Last week dismantled and replaced leaking rear drive oil seal on the K75. For some inexplicable reason could not find the correct allen key to remove the 8 bolts securing the cover plate. I found that a Torx bit was an excellent fit in the cap screw and thus used it to dismantle and rebuild the drive unit. This left me wondering if a Torx bit, that is a good fit in a cap screw is an acceptable alternative or is it preferable to use the correct size allen key? Is there a correlation between Torx sizes and Metric allen keys? Out of interest what size allen key fits these cap screws? I meant to check before assembling back to bike, but forgot. john

The correct size is 7mm.  I bought a 3/8 drive impact socket version just for that job. 

As for Torx* bits I would use one in an emergency only.  Essentially with the design of the Allen head bolt you are after surface area contact between tool and fastener.  Max contact area is best, and a clean bolt head as well, so try and clean any grunge out first. 

The engineers can probably drone on for hours about design of the bolt, torque value, pitch and all that stuff but for me the main concern is undoing the little sucker, first time.   

Dave.

Why do BMW make it a 7mm head? Because they can Please don't ask why because I seriously doubt that even BMW could give a sensible explanation. They would without doubt give you large amounts of waffle and no ice cream.

As for using a torx bit it's not the best way but if needs be and it doesn't damage the head then all's well.
The first time I did a drive I had to buy a 7mm bit from my local auto shop $$$$$$$$$$$$$$$$$$$$$$$$$$$$$.
I have a very large collection of allen keys and yet no 7mm so they are not a common size.

Odd. Perhaps it's an antipodean thing? Every allen key set I have includes a 7mm.

I have a set that was in a plastic roll till I unrolled it and then swept all the small bits of plastic up and had a look at the keys and from what I could see of the fragile roll there was a 7mm in it but it apparently went walkabout some time ago.

MikeP wrote:Odd. Perhaps it's an antipodean thing? Every allen key set I have includes a 7mm.

Perhaps when the ship passes the equator and it turns upside down, the 7mm says to his mates in the tool roll, "Bugger this.  I'm outa here". 

The poor chap then lands on his head, knocking himself unconscious, and when he wakes up, can't remember where he is.  By this time the ship is heading back to BMW land, so the poor little bugger jumps ship at the next port.  He then lives happily ever after in some backward tropical paradise.    

RIP (Rust in Peace).

What a loverly story, You and rick should get together and tell stories, I'm sure Rossco will forget the scones and drift off to KK land instead

Ah That explains it. My set does not include 7mm, nor I notice does the BMW tool role. John

Perhaps it was designed by the same chap that used to work for Shimano and used a 9mm set screw to anchor the cable on a bicycle rear derailleur?  Why would anyone use such an odd size?  It is the only thing you'll find on bicycles with a 9mm head.

You can get heavy duty 7mm allen keys, they were used on a lot of car brake calipers, notably Ford MkV Cortinas and possibly Sierras.

They come with the longer end encased in plastic.

Found them on Ebay
http://www.ebay.co.uk/itm/SYKES-PICKAVANT-FACOM-7mm-HEX-KEY-WRENCH-BRAKE-TOOL-/380912130643?pt=LH_DefaultDomain_3&hash=item58b021da53