Question-------------------------
Followup To
Question -
Hello Whunter:
Background:
I have taken apart a 1980 Mercedes Benz 300SD 5 cylendar turbo diesel engine. According to the service manual, the green MB shop manual for these diesels, the pistons are supposed to have been upgraded to a star shaped pattern on the top. The as found pistons are of the dished type top.
This engine has been rebuilt by someone in California and has many new parts. However, it suffers from low compression in number 1 cylindar. Observation of the rings indicate the rings never seated in number 1. I drove this car across USA. Runs great at HW speed but has a rough idle due to 175# in number and 375# in all others. Also note, the rings in number 1 are pitted by corrosion on the outer diameter surface where they rub on cylendar. Cylendar is in good cndition.
Question: For this model, and year engine, what should the pistons look like. I can provide as many pictures as you need to help out.
Rich
Answer -
Take them to local MB dealer or heavy diesel repair shop.
This will save my fingers, and your time.
I do not like the sound of your description.
Look at the cross hatch pattern in cylinder walls, unless it is perfect, top to bottom, and identical in all cylinders, you will need new sleeves + pistons and match boring.
The cross hatch pattern in MB cylinder walls is critical.
Hello WHunter:
Local Dealers and repair shops do not have expertise. Mostly maintenance types. The cross hatching on number 1 is only barely visible on a portion of the cylendar. Will be sure to cross hatch during assembly.
Cylendar is just within spec as far as diameters, and tapers. You mentioned replacing sleaves. Sounds like a great idea only I can't find a commercially made tool to do it. Shop manual shows a fabicated drift that you are supposed to use a hammer to beat sleave out with. Not enthused about that method. How about using a small saw and cutting out longitudinally. Kind of making sleave into two halves. Local speed engine shop wants to bore them out at $170 per. But that requires complete bare block for their shop. I do not want to mess with injector assembly. Any common knowledge on this methodology?
Found piston ring side gap clearance on number 1 to be excessive. It's running about 13 thousanths, with max spec at 7 thousands(english). Will be buying another piston form web vendor. Probably Mahl brand.
Brings to mind another question: where on the piston do I measure diameter? Spec is 90.9 mm, but piston measurements differ at several locations. For example, top head is 18 thousands small, lower skirt at pin is 6 thousanths small and lower skirt at 90 degrees to pin is just below spec. Is 90 degrees to to pin at skirt the correct location for room temperature measurement?
We really want to do a good job, and also learn as much as possible. My son Mike is working with me on this project. He is a college student studying mechanical engineering at UConn. I am electrical engineer, but enjoy mechanical just as much. We both feel diesels have a very powerful future in USA.
Rich Johnson
AnswerHello Rich Johnson
The #1. cylinder bore is junk. RE: The cross hatching on number 1 is only barely visible on a portion
of the cylendar.
You can NOT cross hatch; it must be rebored, then cross hatched, and may need a new cylinder sleeve.
Replacing sleaves is a precision machine shop job.
Do not attempt the sleeve, allow the machine shop to replace and bore it, $170 per to bore them out is not a bad price.
You must take it down to bare block = no short cuts.
Measure diameter 90 degrees from wrist pin at top, ring groove and skirt = correct location for room
temperature measurement, sounds to me like the current pistons are tapered.
