Connecting rod free-play

[Raymond]

My engine was 3rd oversize and the rings were glazed.  When I removed the crankshaft, it had .25mm over bearings on the connecting rods and main journals, so I just had the crankshaft polished and checked for true and balance.  I have a different block that I had bored for new pistons and align bored for .25mm over main bearings.  

When I use my original connecting rods with new .25mm bearings on my original crankshaft, and torque everything to 90NM for the mains and 61NM for the connecting rods, the crank seems harder to turn than I might expect.  I haven't found a source for plasti-gage yet, but will that be enough to tell me if I have a problem or not?

Is there a way I can use the torque wrench to see if my turning resistance is too much?

Ray
'68 280SL 4-spd Coupe

[ja17]

Hello Raymond,

You should try to do some measurements of the machined parts just to check your shop if possible. You must have the correct factory specs first of all. Harbor Freight has a very nice digital Metric/English caliper which is easy to use and accurate enough for checking things like crank journal diameters. They sell for less than $20.00! See picture below. I have expensive metric micrometers but the digital caliper is much easier to use and seems to have stayed accurate!

Download Attachment: P1010001.JPG
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You will normally have some more resistance to turning during a rebuild due to a new crank seal and new parts creating more drag. However it is important that everything be correct. Something "binding" can cause excessive heat from friction and damage a new bearing or other new part.

Correct assembly may involve some measuring and re-assembly. It is a good idea to plastigage the crankshaft in place without the crank seals and connecting rods first. Dis-assemble and check the plastigage for correct clearances. Clean the plastigage from the parts and re-assemble without the rear seal again and turn the crankshaft. Use a good assembly lube on all the beaarings and crankshaft. Check the thrust bearing clearance with a feeler guage. Now turn the crankshaft. It should rotate freely with very little risistance.

If everything is ok, dis-assemble again and install the crankshaft seals and re-assemble with assemly lube on the crank and seals. The front seal lip should be packed with grease.  Torque everything to specs. You will notice an increased resistance due to the rear seal but you should be able to turn the crankshaft with a 1/2" rachet handle.

The connecting rods and piston assemblies can be plasti-gaged and torqued and re- assembled one at a time, checking rotation resistance with each final assembly. The pistons will cause a noticable additional "drag". The connecting rod bolts should be torqued and rod bolts stretched to the correct "angle of rotation". If you do not understand the process please inquire it is important! The final assembly will be a bit difficult to turn but it should be turnable with a 1/2" ratchet handle. If you cannot turn it with a breaker bar you have a problem.

There are some things to look for if your assembly is binding. Keep checking rotational resistance at each torque pass on the main bearings. Always be sure to install the main bearing caps in the place they were removed!! Make sure there are no dirt or burs on the bearing caps. Check the thrust bearing clerance. Be sure to press the rear seal into it's groove Trim to correct length as per BBB. (you may want to trim after first assembly).

If you still have too much turning resistance, you may have an improperly machined crankshaft, warped crankshaft, the wrong bearings or size or improper assembly.

Plasti-gage can be purchased at your local NAPA store. You will need the "green".

Take your time and do it right ask a lot of questions if you are not sure, or get some experienced help. It is too much work to do over. Phone me or email me if you need intensive care!

Joe Alexander
Blacklick, Ohio

[Benz Dr.]

This could be a case of have conecting rods that are out of round. I always have them checked and usually have them re sized or the bearings will almost always sieze.
 Completely assembled engine should turn easily using a 1/2 inch drive ratchet and the installed crank should spin very freely with the main seals installed. The real seal has to be cut properly or it will leak and it should have a crush fit much like crank and rod bearings .

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org

[Cees Klumper]

For those unsure about what plastigage is: very thin strands of plastic that are placed between bearing and bearing cap. When the appropriate amount of torque is applied on assembly, the strands flatten out very precisely. The width of the depressed plastic exactly indicates the amount of tolerance ("space") between the respective surfaces. This tolerance is important and is found in, among other sources, the technical data manual (still available from DC).

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[Raymond]

Fabulous Joe!  Thanks, Cees & Dan.  I did find the green Plasti-Guage at a different NAPA store and I have a digital caliper.  So now, I need the specs.  I didn't find them in the BBB.  Did I miss something?  Where can I get them?

