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Dan16V 07-15-2002 07:51 PM

MB DOC and BENZMAC - 16V connecting rod question
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MB DOC and BENZMAC and others,

I'm beefing up my 16V engine after the piston failure (see this discussion), and I've got a few questions about the conrods:

1. The rods have studs instead of bolts (see photo). I haven't read anything in the manual about studs. So, is this the stock 16V configuration or have the rods been messed with?

2. If they are stock, should I replace the studs and nuts? How can I check for stretch, etc.? Since I'm turbocharging the engine and am planning on pushing a healthy amount of boost, should I replace them with aftermarket ARP studs?

3. How much horsepower can the stock rods handle? I'm going to have them stress relieved and shot peened. Is 350 or 400 possible without risking rod failure?

4. If you have any other suggestions as to what I should replace and enhance while I have the engine apart for the turbo build, please let me know!!

Thanks in advance!!

Speedtek 07-16-2002 01:37 AM

con rods
The stock 16v conrods should be good for 400 HP using the ARP rod studs. the studs on there now look stock. Piston weight is another factor. What pistons are you using?

Dan16V 07-16-2002 03:48 AM

Thanks Speedtek. I'm going to order custom JE turbo pistons today if all goes according to plan.

One more question:

How do I remove the studs? Are they pressed in? Can I just press them out?

Thanks guys,

Speedtek 07-16-2002 04:30 AM

You need to order the stud kit first and let a machine shop do it.
sometimes they need to be pressed in and in doing so makes the big end journal out of round. So it will need to be honed to make the big end true. You can not see this distortion, but you will see it when you take apart the motor and notice there are wear spots on the bearing. You could get loss of HP and poor oil control and even worse a spun bearing at High RPM.

Are you assembling the motor yourself?

You will need to plastigage the bearings.

Weigh the stock pistons.
Ask JE what they expect the new pistons to weigh.

What piston rings are you planning to use?

I hope you are balancing this motor?

mbdoc 07-16-2002 08:16 AM

You have a very early EURO engine!! These rods are very heavy!!
They are the same as the 1984 190E 2.3 engine!!
Never have seen a rod failure!!
The newer style rods will take all of the pressure that these rods will & they weigh a bunch less. However the later engines guide the rod from the piston & this can be a problem on aftermarket pistons!
Don't WASTE time & money where it isn't needed!!
Even on the EARLY German touring cars they used stock rods!!
ALL of the pressure is on the down stroke so the bolt only has to keep the caps on.
ARP bolts are good but no better than the OE bolts that came on the rods. Mercedes was one of the first companies that used STRETCH bolt technology & they do work well.

pentoman 07-16-2002 03:59 PM

dan16v - Glad to see your Evo II repl. is getting some engine treatment too!


Dan16V 07-16-2002 05:19 PM

Woah! I knew that the engine had been replaced by the previous owner, but I figured they'd install a US spec engine.

Few more questions, MB DOC:

1. DOES ANYONE HAVE THE US OR EURO COMPRESSION HEIGHT FOR STOCK PISTONS?? I haven't been able to find it anywhere!!! I've measured the compression height of my pistons and would like to compare it to Euro and US values so I can find out what my current compression ratio is. MB DOC, can you help me out??

2. Could this be an 85 16V engine as there are quite a few grey market cars floating around? How can I find out? I haven't been able to find the engine number for the life of me. Where is it located on the block?

3. When did Mercedes switch from crank-guided to piston-guided rods on the 8V? Did that happen in 85 because my manual states that 86-87 16Vs differ from 8Vs in that the rods are giuded at the crank and not the piston, so it must have been changed by then. Did only the very early 8Vs (82-84) have crank-guided rods?

4. Can/should I have these rods lightened? Is that even possible? I wasn't planning on going any higher than 7.5k RPMs. I'm guessing this wouldn't be necessary or advisable and might even be impossible, but I'm curious.

I think I'll stick with the stock studs, as these should be able to handle it fine. Yes, I'm assembling the motor myself, and yes, it's going to be dynamically balanced. I'll probably go with Total Seal gapless rings.

Thanks guys, and whoever can give me the compression height values will be my friend for life! :)


mbdoc 07-16-2002 05:42 PM

On the "euro" 10.5/1 engines the installed height of the pistons ARE the same as the 8V after 1987. Projection is .6 to1.0 mm
Most later euro engines 16V are the same as US versions at 9.7/1 & piston projection is even to 0.2mm.

The 8V rods were changed durning the 1984 production run & only the engine number will identify the change.

The crank guided rods have 24mm piston pins & all of the later engines use 22mm piston pins.

