Replacing a missing con-rod cap - OM603
 

I'm about to put together the engine for my '86 300SDL restoration project that has been 5-6 years in the making. The original problem was a connecting rod came loose from the crank and busted two holes in the block. (I bought it this way).

I have the mating connecting rod caps for 5 of the big ends but I need one more. I know in installation it is important to match up each as they came from the factory by matching the scribes, but what about the missing one?

I have some replacements that a forum member sent me, but I don't know how to select the one to use. By what parameters do I use to make this selection? Weight? Measurement?

mplafleur
Connecting rods are bored on the big end as an assembly, rod and cap. If you cannot replace the entire rod assembly and are forced to swap a cap from another rod, at least try to find one that produces a round hole when assembled. A good machine shop might be able to recondition a rod assembly such as this, within limits, I suppose, but then you'd need to balance all of the rods as a set. If you have all of the rods out of the engine, as this would require, I can't see why you wouldn't replace the capless rod with another complete one. This is not a good place to take chances.
Now, if you were stranded in the outback, and this was your one chance to limp to civilization, that would be different. But that doesn't sound like your situation.

Agree with above advice. You need a complete connecting rod, not replacement end cap. If you don't have a whole complete spare rod, seek professional machine shop advice. If you try to match an orphan cap, you will not end up with a round hole when you torque the cap down- that will make the bearing bind.

Yep, either get a complete one, or just pick a cap and have the entire set reconned and balanced. Its only about $15 each around here to have rods reconditioned, not a big expense in the grand scheme.

I have a line item on my machine shop bill that says ""RR Pistons" for $5 each, although they were new pistons and what I got back from him were the 6 pistons with the connecting rods attached. I never gave them the rod ends.

I looked at the end caps. They were marked 391, 2, 3, 4, and 5. (The sixth was the damaged one.)

However with the big ends, I can't make out from the markings which was which. So I can't match them up anyway. They are marked 203, 5-3, 5-3, 6-3, and two aren't marked at all.

I guess I'll just take them to a machine shop and have them figure it out. I do know they all need to be within 4g of each other.

At least I can start by installing the crank, but after that, I'm kind of waiting on the pistons.

Well, that's mighty strange. Even lowly VW rods are serialized, rod and cap, with a three-digit number so that a set can always be re-assembled correctly. I have an old Maserati engine where even the NUTS on the rods are numbered. That's overkill, I know, but everything on that engine is that way. It's over the top.
Perhaps someone out there can fill us in on just how MB does this, or if they don't.
I don't know specifically how MB rods are done, and don't have any available to look at. My advice is offered from a general point of view, having seen and rebuilt a fair variety of engines. Given MB's reputation for engineering excellence, it seems unlikely that they didn't mark the rods for dissassembly/reassembly, unless they intend for you to throw them away and replace them every half-million miles and call it good.
If your pistons were new, for $5 I'd guess your machine shop didn't do anything more than install them onto your rods. Maybe they re-bushed the small end of the rods, but I doubt it, at that price. And if you didn't give them the caps, they obviously didn't do a full re-condition on the rods. They must have assumed you didn't want it done. The big ends tend to go out of round with years of use. A full recon involves grinding a little off of the mating surface and re-boring the big end hole to make it round again.
And you're going to need to take the pistons off again to balance the rods. What an adventure!

Michael, I have the 603.97 rods in the garage and they're marked 8-2, 8-3, 8-4, 8-5 and 8-6. #1 is stored somewhere else so it's not immediately available. It has a 3 digit code rather than 8-1. IIRC the 603.96 rods were marked similarly. My guess is your engine was apart prior to the catastrophic failure.

I take it you didn't mark the pistons.

Sixto
87 300D

Yup. I knew it wasn't original for a few reasons. One being that it had a #22 head and some paint pen markings on some of the parts. I disassembled the engine 5 years ago. I never looked at the numbers on the connecting rod pieces as it wasn't something I thought I should be worried about. They were just put into a box. I later gave the box to the machine shop. I didn't know the caps weren't in the box until long after I picked up everything from the machine shop.

So all I need to do is take the conrods off the pistons and take them toa shop. The problem I have is that 5 caps from a 603.96 ans 5 from a 603.97.

From the attached picture (603.97 on top and 603.96 on the bottom), they are not quite the same size. the 603.97 is wider. It has a bigger shoulder for the bolt for one. Everything else is the same size. I'd use it, but I'd bet it's not within 4g of the rest of them.

So what I need before I take them all in is one more end cap. Then the machine shop can R&R them.

Anybody have one?

My number one also has a 3 digit code. Mine must have been salvaged from other engines. I now must make sure I have 6 matching caps all within 4g of each other and then get them R&R'd.

More money I don't have... (I work for one of those big US automotive suppliers and we just got trimmed to 32 hours)

Wow, those big-end caps are beefy!

Interesting update.

I read that the spec on the connecting rods is a max 4g weight difference.

I weighed the ten big-end caps that I have on a lab scale.

OM603.96
---------
226.4
223.1
225.7
231.0 <= high
218.4 <= low

Spread of 12.6g (out of spec)

OM603.97
---------
229.3 <= low
229.9
235.2 <= high
231.5
229.7

Spread of 5.9g (out of spec)

I see that the weight spread of the parts are both out of spec, but these are for the big-ends only. I will bring in the connecting rods themselves tomorrow and weigh them. The ones from the 603.97 are much closer and if the heavy one was mated with a lighted rod, they might have balanced. I'm not optimistic about the OM603.96 rods and caps. I think someone rebuilt this engine from a bucket of spare parts and didn't match anything up. I wonder if this is why the #6 big-end cap flew off and destroyed the block.

I have disassembled the pistons from the connecting rods and will weigh everything tomorrow. From this I will see if I can come up with 6 sets of connecting rods with less than a 4g weight spread.

Pardon my redneck, but can't you just grind off material until they are all within 1g of each other's weight?

I probably will have to. But not grind. I don't want to overheat the part. If any part turns blue from heat, it's a throw away. I'll file by hand. I'll pair up halves to get them as close as possible first.

Then I'll take them to a shop and have them them checked out for straightness, twist, bore, etc..

..........not if you're going to remove 2-3 grams.......

I'd use a belt sander.........carefully.........you'd have a difficult time overheating them.