Hey everyone! I'm looking for the exact dimensions (thread, length, width) of the bottom bolt on an R56 REAR strut. I think I stripped the bolt... so I'm going to have to find a replacement.

Thanks for your help!

 

Unfortunately, you did not strip the bolt, you stripped the threads in the trailing arm. I know, I have done this on both sides. The bolt is steel and the trailing arm is aluminum. The kicker is the bolt is a special "triangular" bolt which is made to hold. It doesn't like to be taken in and out many times. (Yes, the body of the bolt is not round, but a very soft round triangle).

There is a relatively straight forward solution. You need to get the correct size Helicoil kit (comes with a tap, an insert tool, and several steel Helicoils). You need to drill out the trailing arm to the size the kit tells you. Then you tap new threads into the trailing arm using the provided tap. Finally you screw in the Helicoil.

The helicoil looks like a spring and is made to fit into the threads you tapped and have the correct threads for the bolt on the inside. The trick is to drill the hole out straight and tap the new threads straight. I was putting on new dampers so I just used the stock damper as a guide and drilled and tapped right through it.

I had help on the first one when I took the trailing arm off and took it into work where one of the machinist helped me do it. He said that when he worked on aircraft all of their bolts in aluminum were done with Helicoils as it is much stronger than just threads alone. The second time I did it all under the car without taking off the trailing arm.

Good luck and let me know if I can provide any other info.

Cheers,
Greg

 

Just so we are ready, what size heilicoils?

Reason they are stronger, is they make an effectively larger size bolt.
I also have seen a recommendation to put in a through bolt and nut. Not sure about that.

 

The size of the bolt is M14x1.5x105 according to realoem... And I've used helicoils on robots a couple times with good results. I'm not sure about using them in such a high torque area such as the trailing arm... Also, I see myself removing the bolts from the trailing arm a couple times as well.

I'm considering getting a longer bolt and just using nuts/washers, but I'd like to see if others have done the same and maybe a picture or two before I go this route.

 

tvrgeek,
The "Helicoil" I used is actually a brand called "Recoil" I got from McMaster-Carr. It is M14-1.5 and the drill bit you need is a 9/16ths. I think that AutoZone carries the metric Helicoils also.

Cheers,
Greg

 

@NAProf

Do you have any problems torquing the bolt to spec? Removing the bolt?

 

Hey war6763,
I thought about just using a longer bolt with a nut and washer but was talked out of that by my machinst friend. The outer surface of the trailing arm would need to be ground down to produce a flat for the nut and washer to hold on . The trailing arm has lots of ribs and structural shapes and he was reluctant to mess with the structural integrity of the trailing arm.

I've had the right side with the Helicoil for 6 months and the left side for 2 months. No problems. I was also able to torque to teh requried spec (I think it was 120 ft-lbs, but don't remember exactly).

Remember, the vertical load is prmarily being carried by the socket welded onto the lower part of the damper. The job of the bolt is to hold the damper socket into the slot in the trailing arm. The torque spect is to make sure it doesn't back out.

Cheers,
Greg

 

Ohhhhh ok, makes a lot more sense. I was worried that the bolt would try to carry the load... I guess if the bolt were not torqued enough you would expect it to destroy the helicoil though... You do have a point about the trailing arm's uneven wall though. A friend recommended drilling out the hole, putting in a longer bolt of the same spec, and using a castellated nut along with a cotter key to hold it in. I can see why his approach would be better, but you make a very good point about the wall of the trailing arm.

I think BMW would appreciate the helicoil fix much more than the alternative...

I'm heading to autozone to pick up an M14 set now. I might even go lift the car and install them tonight.

 

Do you think a hand drill will be straight enough? Maybe drilling through the strut might work?

 

Actually I used a hand drill both times. A big 1/2 in shank electric drill. Drill slowly and use some oil. My machinist made a collar out of some scrape brass the first time to help me stay in alignment. But between my two events he threw out the collar. I decided to try using the old damper as a guide. I bolted it up at the top and aligned it so that the socket on the damper fit into the trailing arm slot cleanly. I then used some clamps to hold it in place and drilled right through the damper. I also tapped right through the damper. Needless to say it messed up the damper as it would be too big for a good fit on the actual bolt. That is why I used the stock one which I was replacing with Koni FSD's.

Good luck!
Greg

 

Did you have to rebuild the whole damper assembly or did you just use it directly (like, did you just align and clamp everything together)? How much did it tear up the damper? (I care because in case of an accident or a totaled car, I like to have all stock parts on hand to replace.)

Nonetheless it seems like it's worked well! I'll lift the car tomorrow and maybe take a couple pictures of the process. It seems to be fairly straight forward, just like any other helicoil repair. As long as I can torque to factory spec, I should be good!