I'm sure that the rods can be lightened before you have them stress relieved.

The engine number is on the block just behind the ignition distributor & down just a bit.

Great looking CAR!!

Dan16V 07-18-2002 08:33 PM

Thanks so much MB DOC!
I measured my pistons, and they indeed are the 10.5:1 Euro versions, so my engine is indeed a euro.
Do you by any chance have the following measurements?

1. Headgasket thickness when compressed. From my calculations it should be around 1.5mm.

2. Block height (from center of crank to block surface). My calculations show 227.5mm

3. 16V head chamber volume. My calculations come up with 55cc.

How close am I?

Thanks again!!

omegabenz 07-19-2002 03:12 AM

Your studs are made by kamax which makes mercedes fastners. So I would say that its a safe bet that they are stock studs.

If you lower your compression you would be doing yourself a favor because if you are going to turbo the engine, you want to be able to shove more air in without making the fuel pre ignite which causes pinging, etc. With a higher compression piston you can put as much boost into the engine. I hope this makes sense.

As said earlier, let a machine shop put in new studs if you are going to do it. They have to be perfectly straight and you can hurt the rod.

Good luck on your project...Try to document everything for when you are done. Try talking to Adam with the 300E turbo, he might be able to give you some good info.


Dan16V 07-19-2002 09:09 AM

I'm going with 8.5:1 custom JE Pistons. That should let me get a decent amount of power :).
Is there any special trick to removing and pressing in new studs? I know that I'll have to have the big end honed. My father has a machine shop, but we don't do automotive work. So we have the equipment, just not the know-how. Any special tips?

I'm still looking for the following measurments:

1. Headgasket thickness when compressed. From my calculations it should be around 1.5mm.

2. Block height (from center of crank to block surface). My calculations show 227.5mm

3. 16V head chamber volume. My calculations come up with 55cc.

Thanks guys!

Speedtek 07-19-2002 01:59 PM

Compression Ratio
How much boost are you planning to run?

What turbo are you using?

I would run 9.2 to 1 Comp Ratio on a 16v motor. 8.5 is too laggy
on the lower rpm scale for turbo. Unless you are planning to run over 22 psi? I can go more into details on this if you want.

Ask JE what the estimated piston weight will be. you will not want to stress the rods more than stock pistons. JE is known for making heavier pistons. THe main thing on turbo pistons is the head is at least 5mm thick on the compression chamber side.
the skirts can be lighter. Also since it is a large bore you can set the piston to wall clearence at .00175" would be optimal. On the .0025" is too loose. JE Pistons made a few years ago would run looser clearence.

I use to use a lot of JE but have switched to ROSS pistons. They are a lot lighter. We have used them on our brand 'H' motors running over 580 HP with no problems.

mbdoc 07-19-2002 05:25 PM

MB doesn't list block height from crank centerline, but from sealing surface to sealing surface. Height when new is 292.5 +- 0.05mm & min height is 292.2mm

Follow piston mfgr for piston clearance! Also think about having the piston tops ceramic coated & the skirts moly coated!!

I tried Ross pistons & they needed .006" skirt clearance!

Dan16V 07-19-2002 06:02 PM

Thanks the info!
Speedtek, I ordered 8.7:1 pistons. I was planning on pushing about 15-18 pounds of boost. Do you think it would be safe to go to 9:1? I could probably have the pistons changed yet. I agree with you that 8.7:1 will be a bit laggy, but I was willing to sacrifice that for more total power. I'm thinking about having the cams reground to cut down on overlap, and that should help low-end out. Which turbo would you recommend to achieve the flow I need? Would a T3/T4 hybrid be good? Which A/R values? A regular T3 (.42 intake, .49 exhaust) would be too small, right? I want to keep the turbo as small as possible to reduce lag and build boost as low in the rev range as possible.

MB DOC, thanks again for your help! I was aware of that height, but wanted the other measurement so I could confirm my compression ratio. I calculated that value to be 227.625, which should be very close.

Thanks again, I'll update you as I progress in this project!

omegabenz 07-19-2002 06:07 PM


8.7:1 is good. I talked to my brother about turboing it. He said you can probably run that to about 10 psi, and 12 psi with an intercooler.

You should even try to go lower. I think that 8.7 : 1 is a good compromise though.

See dan, if you set up your turbo right to spool fast, you wont have to worry about off the line preformance because if it spools fast, then it will take care of the low compression.

If you did 9. :1 then you can only run a few pounds of boost.

Dont grind your cams. And if you grind them how you said to decrease overlap you will actually loose low end torque.

Stick with the 8.7:1. For the turbo....are you going to do a single turbo or are you going to biturbo your car???


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