OM617 NA rebuild - piston, rings and bore clearances
OM617 NA rebuild - piston, rings and bore clearances
First some background, please bear with me!
I have an engine which was after-market turbo charged in the 1980's, some 300000km ago. It is in a 300GD. I bought it barely running. The engine always had blue smoke, which initially I thought was coming from a leaking turbo. When I ran the engine briefly without inlet or outlet manifolds there was still a good amount of smoke.
I then adjusted the valves and did a compression test, which showed low values on all 5 cylinders. When running, the oil filler cap does move around a little bit, then closing off the valve cover breather shows there is definately blow-by.
Now I have the engine in bits, I have made some measurements but I need assistance in interpreting the results.
Firstly, the engine appears to have been machined 0.5 mm oversize previously. The pistons are stamped 91.47.
The pistons measure 91.36 across the skirts. Appearance is good.
The gap between top ring and piston groove is 0.25mm (service limit 0.2)
The gap between middle ring and piston groove is 0.17mm (service limit 0.15)
The gap between bottom ring and piston groove is 0.09mm (service limit 0.1)
The ring gap with the top ring is 0.65mm (SL 0.15)
The bores had no step at the top, and honing pattern was visible. They are all consistent at 91.56mm.
The cylinder to piston gap works out at 91.56 - 91.36 = 0.20mm (SL 0.04mm) This measurement is thus way-out of spec.
Is this the reason for poor compression? Was the rebore too large even for the oversize pistons? I aim to get the engine running well, without the smoke which prevents passing a roadworthy examination.
Comments please!
Thanks
Adrian
Goodday All
First some background, please bear with me!
I have an engine which was after-market turbo charged in the 1980's, some 300000km ago. It is in a 300GD. I bought it barely running. The engine always had blue smoke, which initially I thought was coming from a leaking turbo. When I ran the engine briefly without inlet or outlet manifolds there was still a good amount of smoke.
I then adjusted the valves and did a compression test, which showed low values on all 5 cylinders. When running, the oil filler cap does move around a little bit, then closing off the valve cover breather shows there is definately blow-by.
Now I have the engine in bits, I have made some measurements but I need assistance in interpreting the results.
Firstly, the engine appears to have been machined 0.5 mm oversize previously. The pistons are stamped 91.47.
The pistons measure 91.36 across the skirts. Appearance is good.
The gap between top ring and piston groove is 0.25mm (service limit 0.2)
The gap between middle ring and piston groove is 0.17mm (service limit 0.15)
The gap between bottom ring and piston groove is 0.09mm (service limit 0.1)
The ring gap with the top ring is 0.65mm (SL 0.15)
The bores had no step at the top, and honing pattern was visible. They are all consistent at 91.56mm.
The cylinder to piston gap works out at 91.56 - 91.36 = 0.20mm (SL 0.04mm) This measurement is thus way-out of spec.
Is this the reason for poor compression? Was the rebore too large even for the oversize pistons? I aim to get the engine running well, without the smoke which prevents passing a roadworthy examination.
Comments please!
Thanks
Adrian
If u want to have the engine running for a few Km more, just get some new rings inside and hone it (by hand) assemble it , and after a few hours running low , compression will increase and the smoke will be partially gone. its amasing how much play u can have in this engines before u have to rebore and rebuild the cylinders...
If u have a good budget, the way is to install new skirts and STD pistons , for sure u can find there someone willing to install skirts and cut the bores to STD and for the piston u can find repair kits with piston , rings, bearings all u need to assemble the engine back toguether.
Avoid MB as much as u can, they can be quite expensive in some goods....
The head assy , should be done too... valve seats, valve contact surface , valve stem runners and rings , those u can find in kits too.
Timming and so on ,
3rd option , buy another engine. it can be quite costly repair one engine like the book say, at least in my planet....
good luck with it ...
FD,
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hy there,
If u want to have the engine running for a few Km more, just get some new rings inside and hone it (by hand) assemble it , and after a few hours running low , compression will increase and the smoke will be partially gone. its amasing how much play u can have in this engines before u have to rebore and rebuild the cylinders...
If u have a good budget, the way is to install new skirts and STD pistons , for sure u can find there someone willing to install skirts and cut the bores to STD and for the piston u can find repair kits with piston , rings, bearings all u need to assemble the engine back toguether.
Avoid MB as much as u can, they can be quite expensive in some goods....
The head assy , should be done too... valve seats, valve contact surface , valve stem runners and rings , those u can find in kits too.
Timming and so on ,
3rd option , buy another engine. it can be quite costly repair one engine like the book say, at least in my planet....
good luck with it ...
FD,
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Thanks for your encouraging reply.
I will certainly be reconditioning the head, but I want to be sure that the bottom part of the engine is serviceable.
I would like to know if the piston/rings/bore clearances are the likely cause of borderline compression!
Regards
Adrian
Barrote
Thanks for your encouraging reply.
I will certainly be reconditioning the head, but I want to be sure that the bottom part of the engine is serviceable.
I would like to know if the piston/rings/bore clearances are the likely cause of borderline compression!
