It's happened. In hindsight, I should have stayed away from that bolt.
I've been going thru my newly acquired 87 K100LT and it's been good so far. While in the processes of changing the fork fluid I found a drain bolt on one fork stripped.
I see there are three options I can try before I ....order a used lower fork section.

I'm going to try a slightly longer bolt before I go with the other two options which is to drill out and install a helical and the other is to drill out the threads on the lower section of the fork and use a tap for the next size bolt and step up in the bolt size and the washer.

I've read that there is a valve in the lower section of the fork that I need to stay away from. Does anyone know what length M6 bolt I should try before going to the other two options? Without looking at the OEM bolt I'm assuming it is a fine thread bolt.

I did the same on my '87 RS so I bought a Helicoil kit and brought it back to OZ with me on one of my visits, rethreaded the hole using the Helicoil spring thread and the stock drain plug screw. Easy fix.

It's 6mm x 1.0

It has never leaked.

If a replacement is needed: BMW part number 
07 11 9 913 408 screw 6x6mm. The crush washer is a 6 x 10 and part number 07 11 9 963 030. You can't go any deeper with a new screw than 6mm, maybe 7mm, or you will make contact with fork internals.

I have no experience with helicoils. Is the 6mm x 1.0 in your reply the Helicoil?
If I'm not mistaken, I would need to do some drilling to install the Helicoil right and should the helicoil be stainless steel?

DanT56 wrote:I have no experience with helicoils. Is the 6mm x 1.0 in your reply the Helicoil?
If I'm not mistaken, I would need to do some drilling to install the Helicoil right and should the helicoil be stainless steel?

6 in millimetres, 1.0 in thread pitch. It's not fine but standard pitch for a 6mm screw. The above image is a typical Helicoil kit. You drill out the damaged threads in the fork leg. You re-thread the enlarged hole with the provided tool. You screw the spring-steel threads into the newly enlarged hole with the provided tool. In some cases you snap off a tiny end bit of the new threads, then re-insert your original - undamaged - screw with a new crush washer. Voila!

If strength is a concern, years ago I was with an old time mechanic after hours at work and worried about him using Helicoil inserts in my recently freshened, dual-plugged cylinder heads in the now stripped out spark plug holes. I wanted them welded up and re-threaded. He turned to me and asked if I'd ever studied a helicopter's aluminium frame. I hadn't. He stated that every screw holding on a part in the main frame is inserted into a Helicoil-like device. I nodded in agreement. He performed the task. Those four spark plug thread inserts are still functioning two decades later.

This is excellent and unexpected. Thank you.
Now I feel like I can tackle this and hopefully have success doing it.

DanT56 wrote:This is excellent and unexpected. Thank you.
Now I feel like I can tackle this and hopefully have success doing it.

Yeah, it's a straightforward procedure. The Helicoil kit will inform you of what size drill bit to use. I did not remove the fork leg. I did it in situ, and used wheel bearing grease on the tip of the bit to capture any stray aluminium particles. I was wanting to do a regularly scheduled fork fluid change so did that upon completion of the thread repair.

Look around a bit as Helicoil kits are not inexpensive, tho' cheaper than the alternative of locating a good, used fork slider, which itself may have compromised threads or other issue.

Good luck.

You'll need a 6.2mm driil. Better buy a kit complete with the drill, tap, driver, snap tool and helicoils. Example:

https://www.ebay.co.uk/itm/BRAND-NEW-6mm-Helicoil-Type-Thread-Repair-UK-Seller-Freepost/274600538112?hash=item3fef77b000:g:G0UAAOSwIqlfyfHJ

You will also need a tap wrench if you don't have one. Example:

https://www.ebay.co.uk/itm/FACOM-Professional-RATCHETING-T-HANDLED-TAP-WRENCH-For-Taps-up-to-6mm-830A-5/131312899794?epid=1445860910&hash=item1e92dbd6d2:g:OGEAAMXQxzZRcd4T

Having a set in the wokshop is a damn good idea because I can (almost) guarantee that the fuel rail threads will strip at some point. Low Flying Brick has around a half-a-dozen helicoils holding the cam cover on, plus both fuel rail mounts. It's an easy enough job if you follow the instructions. When drilling out the remains of the old threads, let the drill lead itself in and it should go in parallel to the hole. Most importantly make sure that the tap goes in exactly perpendicular to the hole.

Dai wrote:You'll need a 6.2mm driil. Better buy a kit complete with the drill, tap, driver, snap tool and helicoils. Example:

https://www.ebay.co.uk/itm/BRAND-NEW-6mm-Helicoil-Type-Thread-Repair-UK-Seller-Freepost/274600538112?hash=item3fef77b000:g:G0UAAOSwIqlfyfHJ

You will also need a tap wrench if you don't have one. Example:

https://www.ebay.co.uk/itm/FACOM-Professional-RATCHETING-T-HANDLED-TAP-WRENCH-For-Taps-up-to-6mm-830A-5/131312899794?epid=1445860910&hash=item1e92dbd6d2:g:OGEAAMXQxzZRcd4T

Having a set in the workshop is a damn good idea because I can (almost) guarantee that the fuel rail threads will strip at some point. Low Flying Brick has around a half-a-dozen Helicoils holding the cam cover on, plus both fuel rail mounts. It's an easy enough job if you follow the instructions. When drilling out the remains of the old threads, let the drill lead itself in and it should go in parallel to the hole. Most importantly make sure that the tap goes in exactly perpendicular to the hole.

As he's in America Land where metric is still almost frowned upon as spawn of Satan and socialist (I kid, I'm a kidder), he'll find that a 1/4" bit does the job for 6mm re-thread jobs since a 6.2mm bit will be hard to find. Also, that square-section top tip of the threading tool is common enough for household hardware tools bit holders to fit. If doing a lot more Helicoil repairs the proper tools, as Dai has shown, are highly recommended.

Also, to do a proper threading job on a blind hole or in a place where there is an obstruction behind the hole like a damping valve, a set of taps including a "bottom" tap is necessary to cut threads all the way to the bottom of the hole.  The better sets will have three in them.  A "starter", "intermediate", and "bottom" taps.  It's not a bad idea to search out a set whenever you need to tap a hole.

The starter has a long lead that helps get the tap straight in the hole, the intermediate cuts the threads into the hole and the bottom tap carries the threads all the way to the bottom, just as it's name implies.  Each one does an important part in getting the hole tapped properly.

These are important, for example, when tapping for a cylinder head bolt that requires thread engagement all the way to the bottom of it's hole in order to hold the torque on it.

Thanks for all the info and pointers on the helicoil and the proper way to drill for them. 

I may have lucked out this afternoon by getting the next size M6 bolt and cutting a slight bit off the end of the bolt. I placed a new M6 washer and some lock-tight on the bolt and was able to get it good and snug using a 10mm wrench. I didn't bother with the torque wrench but feel it wont give me a trouble. I'll know tomorrow how good it is when I but some fork fluid in it.

I'm also going to get one of those helicoil kits as a back up and it sounds like I may need it in the future.