I can turn the whole assembly with a 10-inch long wrench and one hand, but it's tight. The resistance feels consistent throughout the rotation.  It's just that in the 30 years since I last built a V-8 for a hot rod Chevy, my dusty memory made me expect this 6 to turn very easily...like with a couple of fingers.

I'll guage all the bearings tomorrow and record the measurements.  I have been using straight 50w oil for the reassembly.  Do you have another recommendation?

Thanks,

Ray
'68 280SL 4-spd Coupe

[ja17]

Hello Ray,
Yes the 50w oil should work fine.

Specs for M130 engine (280-SL)

Std. main bearing journal size = (59.965mm to 59.955mm) 1st repair = (59.715 to 59.705)
Std. rod bearing journal size= (47.965mm to 47.955mm) 1st repair = (47.715 to 47.705)

Clearances are most critical for good oil pressure and engine longevity. Using "plastigage is a good way to confirm correct clearances.

Permissable main bearing crankshaft clearance = 0.045mm to 0.065mm
Permissable rod bearing crankshaft clearance = 0.035mm to 0.055mm
Permissable clearance between flanged bearing surface and thrust surface on the crankshaft = 0.10mm to 0.175mm (this can be checked with a feeler gage.

Permissable piston protrusion from block at tdc = 0.2mm to 0.7mm or on USA versions 0.3mm to 0.8mm

***specs from 1969 Mercedes Technical Data booklet








Joe Alexander
Blacklick, Ohio

[Raymond]

Okay. Riddle me this.  
I started burning oil, but the engine ran smootly even at 5000 RPM on the Interstate. No overheating issues since last year when I blew the head gasket.  I took the crankshaft and connecting rods out of my engine and it had .25 oversize bearings on all.  I re-used my connecting rods and had the crank polished. And used new Kolbenshcmidt .25 over bearings.  My caliper shows the main journals .015 too fat and the rod journals just at the fat limit.  The plastigage confirms the measurement of the mains. On the rods, I get a clearance that is way to tight.  

Caliper only goes to hundreths but seems to measure that accurately.

Am I in the Twilight Zone?

Ray
'68 280SL 4-spd Coupe

[ja17]

Hello Raymond,
Something is not right here. Make sure you are not getting your metric and inch measurements confused.

If you kept your old bearings confirm that they are the same size as the new if you have not ground the crankshaft. You can measure the thickness of the metal at the edge of the bearing to confirm that the bearing sets are the same size.

Normally use the caliper to establish crankshaft condition (diameter, wear, taper etc.).  

Crankshft clearance is what effects the oil pressure and longevity of the engine the most. Using a strip of plastigage across the bearing surface will give you the critical clearance figure. Make sure you are using it properly.

http://www.3sxperformance.com/faq/plastigage/index.asp

Joe Alexander
Blacklick, Ohio

[Benz Dr.]

Sounds like you have out of round big ends on the conecting rods. We're only talking about .001'' clearances for most of these things so there's very little room for wear or error.

Also sounds like the crank journals are .25mm but the rods journals are more like standard. Pistons are stated as over size and bearings are stated as undersize or 1st , 2nd, 3rd repair size.
 
These sizes are usually,
standard, .25 , .50, .75 and 1.00 mm undersize.

Early engines use a thick bearing shell. I wonder if you got a set of those by mistake.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org

[Raymond]

Follow-up

The machine shop finished with my engine parts yesterday.  The machinist guessed that in a previous rebuild, the connecting rods were mistreated when removing the pistons. (FWIW, I used a wooden dowel and pushed on the bottom of the pistons by hand.) As Dr. Benz suggested, a couple of big ends were out of round.  That was enough to cause the problem. The shop cut all of the rods evenly and re-bored them to fit my crank and bearings.  I'll start putting it all together this weekend.  

The paint shop started stripping the first panel yesterday.

Ray
'68 280SL 4-spd Coupe

[Benz Dr.]

Well..... there ya go.  The line boring is done to standard measurements and it's the crank that is ground to a smaller diameter to get the .25 mm  undersize.
Big ends are always honed to standard size as well as piston pin bushings. These are very important steps - you were smart enough to realise that something was wrong and fix it BEFORE you had a problem. I'd hire a guy like you.

It doesn't always matter if you know what you're doing, you just have to pay attention to details and ''see'' things.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org