w123 A/C bracket: all 3 bolts broken - PeachParts Mercedes-Benz Forum

whyameye · #1 08-22-2014, 11:51 AM

1985 w123 300D: All 3 bolts holding the A/C bracket to the motor housing have broken inside the threaded holes of the motor housing. Focusing on one hole for the time being, I first tried a left-handed drill bit set. When the drill bits offered little hope I redrilled the hole with a regular bit and tried a screw extractor. I can get the screw extractor to bind but I can't get the broken bolt to budge. Eventually the extractor breaks free somewhere around when I have used all my strength up anyway trying to move it.

The bracket is fine. I'm not sure how this happened but I don't know the history of the car and wonder if a couple of these bolts might have broken before I owned it.

While I'm working on this the compressor is being supported by rope and bungee cord. The compressor is fully charged with R12 and functional; A/C was great in the car until this happened.

Any advice is welcome.

-John

leathermang · #2 08-22-2014, 01:29 PM

The left handed drill bit is a great tool...

once you have a clean hole....

use a LEFT hand tap ... might need two.... ' gun ' type.. easier to get started true...
then a ' bottoming ' one to give you enough threads to engage.

then put a left hand threaded bolt into the broken bolt. As you tighten it will unscrew the original bolt. Use high grade bolts ....like grade 8...and research the pilot hole size carefully...

t walgamuth · #3 08-22-2014, 01:37 PM

Wow. What a difficult situation. No room to work, is there?

If I had this problem I'd take it over to my best friends Machine shop and ask him to do it.

That one that is drilled out but off center looks a bit problematic to me.

Good luck!

BillGrissom · #4 08-22-2014, 02:27 PM

The last photo of it drilled off-center is the biggest problem. If you can get a centered pilot hole drilled, that is a big step. My best luck has been in drilling until I get just shy of the (internal) threads in the block, then either folding the shell of remaining bolt inward, or removing it by tapping the hole (like thread-chasing).

I don't like screw extractors because they usually never work and they can break off in the hole, leaving a hardened piece you can't drill out. I have a buggered-up valve cover hole in my 65 Newport's engine like that right now. I suspect a screw extractor works best if the bolt broke from over-tension or bending, not when it broke in torque because bound by rust (usually my problem). The former is your problem, so you have a chance.

Many people have had success by welding a rod to the exposed bolt. This probably works because the heat helps loosen the threads and the rod welds to the bolt since it gets hot first, while the cast-iron doesn't bond easily.

Good luck. No idea why the bolts could have broken. They even seem overly large for that application. Perhaps they were over-torqued when installed and just waiting to break.

whyameye · #5 08-22-2014, 03:08 PM

I wish I hadn't missed so badly in drilling that hole offcenter. I didn't think it looked too bad when I first drilled it, but as I moved up drill sizes it seemed to get worse. I tried to stamp a center before I drilled but I couldn't move a hammer in the tight quarters.

To try to "fix" that hole, would a dremel cutting tool work, or will I just cause more trouble?

I'm thinking to try drilling until I get just shy of the (internal) threads in the block, as BillGrissom suggested. With that off-center hole that will be a challenge. And with the others not having holes at all yet...maybe my luck (and skill) will change. :-)

I'm having trouble finding left-handed taps or left-handed hardened bolts in town. I may also try the welding technique --- a friend of mine has offered to help with that. I'm a bit nervous about the tight quarters and the flammable liquid residue on the engine...

-John

leathermang · #6 08-22-2014, 03:15 PM

Those bolts might have been in there 24 years.
' Screw extractors ' are terrible... do not grab properly...and put pressure outward..
If you can weld a nut to the bolt ( there are copper shields for not welding to the head or block ) use ' high alloy ' rod.... pretty small... and a little hot...for cutting through the slag you are putting into the hole...does not need to look good...only be strong.

If you want to take out that offset drilled bolt.... you can get ' drill stem tube' I think it is called... get the size of the threads... it is hollow.. they you make some teeth on it....and cut around the bolt.. or at least into the threads...

whyameye · #7 08-25-2014, 03:48 PM

I think I'm nearing a happy ending. By undoing the oil cooler hoses I had a bit more room to work. I also sprayed the holes with PB blaster several times over a couple of days. Then with the two broken bolts I hadn't drilled into yet I was able to get them out with a left-handed drill set; as I moved up in drill bit sizes at some point they both came spinning out. The m8 threads in the holes were intact.

