[NGEE]

Zipping up my sub-frame, on the very last bolt, I appear to have somehow cross-threaded it. One of the three bolts on the passenger side - the short rear bolt. I can't imagine how it happened - I'm always super careful about starting a bolt by hand-threading it.

I got it 1/4 of the way in before it became obvious, and now I don't want to run it the rest of the way in at this point.

Does anyone have experience with this sort of thing? I've never used a tap before, and I don't know if that's what's called for here.

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Solution is in post #12: https://www.e90post.com/forums/showp...6&postcount=12
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NOTE: IF YOU ARE HERE TO RESEARCH DROPPING YOUR SUB-FRAME - READ THIS:

There are DIYs on dropping the sub-frame, and this isn't one of them. However, you will do yourself a huge favor by following this advice.

When dropping the subframe, do this:
The only sure way to avoid buggering the threads while dropping the subframe is to use a proper cradle to hold the subframe tight while you remove the bolts. Also, in post #14, user mainbearing says to:

Originally Posted by mainbearing

"remove these two rear bolts first, then move to the middle ones and finally the front two bolts. Reason is the pressure on those rear bolts when removing the subframe could bugger the threads."

When reinstalling, do this:
The sub-frame bolts should hand-thread ALL THE WAY into the frame bushings. At a sloppy 1.5 pitch, there should be no resistance all the way up. So before you begin reattaching your sub-frame - try it. Just run each one all the way in, by hand. If you have any resistance at all, then STOP. Clean up your threads. You have options:

- soak your bolts in pblaster, then clean them up good with a brass wheel and dry them
- purchase new bolts, they are each less than $10 at the dealer
- purchase a thread chaser set like this to clean the bushing threads: https://www.ecstuning.com/b-schwaben.../006258sch01a/

At this point you should have no trouble running each bolt all the way in by hand. Now you can proceed to re-install the sub-frame knowing that EACH BOLT SHOULD GO IN BY HAND.

Again, using a proper cradle to hold the subframe tightly in place will avoid most problems. The sub-frame is a big, unwieldy hunk of aluminum. If you are like me and have no help and no proper cradle, you'll be using a hydraulic jack to jigger it into place. Because it is so big and unwieldy, even if you get the front sub-frame holes slotted onto the frame bushings, there will be play. That play can cause any of the bolts to bind on the sub-frame hole when you attempt to thread it. If you encounter any resistance, YOU KNOW THAT SOMETHING IS WRONG because you have already verified that they should go in easily by hand. STOP. Make the necessary adjustments until you can thread by hand.

In my case the right rear bolt simply would not clear the sub-frame hole without binding. I got it in 3 or 4 hand twists then met resistance. I made a bad assumption that I was good to go. Continuing to crank the bolt with a wrench, it got harder with each turn - classic sign of cross-threading. At this point if I'd decided to just power it in there, I likely would have met a point of no return - it won't back out or go in further without snapping the bolt. Thankfully I had the sense to stop.

Once I re-tapped the hole until the bolt went in cleanly by hand, I installed, dropped and realigned the sub-frame three times before I finally accepted the fact that the right rear bolt was not going in cleanly without binding.

To solve this, I used this tool to expand the sub-frame bolt hole just enough to gain clearance so that right rear bolt would not bind: https://www.amazon.com/Milwaukee-48-...1079811&sr=8-5

Once you get it aligned so that the bolts thread by hand, you are safe to torque it in.

[NGEE]

So it seems obvious that I'll need to tap the frame. I could use some over-the-shoulder advice from those who have been there.

Step 1

The original bolt, according to RealOEM is M12X1,5X53-10.9. I believe I should start with the correct tap for that bolt, assuming I can just refresh the threads and carry on.

How do you make sure it's tapping straight? Apart from eyeballing it, does it sort of straighten itself after you get it started?

I heard someone talking about a M12x1.5 restoration tap - anyone have experience with these?

Powerbuilt makes a back-tap for spark plug holes - expands into the good threads near the chamber, and cleans bad threads as it is backed out. https://www.ebay.com/itm/Powerbuilt-...-/273136042962 Does anyone know of a tool like this for M12X1.5?

What I'm really curious about, is how the tap treats the good threads (see diagram below). I understand it cleans and re-threads the messed up threads, but when it gets to where the good threads are I'm assuming they won't align and it will rethread those as well? I'm sure that's a REALLY dumb question but I just have no experience with it.

Should I just hire a pro?



Step 2

If that tap/restoration doesn't work, it looks like I'd be going helicoil, but I don't know much about helicoil and don't know what size and whether I'd have to use a new/different bolt.

[IllSic_Design]

You want to use a chaser/follower tap to clean/fix any threads that were damaged. You will need to make sure when you use the tap that it is going in straight, otherwise you will make a mess out of the hole. If you tap it straight then it will just follow/clean up the "good threads" in the hole.

[NGEE]

Originally Posted by IllSic_Design

You want to use a chaser/follower tap to clean/fix any threads that were damaged. You will need to make sure when you use the tap that it is going in straight, otherwise you will make a mess out of the hole. If you tap it straight then it will just follow/clean up the "good threads" in the hole.