 

Thanks. Good info to be posted. If one has this problem, it could save a day or so in getting the info to do the fix.

The ONLY concern I would have is the length of the helicoil. The arm has a very long thread. Inserts are not that long. They are stronger per inch, but if you reduce the length, then the total solution could be weaker.

It all comes down to this being a less than optimim design. A bolt in cantlever into an aluminum arm carrying the entire suspension load. Good enough for a street car you never touch I guess. Not the worst thing I have ever seen in a production car. Caroll Smith would cringe!

 

BTY, it is very good to see this thread acknowledge the function of the spacer is to transfer the load into the arm without cold flowing the threads. Another thread has some believing is is nothing but a spacer.

 

war6763,
Yes, I rebuilt the whole strut assembly so I could bolt it up at the top to help get it aligned properly. That way I only needed one clamp at the bottom to help hold it in place. But I have changed out springs and dampers so often that the rebuild took all of maybe 20 minutes.

tvrgeek,
I saw the other thread you are in about this kind of problem. I wish I had seen it last month when it was current. It seems that more than a few people are running into this problem. When I stripped out the first one I felt like an idiot. But when my machinist friend looked at the bolt and explained the odd shape to me I realized that more than a few folks will run into this problem.

As for the length of the Helicoil - I only used one. If you look at the length of the bit in the trailing arm that is threaded it isn't that long. If you set the Helicoil in at least one thread from the top of the hole (as it says in the Helicoil instruction) the other end of the Helicoil is only 3 or so threads from the other surface. I drilled and tapped all the way through the trailing arm so I could see the bolt and the Helicoil when I threaded in the bolt. I think the Helicoil is long enough.

Cheers,
Greg

 

Great info for everyone so we can be prepared. I expect to have my shocks on and off several times before I am happy with the chassis setup. I am for-warned!
Three cheers for the power of owner forums.

 

The helicoil kit should be here tomorrow... I'll lift the car and take some video just in case anyone wants to see how the procedure works out. I'm sure LOTS of people will eventually run into this at one time or another.

 

Helicoils work great! Went in easily enough, was able to torque to spec, and seem to be holding up VERY well! Thanks for the suggestions everyone!

 

So, I've had the helicoils in for about a month and they seem to be holding up very well. I will be lifting the car this weekend to check torque specs, but I don't think there will be a problem.

 

I've used helicoils for 40 years, and they work well, especially in aluminum or other soft metals.

You can buy them in all different lengths, so if you need a longer one, they're out there but it seems to me the standard is three times the diameter of the bolt in length, and that's probably how long the ones in the kit are.

Why does MINI use this odd bolt configuration? Is it just for ease of assembly on the line? Or is there some other engineering reason behind the "triangular bolt"?

 

Triangular bolt is suitable for machine insertion and is self locking. It seems, once. Once is all BMW cares about as this is not a usual maintenance item. Only us crazy people who won't accept their terrible Delphi shocks ever find out about it. 99% of their customers just drive the car. ( I am guessing a high 1% mods here, where the typical is more like .01% mods)

Mine were only at about 80 Lbs from the factory. I torqued them to 100 out of fear from this thread. ( bad pun, sorry). Spec is 120. Without the drill and heli-coils, I was just chicken. I'am wondering if the folks who stripped them put them in with an air gun and then went to torque them? Or does the plating on the bolt embrittle the threads in the trailing arm? So many possible reasons. Bring me 50 arms and bolts ans we can do some experiments.

 

This is all good info... I have had mine out 3 times so far, and with this last go-around about a week ago I noticed while putting it all back together, a lot of shavings comming out of the end of the trailing arm. The werent huge, but just a dust.

They torqued to spec just fine, so im not all too concerned, but it is scary to think about.

Ill add helicoils to my next suspension mod plans

 

The Helicoil instructions say that if you need more thread, you can stack the Helicoils one on top of the other, just break the tab at the end of the Helicoil off with a punch after it is screwed in on top of the first Helicoil. They way they're designed, the threads line up continuous.

I've seen Helicoils installed by the manufacturer on large marine diesels, because they are stronger than drilling and tapping the native metal in a casting.

Dave

Dave

 

Used to be SOP in rebuilding old VW and Porsche cases. Reason being it provides effectively a larger diameter thread, so the force is over a larger area. Why not a bigger bolt? Usually because it won't fit.

 

having the same stripping issue. Anyone have pictures of the helicoil installed?

 

I have used a through bolt on my drivers side rear trailing arm for over a year now. No problems. You do have to grind down the outside of the trailing arm but the area that gets ground really should have no affect on strength.