What an idiot...
 

So, when I went to buy the tap, I went to the nut and bolt shop and brought a Head Bolt with me to make sure I got the right one. What I didn't know at the time was the guy sold me the wrong tap. The head bolt thread is M12x1.75, but what I got for a tap was M12x1.50. This caused the problem and has now destroyed my engine. I think I may have to go with Time-Certs or Helicoil to see if I can salvage it. I'm pissed, but more pissed at myself for not double-checking the tap size. I'm thinking about a small claims suit against the nut and bolt shop. First things first though, the repair and recover.

Oh man, this is painful to hear !!!! good luck on this man....

Question: is it possible to get custom head bolts that would match the new thread caused by the wrong tap you used? does anyone know if this is possible? it could make repairing this problem easier....maybe some of those aftermarket ARP studs/bolts or something like that could work......even some regular non-stretching bolts.....you always re-torque them after a couple of hundred miles ....

good luck

Section 01-1220 in the manual details how to repair the threaded head bolt holes. It says to use heli-coil and gives specific directions.

I don't know if there are larger bolts available or not. Likely not.

The owner of the nut and bolt store is telling me that he's never heard of an M12x1.75 thread pitch head bolt. I'm going to bring the pages from the manual to show him.

He asked that I stop by to talk with him so we can resolve this. We'll see how it comes out. More later.

The owner of the Nut and Bolt shop provided me with what I need to perform the repair (free). Though it's not really free. And he sent me to a local machine shop to talk with the guy there. He said NOT to use the head bolts again and said heli-coil would hold the torque just fine. Not having any more new head bolts, I decided to measure the old ones. Each one measures between 160.5 & 161 mm, which is well within spec. So, I will be using the old head bolts.

More later as this progresses. Hopefully my next post will be to say it's done. We'll see.

pitch the old head bolts and get new, you compromised the threads on the old ones

don't cheap out now, they aren't that expensive

If there's anything I haven't done on this job is cheap out.

The new bolts that I bought were compromised. The old ones were not. Therefore, since the old ones are within spec, I'm going to use the old ones and pitch the new ones. With the exception of two, which I'll cut off the heads and use as guides for placing the head carefully on the block.

Complete...
 

Nine weeks later, the automobile is running great. To bring closure to this repair and to post what I learned through the process, here are some things from the "what it's worth dept."

Note to self; Head bolts are M12x1.75x160mm. Second note to self; Be sure to check the thread pitch of the tap.

It was recommended to use a mag drill to be sure the drill and tap were on centerline, however not having such equipment, I bought a 1/2" drill with a side handle and used 3 different size drills so as not to lose the hole centerline and to step up to the size necessary for the heli-coil tap (as recommended by the local machinist). I also used a large socket over the bit to keep it at the proper depth. The challenge was keeping the drill straight while it was cutting. I checked the tap several times from the front and sides to be sure it was on centerline as I cut the new threads. If the threads ended up off centerline, there was very little variation, because the bolts screwed in without much resistance.

Others were emphatic about taking the manifolds off with the head, however, having the luxury (if you can call it that) of doing the job twice, I can tell you it is easier to remove the manifolds separately. Though you don't have to remove the intake, you should remove it after the head is off to access the little hoses under the manifold and to clean the egr tube, plus the water pump is a whole lot easier to replace (if it needs it). The second time, I removed the exhaust manifolds, unbolted the intake and removed everything necessary and was able to lift the head off by myself.

Then, using the old head bolts (with the bolt heads cut off and a slot cut in the top for a screwdriver) as dowel guides, the reinstallation of the head went a whole lot smoother because there was no chance of damaging the head gasket because the head slid down straight and flush. What I had to do though was remove some of the plastic firewall stuff to allow room for the head to slide straight down. Once the head was in place, I unscrewed the home-made dowel pins, installed the head bolts and torqued the head to spec. Also, this time around I used Copper Spray-A-Gasket as additional insurance.