Thanks. I'm afraid I don't understand "follower tap". The chaser tap is a specific kind of tap apparently designed to clean rather than cut threads. This makes sense to me, since I don't know how you'd cut m12x1.5 threads in an m12x1.5 threaded hole - seems like it would make a mess. But I don't know how a chaser tap would work on threads that are messed up, and even if it could somehow restore those messed-up threads, I don't understand how it would handle the transition from the messed up portion of the threads to the clean portion of the threads - seems like it wouldn't transition cleanly (see my diagram).

[NGEE]

Another option might be to drill out the messed-up portion of the threads. In my diagram, I could drill the messed up portion to a wider diameter so that I could get access to the clean threads, which would be more than enough to secure the bolt. Any thoughts on this?

[slupie]

You need a thread chaser or restorer. AutoZone or other parts stores might rent these for free. This will clean up the damaged threads without ruining the ones that are still good. If you use a tap, you have a high probability of damaging the good threads since a tap is designed to cut threads no matter what.

https://www.autozone.com/test-scan-a...set/391372_0_0

[nsjames]

do not use a tap or you will probably irreparably damage it.

https://www.amazon.com/Mstc-RC7-Metr.../dp/B000AEX8DW

that's what you need.
either buy a new bolt of buy a thread repair die.
a triangle file is also useful to fix boogered threads on all sizes of bolts but it takes more effort.

that's what you need to try first.
if that fails then you can think about drilling out the hole and repairing it with a timesert kit.
timesert kit will probably cost you 80 bucks or so. at least the last one I bought did.

[NGEE]

Originally Posted by slupie

You need a thread chaser or restorer.]

Originally Posted by nsjames

do not use a tap or you will probably irreparably damage it.

Thanks. I ordered the Schwaben thread restorer set to attempt the repair.

[Welcome to NBA Jam]

Let us know how it goes. This is my nightmare.

[NGEE]

Originally Posted by Welcome to NBA Jam

Let us know how it goes. This is my nightmare.

When my son replaced the bushings on his E36 lower control arms, I told him 50 times: DO NOT CROSS-THREAD THOSE BOLTS==>THEY GO INTO THE FRAME!

He is secretly enjoying all this.

[mweisdorfer]

I'd use a tap & die to clean up the hole & bolt. A "quality tap" will do you good. Irwin would be my choice.

If you take too much meat off the threads of the subframe bolt hole, you'll probably have to look into a Heli-coil or a new subframe

[NGEE]

Originally Posted by mweisdorfer

If you take too much meat off the threads of the subframe bolt hole, you'll probably have to look into a Heli-coil or a new subframe

The sub-frame just has holes in it that the bolts pass thru - so maybe you meant new frame?

Originally Posted by Welcome to NBA Jam

Let us know how it goes. This is my nightmare.

I have solved the problem and got it all zipped back up. Only set me back a week

First I purchased the Schwaben thread chaser set ($32 at ECS). This type of tap does not cut - only cleans. I believe it is meant to be used in a situation where threads are simply corroded but not mangled. Or situations where you can thread it into good threads on one end, and clean the bad threads on the other end as you run it through. Not the case here and I could not get it to bite into anything.

Then I ordered an Irwin tap and die set with a 12x1.5 tap ($100 at Amazon). I ran the tap in, at first just wanting to try and tap the bad threads but not go all the way in and re-tap the good threads. As I suspected, the transition from bad, re-cut threads to good original threads was not clean. So I ended up running the tap all the way in. That worked.

I also figured out why the damn thing probably got cross-threaded to start with. My sub-frame may be warped or otherwise out of alignment. When I tighten the two fore bolts and the two middle bolts, the two rear bolts should line up perfectly with the frame bushings. But while the left side rear of the sub-frame is tight and aligned with the frame bushing, the right side is drooped down. At any rate, when I tried to run the bolt up into the newly-cut threads, it would not go. It was binding on the hole in the sub-frame, which wasn't 100% aligned with the bushing. I would drop the sub-frame and verify that the bolt would hand-thread cleanly. But when I tried to reinstall the sub-frame, it would bind. I actually ended up cutting the sub-frame hole a bit larger using an electrician's step-bit, and then the bolt ran in like a champ.

I torqued all the bolts to 80 ft lbs, except the re-threaded one which I just ran in PDT.

Thanks everyone for the inputs.

[DieselOG]

Strongest thread is the cross thread

[mainbearing]

I am pretty careful with hand threading bolts first. However, one of the rear bolt (one that goes 45 degrees into the chassis) had damaged threads. They felt fine while removing, and I could not understand why the threads were damaged.

I suspected that even with a jack supporting the subframe there might be some downward force on those rear bolts just before coming loose that buggered the threads.

Anyway, can be fixed by a tap and die set $20 at Harbor Freight (https://www.harborfreight.com/carbon...-pc-62832.html). The first turn of the thread was slightly buggered that a milder "thread chaser" would not thread either. I believe it was the M12-1.5.

Therefore I suggest removing these two rear bolts first, then move to the middle ones and finally the front two bolts. Installation iirc Bentley says to install the fronts first. Therefore reverse the order.