Regards
Adrian
Once the cylinder wall is marked at the top, like corrugated , that was due to 2 causes:
1-damage due to intense work, mileage , or defective lubrication, age and hard work related. that is due to beating on the cylinder wall ring up and down show wear and cylinder becomes oval.
2-incorrect ring use, polluted fuel or uncoventional fuel like heavy oil, vegetable oil , overheating and many others. This causes the rings to exert too much strenght against the wall or by being too hard metal or by carbon deposit in the grove wich force them out. This case cylinder is not ovalized , and the wear is very visible.
Anyhow as i said , theres only one way to go, new skirts and cut for STD piston, or check oversize piston availability and compare to how much need to be cut.
Of course u can just handcraft the corugated surface, and install a new rings and use it for some more km.
The numbers work pretty much like this: exemple , cylinder is cut to 89mm more less .008, a measure is done and the cylinder will assigned a code X, B, A . X is 89mm more or less .08 if cylinder is 89.016 code B is asigned . then u chose a piston with this code.
cylinder max permissible wear is 0.2mm. in one of the 3 measures botom midle and top , measure is taken where the ring set changes direction.
if u work in a oversize piston the numbers are the same, except u can have one .25, and another .50. the person that cut the cylinder should mark it in the block. is the same .20mm wich is a lot.
the piston grove measure, is like what is stated on the manual. but usually unless u see too much wear the new rings come with a slightly oversize, thats why u see now diff measures.
in my personal opinion no one in good mind will rebuild a engine without having the block in good shape, like new, so skirts std pistons and rings, main bearings and crank bearings checked. sometimes the crankshaft exibits wear , like ovalization on the rod bearings, or the main ones, since its a high mileage engine.
anytime...
regards
FD,
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Hello,
Once the cylinder wall is marked at the top, like corrugated , that was due to 2 causes:
1-damage due to intense work, mileage , or defective lubrication, age and hard work related. that is due to beating on the cylinder wall ring up and down show wear and cylinder becomes oval.
2-incorrect ring use, polluted fuel or uncoventional fuel like heavy oil, vegetable oil , overheating and many others. This causes the rings to exert too much strenght against the wall or by being too hard metal or by carbon deposit in the grove wich force them out. This case cylinder is not ovalized , and the wear is very visible.
Anyhow as i said , theres only one way to go, new skirts and cut for STD piston, or check oversize piston availability and compare to how much need to be cut.
Of course u can just handcraft the corugated surface, and install a new rings and use it for some more km.
The numbers work pretty much like this: exemple , cylinder is cut to 89mm more less .008, a measure is done and the cylinder will assigned a code X, B, A . X is 89mm more or less .08 if cylinder is 89.016 code B is asigned . then u chose a piston with this code.
cylinder max permissible wear is 0.2mm. in one of the 3 measures botom midle and top , measure is taken where the ring set changes direction.
if u work in a oversize piston the numbers are the same, except u can have one .25, and another .50. the person that cut the cylinder should mark it in the block. is the same .20mm wich is a lot.
the piston grove measure, is like what is stated on the manual. but usually unless u see too much wear the new rings come with a slightly oversize, thats why u see now diff measures.
in my personal opinion no one in good mind will rebuild a engine without having the block in good shape, like new, so skirts std pistons and rings, main bearings and crank bearings checked. sometimes the crankshaft exibits wear , like ovalization on the rod bearings, or the main ones, since its a high mileage engine.
anytime...
regards
FD,
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It transpired that the old pistons were cracked (on the top ). I have ordered new std pistons. The machinist is wondering what size to make the bores, and whether:
1. he should wait for the pistons to come and measure them up
2. he should modify the bore size considering that the engine has an aftermarket low-boost turbo on it? (The factory turbo engine has different bore sizes).
Does anyone have experience of this or comments please?
Regards
Adrian
Hi All
It transpired that the old pistons were cracked (on the top ). I have ordered new std pistons. The machinist is wondering what size to make the bores, and whether:
1. he should wait for the pistons to come and measure them up
2. he should modify the bore size considering that the engine has an aftermarket low-boost turbo on it? (The factory turbo engine has different bore sizes).
Does anyone have experience of this or comments please?
Regards
Adrian
u must follow the repair manual, they tell u what is the limits for the bore size, the engine being a turbo or not maters nothing.
usually u have , cast pistons and forged, its very unlikely u have forged pistons for this engine , the forged piston tend to grow in size i lit bit more , so room for the piston has to be made in the cylinder.
its a good idea to have the piston measured, and then cut the bore, be aware that the piston is not round due to diferent expansion rates.
the bore should be round,
as i said , mb repair manual and leave the bores according to specs.
regards
FD,
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Hey man,
u must follow the repair manual, they tell u what is the limits for the bore size, the engine being a turbo or not maters nothing.
usually u have , cast pistons and forged, its very unlikely u have forged pistons for this engine , the forged piston tend to grow in size i lit bit more , so room for the piston has to be made in the cylinder.
its a good idea to have the piston measured, and then cut the bore, be aware that the piston is not round due to diferent expansion rates.
the bore should be round,
as i said , mb repair manual and leave the bores according to specs.
regards
FD,
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but as i said u must guide by the manufacturer manual....
