fkellees

Hello,

Im new to this forum and am looking for accurate information. I have been having a difficultly finding much of anything on the internet regarding specifications on my engine.

I have a 1991 420 SEL that has 173,000 miles. I purchased the car for school in non-running condition. The timing chain guides had broke. In a moment of brilliance I decided to rebuild the engine in class. The entire engine is torn apart and has been cleaned. I'm now ready to take measurements and decide what needs to be done regarding cylinders, pistons, and related parts. All of the parts were systematically removed and I know the original locations of all them.

Hopefully everyone will not mind my questions and ongoing use of this thread and I'm sorry for "stupid" questions. I am new to Mercedes Benz.

1. All of the cylinders have a ring ridge of .002". Is this acceptable to leave in the cylinder? Can it be honed? If honed, do I need oversize pistons or would wider rings work if available? Can the sleeve be bored (trying to avoid)?
-I don't want to resleeve the block.

AH1W-COBRA

MBUSA may help if you describe the situation as a educational/school auto shop project.as you can allready tell there is nothing that tricky about a benz motor.that .002 at the top of the bore has got to go,it's not so much getting the new rings to pass by it on install.it's that new rod/crank bearings will change the height dimension of the rod and piston in the bore and the top ring will end up banging that ridge,your top ring will be damaged,or worse the ring groove in the piston.use a good ridge reamer and then a bottle brush style hone unless you are really good with a stone hone.id recommend letting a pro shop do the ridge and honing but that takes $ and you would still have to check the bores for concentricity/ roundness after they were done.ask MBUSA to donate the tech info for the project,to be return when completed.

hineywineries

First of all I think you're either slightly nuts or have too much time on your hands. That engine is so well engineered, there is no reason to ever rebuild one unless you've got a rod sticking through the oil pan. Did you notice that the cylinders are aluminum. There are no sleeves. You won't be able to duplicate that silicone finish in a hobby shop. That being said, reassemble the engine and just replace the bent valves on the left bank. Since it's apart, you may as well do a valve job on the right bank. The book you need is the M116/117 book for the 420 and 560 engine. Don't get the M116/117 book for the 350 and 450 engine. This is an MB pub and often shows up on ebay. Good luck.

AH1W-COBRA

im constantly learning new things on this forum but my old school mentality wont let me vision an alulminum cylinder wall.reguardless of the teflon coating,if the friction didnt destroy the piston to cylinder wall tolerance than corrosive combustion chamber qualitys would.am i just misunderstanding?

AH1W-COBRA

if HW is correct you cant restore the cylinders in any way. my bad."cobra"

hineywineries

Aclually you can rebore and buy over size pistons, but like I said you need to restore the special surface on the cylinder walls. One of the biggest problems MB first encountered with aluminum block and heads was the head bolts pulling the threads out of the block on the 380 even considering the low clamping pressure. The low clamping pressure was compensated by using 18 bolts per head. The problem was solved with the advent of the 420 and 560 by using bolts made of a special material. They are extremely expensive. It does sound unlikely to have aluminum bores, but MB engineers did it. Trust me. I've been doing this mechanicing thing for 30 years.

vettdvr

For you project I think a ridge reamer would work. Previously covered if you don't remove the ridge you can break the top ring on the piston when you run the engine. Ok,, maybe there is a special coating on the cylinders, but for ring breakin I would only use very light 600 grit paper with oil just to very lightly scuff the surface without removing lots of metal. If you have too much taper the rings won't hold. But if you only have .002 you probably could get away with new rings, reuse the pistons. Granted there are some engines with 1 time use headbolts. But that is why they have torques specs. If you can get access to a manual and confirm the specs you should be able to determine if the bolts are 1 time use.

Given this is a learning exercies and not a blueprinted race engine. I would let the students learn as much and then see the results.

Best of luck.