The 3rd hole, the one with the off-center hole that I drilled earlier, has been a bit trickier. I finally drilled it out completely with a 5/8" drill bit and tapped it with a 3/8 x 16 tap --- first the "gun" type then a "bottom" type.

The bracket now now mounts. The 2 m8 holes work great. The 3/8" hole binds just a little with the bracket on (I probably didn't get the threads perfectly aligned with the bracket hole) but seems to snug up OK. I bought grade 8 bolts for the bracket.

My question is how much torque to put on the 3/8" hole and if I should use threadlocker blue. I'm concerned that, given my lack of experience tapping and the difficulty of lining things up in this tight area, the threads aren't perfect. I'm worried if I snug it up as tight as I might for the factory-made m8 threads, I might strip it through. Is this a legitimate concern or should I just "go for it" as it should be a strong hold anyway? What would be a reasonable torque to put on a mounting bolt like this? And, would threadlocker blue help me justify not putting full torque on it or am I just making a mess for later?

Thanks in advance,

-John

leathermang · #8 08-25-2014, 03:58 PM

Wow, you really listened to my suggestions...
The left hand drills are just wonderful... it worked just as it was supposed to.. as you get the pressure off the threads.. it just backs out...

Fine thread compared to coarse thread will affect the proper torque..
Which is it ?

Don't bother with the blue locktite... use the red... stud and bearing mount...
There is not that much difference..... in take apart pressure needed.... and this is a high stressed place you do not want to come apart... ' stud and bearing mount' sounds almost permanent... it is not... but the blue is underpowered for where this is and what it is holding.
It also makes a difference in just how clean you get the hole and bolt ...and put it in the hole and one the bolt as you screw it in.

Something else that makes a difference... is make sure the surface you are pulling up against is level with the bottom of the head of the bolt... and you can use a good grade 8 washer under it for good measure... you do not want off center pressure on the bolt...

Not many people have the gun and bottoming... so I am going to ask this question...
do you know which percentage of thread you will have .... that can make a difference also...
I am looking for my little book on this ... but you can probably find it on the net also..

greatsuccession · #9 08-26-2014, 05:48 PM

Does anyone know what the bolt dimensions are or where to find them? I have two broken bolts and the compressor is just rattling against the bracket. I've looked all over the web and I cannot locate the info.

Thanks,

Stacy

whyameye · #10 08-26-2014, 10:50 PM

Great Succession: Sorry I'm just seeing your post so I didn't measure the bolts for you when I had them out. If you are talking about the bracket bolts to the engine, I just brought the bracket into the hardware store and matched up the lengths that way. They are all m8s with 1.25mm thread pitch. If your bolts are broken like mine were you're going to have to take the bracket off anyway, so you can bring the bracket in and match the lengths that way.

If you are talking about the bolts from the bracket to the compressor, they are M12s. One is an M12x160mm and one an M12x170mm. I don't know the 3rd but it is the shortest. I also don't remember the thread pitch but I'm willing to bet it's 1.75mm. If you need I can figure that out from a nut I have that fits the bolts.

Anyway, my story has a happy ending. The bracket is mounted, torqued up, and secure. The compressor is also reattached (I had to get another M12x160mm bolt for that --- it only had 2 of the 3 bolts.) Everything is solid, no rattles, no belt slipping, AC is nice and cold again. The biggest hassle in reassembling were the oil cooler hoses and their brackets --- messy and hard to tighten inside the engine compartment.

Leathermang: yes I did do my best following your suggestions and thanks for offering them. I used a course thread grade 8 3/8" bolt so I was able to Google that to find the torque. The bolt torqued up just fine. I also used the grade 8 washers like you suggested. You asked: "do you know which percentage of thread you will have?" I'm not sure what you are asking?

leathermang · #11 08-27-2014, 12:10 AM

Since you had the gun and bottoming taps... I thought you might be a machinist ... or enthusiast... taps can be bought with either very tight or looser... tolerances with regards to the bolt you are putting in the hole...
and since tightening torque is really an approximation based on the resistance to the turning of the bolt.... not an actual measurement of the ' pull down ' power being exerted on the item...
off the top of my head... like 80 something percent... or 90 something percent...
The 80 of course would strip out sooner if you are too aggressive... and the ninety.. which is not the norm... would require more ft lbs on the torque wrench to get the same ' pull down ' power.... due to its increased resistance....