I didn't know this, but when the head bolts failed, the exhaust system must have filled with coolant and oil. This made for a very interesting evening as I filled the neighborhood with smoke for about an hour. No one called the fire department, which was a good thing. The smoke from the tailpipe didn't seem to want to go away after an hour of running and in fact this morning I was a bit worried because it was still putting out some significant puffs. The oil level wasn't diminishing nor was the coolant level, so my wife and I decided to take a country drive and after about 40 miles, the smoke diminished and then finally stopped. Hooray!

The saga has ended. Praise God.

Congratulations! What a sigh of relief. I know you're glad to get that behind you.

I am now going home to check the head bolt tap that I bought to make sure it is a M12x1.75. I used a 1/4" drive ratchet as a tap handle and plenty of oil when I chased my threads. Some of them were kinda tough during the chasing but no metal shavings came out.

How did you make enough access for the drill motor to clear to get at the rear-most head bolts? Just by removing the plastic stuff - that was enough?

Knowing it's behind me is a sigh of relief, however, I'm still stressing over a couple of things... The cooling system won't build pressure, so I'm thinking the radiator cap failed when she blew all the coolant out through the overflow hose. And there's oil floating in the coolant plastic tank, not much, but enough to warrant a flush. The other thing is... I asked every parts store I knew for longer heli-coils in the M12x1.75 size and they all said, "nope, don't have that size". Well, I found out - after the job was done - that there are longer heli-coils available. So, now I hope the ones I have in there hold. If this were a car with an M103, I'd be selling it, but have no intention of it. If she breaks again, I'll be fixing it again.

I was concerned that I wouldn't have enough room for the drill and had no choice but to remove the plastic firewall pieces to allow access. If the wiper motor had been in the way, I would have removed that too. Removing the engine was not an option at this time. I suppose I could have jacked up the tail of the transmission to give more clearance for the drill, but didn't have to. I did however have to remove the side handle from the drill to allow room.

Back in the late 70's and early 80's I was a mechanic then I got into appliance repair and now I'm teaching back-up generator repair and basic electricity. So, even for those of us with mechanical experience I must say this repair can be difficult and is not a somthing for newbs.

The lessons learned here (for those just reading this thread) are this; just as it takes all of the ingredients to make a cake, it takes all steps to be done correctly to ensure that the job is done properly. One mistake with the cake ingredients and you get soup. One mistake with thread pitch, you end up with near disaster. Check and double check everything yourself.

Thanks. Excellent, excellent bits of wisdom.

As for your oil in the cooling system, remember that the system ran with oil in it for awhile, so there is now an oil film spread throughout all portions of the entire cooilng system: in all the hoses, the heater core, radiator, water jackets, etc. I bet you are seeing this start to break loose and float up. So MB makes a chemical to get rid of the oil. There are two chemicals aactually - one is an oil removal chemical and the other is a citric acid chemical (I assume that one is for rust and hard water scale deposits). I have the oil removal stuff at home so I don't have the MB part number here with me. It was about $12 for the bottle. It is a powder you mix with water, dump it in, run the engine, then drain it. I tried to find the P/N doing a web search but no luck.

It would probably be wise to disconnect the heater core and blow a bunch of water through it and then blow air through it also. There is a fitting that they make that you can attach an air hose to that will enable the blowing out of the heater core. You could probably rig one up easy enough. Me, I am replacing every one of my hoses ($$) since a rubber hose becomes structurally compromised when it is exposed to oil in the cooling system and I don't want a hose blowing up on me.

I'll find the oil removal chemical P/N and post it up here for you.

Found it. The de-oiling chemical is P/N A.001.986.21.71, 500g container (mix 50grams/liter). The citric acid powder is P/N A.000.989.10.25, also
500g container. Happy chemickaling!

Thanks! I'll stop by Arrowhead Mercedes today and see if Don has this in stock.

Regards,
Michael.