[NGEE]

Originally Posted by mainbearing

I suspected that even with a jack supporting the subframe there might be some downward force on those rear bolts just before coming loose that buggered the threads.

Thanks, I updated my OP.

[TyroneShoelaces]

Originally Posted by DieselOG

Strongest thread is the cross thread

As an engineer/precision machinist, that's a type 4 interference thread

Honestly from wrenching on cars for 30 years with my early years building air cooled vw drag bugs and offroading jeeps I have ALWAYS started all bolts by hand. Never assume you have clean non rusty threads. I cringe at watching youtubers and tire techs running bolts in from start with electric impacts. Why everyone think they're nascar pit crew? OP glad you got it sorted.

Does everyone not run a floor jack with cribbing tight to the frame, pull all bolts then lower? This business of jackstands doesn't make sense as mine are always a click too low and no angled section to slide them up snug. If the first twist of the floor jack doesn't have you wondering if the thing is going to come down like an asteroid from space tumbling to the ground then what kind of fun are we having?

[Efthreeoh]

Originally Posted by TyroneShoelaces

As an engineer/precision machinist, that's a type 4 interference thread

Honestly from wrenching on cars for 30 years with my early years building air cooled vw drag bugs and offroading jeeps I have ALWAYS started all bolts by hand. Never assume you have clean non rusty threads. I cringe at watching youtubers and tire techs running bolts in from start with electric impacts. Why everyone think they're nascar pit crew? OP glad you got it sorted.

Does everyone not run a floor jack with cribbing tight to the frame, pull all bolts then lower? This business of jackstands doesn't make sense as mine are always a click too low and no angled section to slide them up snug. If the first twist of the floor jack doesn't have you wondering if the thing is going to come down like an asteroid from space tumbling to the ground then what kind of fun are we having?

uTube is the LAST place I go for vehicle repair advice. And seriously are people actually taught not to hand thread bolts at the start?

[mweisdorfer]

Originally Posted by Efthreeoh

uTube is the LAST place I go for vehicle repair advice. And seriously are people actually taught not to hand thread bolts at the start?

I'm not sure they are taught anything these days. About 16 years ago, Federal Government in its infinite wisdom did sway with auto shop classes & the like, which were for the most part at the HIgh Schools kids attended. You can still attend these classes, but you probably have to get on a bus and attend a regional facility.

It seems every kid will be a mathematician or a scientist due to common core initiatives. They even had special education kids taking Algebra 2, against the advice of 99% of all teachers. That part didn't last too long, but it was painful to watch 🙄

So, unless dear old dad worked with their kid, on Saturday morning, changing the oil or doing the brakes on the family car, it didn't happen!

[mainbearing]

I think the problem happened NOT when threading the bolts back in, but rather when taking them OFF.

Three members (maybe more) seeing the same thing only on the same bolts there was a pattern. I initially thought it might have been a one off mistake putting the bolt back, but no, now I think it was taking them off. Even if the bolts seemed to come off smoothly.

Yes I agree make sure the floor jack(s) are tight to the subframe before removing the bolts. Probably not to the point of lifting the car off the stands (like loading the suspension before tightening bushing and shock bolts).

Anyway remove the slanted rear bolts first and install them last would be my suggestion.

[Dave92N54]

Originally Posted by mainbearing

I think the problem happened NOT when threading the bolts back in, but rather when taking them OFF.

I would say you're correct. Mine did the same. Used a die to clean up the threads and all seemed okay when putting it back together. Guess we'll find out when I drop it again in a few weeks. I ordered new bolts this time, will every time from now on, I know BMW recommends it, but seems many here do not.

[mainbearing]

I did what one member recommended and reused the subframe and V-brace bolts. They seem to be working fine, but if I were to remove them once more then I would put new ones on there next time (hopefully not again).

On our Audis I do use new bolts and nuts as spec. I am not sure why German manufacturers are so keen in torquing to yield. Japanese do not typically TTY and their reliability generally are just better.

[Dave92N54]

Originally Posted by mainbearing

I did what one member recommended and reused the subframe and V-brace bolts. They seem to be working fine, but if I were to remove them once more then I would put new ones on there next time (hopefully not again).

On our Audis I do use new bolts and nuts as spec. I am not sure why German manufacturers are so keen in torquing to yield. Japanese do not typically TTY and their reliability generally are just better.

It's weight savings and metallurgy for the most part. Steel bolts and an aluminum block (N54, Magnesium/Aluminum composite for N52) do not go well together. What happens is electrons will transfer from the aluminum into the steel, weakening the aluminum.

Personally i'd take the much, much lighter aluminum subframe paired with bolts that need to be replaced than an incredibly heavy subframe so we can use steel bolts. I'd have to say as for the japanese, that's only true of older ones. Toyota uses lots of aluminum these days.

It's very interesting to see other manufacturers taking cues from BMW, and the other germans. It's almost like they wait a few years and start doing the same. For example, If you were to set a modern chevy coolant hose next to one of ours, you'd probably think they both came off a BMW. Same with the valve covers, water pumps, etc.