouncing off & going every where.
I dont know who told me about the ring gap thing. I have used it a few times. It wont tell you much about oval wear though. If you have a problem with that you will be able to see that the ring doesnt contact the bore all the way around.

rrgrassi · #**128** 04-02-2010, 11:10 AM

The scuff marks could be piston slap. That can sound like a knock.

barry123400 · #**129** 04-02-2010, 12:14 PM

You might try two different pistons in the first bore upside down. One at a time of course. Compare the skirt clearances with a feeler gauge. That way if the number one piston has a lot more wear clearance it could very well be the problem. Did you plastigauge the other rod bearings to see if they were a lot tighter earlier than the number one? They very well might have been.

konstan · #**130** 04-02-2010, 01:45 PM

@rrgrassi - yes, I thought of that too... can't wait to get home tonight and do some measuring

@barry - I did #2, and plastigage-wise it was not real clear if it was any different.... maybe a different color plastigage should be used but I only have red... and I will try different pistons in the #1 bore

DeliveryValve · #**131** 04-02-2010, 04:11 PM

I asked this earlier, but I don't think you mentioned this. But what was your compression numbers for each of the cylinders?

.

konstan · #**132** 04-02-2010, 04:38 PM

I don't have the actual compression numbers... I took it to a guy here in town to see if he could find the knock, he did the compression test and said that the compression numbers looked "really good" but I doubt he remembers what they actually were...

konstan · #**133** 04-02-2010, 11:09 PM

Measured bore diameter. I used a HF hole gauge, they taught us how to use that in machining class... I measured top, middle and bottom, several times each (to measure for out of round), for every cylinder.... long story short, the measurements are right on 90.9mm (3.578").

This is really good news, right???

Next going to clean the pistons and measure them.

jt20 · #**134** 04-02-2010, 11:19 PM

I am very impressed with your thoroughness.

Usually, I cut whatever corners are reasonable on my own engine. (not sloppy, but I let things go to their limits and make assumptions without scientific basis.)

nice work

barry123400 · #**135** 04-03-2010, 12:13 PM

If you are well satisfied with the bores for reuse. If the number one piston shows excess wear. I would have the number one rod checked for trueness on a jig at an automotive machine shop. A slight bend from being hydrolocked for example at one time might have tweaked it enough to ruin a piston over time.

Or if you decide on a good used piston it will probably come with the connecting rod.