Rebuilding Engine in '91 420 SEL
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1991 420 SEL
Rebuilding Engine in '91 420 SEL
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.
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.
Super Member
Joined: Jan 2008
Posts: 508
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From: california high desert
2003 S500, 03 C240, 03 E320, 03 tahoe
you have got to get the tech pubs
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.
Last edited by AH1W-COBRA; Mar 29, 2008 at 11:56 AM. Reason: typo
Senior Member
Joined: Jun 2006
Posts: 303
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From: Northern California
It changes all the time
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.
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.
Super Member
Joined: Jan 2008
Posts: 508
Likes: 1
From: california high desert
2003 S500, 03 C240, 03 E320, 03 tahoe
aluminum clyinders?
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.
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.
Super Member
Joined: Jan 2008
Posts: 508
Likes: 1
From: california high desert
2003 S500, 03 C240, 03 E320, 03 tahoe
disreguard all this,if hineywineries is correct
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.
"
Senior Member
Joined: Jun 2006
Posts: 303
Likes: 1
From: Northern California
It changes all the time
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.
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Joined: Mar 2006
Posts: 3,254
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From: Louisiana
03 E500 and Corvette
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.
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.