Cylinder cutting is delicated machinning, so the person who runs the shop usually have tables for every engine according to piston manufacturer, when u have a job like this done , the installation of bearing caps, crank , piston, rods, dadad should be done by the machine shop people. just cause they are more used to do that , and have the correct measuring tools.
regards
FD,
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The measure about the Groove is too much , in a new configuration 60X engines the play in the grooves should be less than .1mm .
but as i said u must guide by the manufacturer manual....
Cylinder cutting is delicated machinning, so the person who runs the shop usually have tables for every engine according to piston manufacturer, when u have a job like this done , the installation of bearing caps, crank , piston, rods, dadad should be done by the machine shop people. just cause they are more used to do that , and have the correct measuring tools.
regards
FD,
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I have a question regarding the conrods: does it matter if they are not all facing the same way? In other words can a conrod be fitted either way?
This is for a non-turbo Om617 of course.
I won't say why I ask this question.....
In the end the shop fitted new liners so with the new pistons there should be no issues.
I have a question regarding the conrods: does it matter if they are not all facing the same way? In other words can a conrod be fitted either way?
This is for a non-turbo Om617 of course.
I won't say why I ask this question.....
for instance there are some rods wich develop lubrication problems in the wrist bearing, and have a hole facing the squirter, if u install one of this the oposite way, u know late in time play will apear due to unsuficient lub and cooling.
the other face is the cap Groove, the caps may tend to rotate if assembled the wrong way.
why u want to do such a strange thing? u lost the side ?
the maintnance and assy manual tell´s u if they have a right or left side.
regards
FD,
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well , theres nothing that really say´s otherwise.... but they should be installed the way they were made to,
for instance there are some rods wich develop lubrication problems in the wrist bearing, and have a hole facing the squirter, if u install one of this the oposite way, u know late in time play will apear due to unsuficient lub and cooling.
the other face is the cap Groove, the caps may tend to rotate if assembled the wrong way.
why u want to do such a strange thing? u lost the side ?
the maintnance and assy manual tell´s u if they have a right or left side.
regards
FD,
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(01-04-2016, 08:58 AM)barrote well , theres nothing that really say´s otherwise.... but they should be installed the way they were made to,
for instance there are some rods wich develop lubrication problems in the wrist bearing, and have a hole facing the squirter, if u install one of this the oposite way, u know late in time play will apear due to unsuficient lub and cooling.
the other face is the cap Groove, the caps may tend to rotate if assembled the wrong way.
why u want to do such a strange thing? u lost the side ?
the maintnance and assy manual tell´s u if they have a right or left side.
regards
Hi Barrote
I examined the conrods and they look symmetrical, and both the MB workshop manual and the Haynes one say nothing about this point.
So if I put new conrods in, there is no particular orientation mentioned by the Manuals.
Is there some other point that I have missed?
Thanks
Adrian
(01-04-2016, 08:58 AM)barrote well , theres nothing that really say´s otherwise.... but they should be installed the way they were made to,
for instance there are some rods wich develop lubrication problems in the wrist bearing, and have a hole facing the squirter, if u install one of this the oposite way, u know late in time play will apear due to unsuficient lub and cooling.
the other face is the cap Groove, the caps may tend to rotate if assembled the wrong way.
why u want to do such a strange thing? u lost the side ?
the maintnance and assy manual tell´s u if they have a right or left side.
regards
Hi Barrote
I examined the conrods and they look symmetrical, and both the MB workshop manual and the Haynes one say nothing about this point.
So if I put new conrods in, there is no particular orientation mentioned by the Manuals.
Is there some other point that I have missed?
Thanks
Adrian
usually they are not symetrical, u can install them in the same sense using the cap Groove as reference.
up to u, but as i said earlier the engine will not destroy itself or be week cause of that. balance is another question but since it will be a low revolution engine, low i mean 3.5 k most of the time that shouldn´t be very dangerous.
regards
FD,
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rod balance , the total weight of the rods, and weight distribuiton, the manual stipulates how it is done and the tolerances,
usually they are not symetrical, u can install them in the same sense using the cap Groove as reference.
up to u, but as i said earlier the engine will not destroy itself or be week cause of that. balance is another question but since it will be a low revolution engine, low i mean 3.5 k most of the time that shouldn´t be very dangerous.
regards
FD,
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The wrist pin bearing is the other factor, usually lub is done by gravity, and holes in the bearing were made to orientate the faling oil from the squirters / piston crown into the bearing....
Being the rod balance, secondary, they need to be within margins and balanced same way.
Regards
FD,
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as i said the engine will start and ride and everithing... but there´s a reason why the conrod have right and left, usually is due to the combustion stroke momentum tend to rotate the cap under high load conditions and for higly used engines, all the unrepairable wear i seen in those journals were due to cap sliping sidewards after rotation.
The wrist pin bearing is the other factor, usually lub is done by gravity, and holes in the bearing were made to orientate the faling oil from the squirters / piston crown into the bearing....
Being the rod balance, secondary, they need to be within margins and balanced same way.
Regards
FD,
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