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Joined: 04 Nov 2010
Posts: 155
Location: Ely, Cambs

PostPosted: Tue Oct 31, 2017 1:34 pm    Post subject: 986 Boxster - Replacing Rear Brake Caliper Mounting Bolts Reply with quote

When the time comes to replace the rear discs on your 986 Boxster (and other models inc GT3 if the internet is to be believed) you will need to remove the two hex socket 12 mm full thread cap screws sets that hold the rear brake caliper onto the aluminium housing.

If they have not been recently removed, this is where your problems will probably start.

It could be corrosion of dissimilar metals or the rust of exposed part of the cap screw but whatever the cause it will require a extended 600 mm long breaker bar to undo the cap screw and there is every chance that you by using such force you will strip or severely damage the thread in the aluminium housing.

Whilst I do not know the cost of the housing - it is made by Porsche and I speculate there will be at least four figures in the £ column. Hence you might consider using a thread repair product of which Helicoil (aka Recoil) is the best known.

The cap screw size is M12 x 1.5 - This not a very common thread size, so availability can be difficult.

I purchased one from RS Components (part no 703-7179) on a next day delivery at an approximate cost of £50:00

The kit consists of a 13/64” drill, a thread tap, 10 x M12 x 1.5 inserts and a tap holder/ insert “inserter”.

Internet searches showed that inserts were in theory available in 12mm, 18mm and 24 mm lengths. The kit came with 12mm long inserts and those were what we used as the car was needed for a track day the next day.

With more time I would have sourced some longer ones as it was going into aluminium and Porsche choose to use a long cap screw.

The first job is to drill out the remains of the old thread with the 13/64” drill - but there is not enough space in the wheel arch to get a conventional electric drill in.

You will need a right angle drill.

The right angle drill I had to hand (and I speculate most of those readily available) had a 3/8” chuck which meant I then had to reduce the size of the drill shank so it would go in the chuck.

With the benefit of hindsight I would also change the cutting tip angle from the normal 135 degrees to 118 degrees which results in significantly less “snatch” when cutting into aluminium.

It was suggested by others that if the thread was fully stripped in the aluminium housing then you did not need to use the 13/64” drill to prepare for the “tap”.

In my opinion this leans heavily to a “bodge” or "lack of equipment necessity", whilst drilling out with the twist bit is “engineering” and do not forget we are dealing with the brakes on our own Porsche.

I made a jig to ensure that the thread removal by drilling was accurate - but did not use it as the drill tends to follow the old hole.

Now it is time to use the thread tap. As with most things in life a little lube helps. I used some Ambersil Metal Cutting Compound as I had some. There are specific products for aluminium but it seems a little overkill to purchase some specifically for two cap screws.
I am told that bee's wax is a good aluminium cutting compound.

Put the tap in the handle supplied, make sure it is square to the hole and rotate it a couple of times until it bites and cuts a couple of threads - then back off 1/3 to 1/2 revolution to break off the waste bits as you go. It is ESSENTIAL to back off (1.3-1/2 turn) after every 1-2 turns to ensure that the waste material is cleared.
You should not need to use too much force, it is supplied with a short handle and you are in cutting soft aluminium.

Continue this until the tap appears at the bottom of the hole. Be careful not to go too far down and “bottom” the tap.

I used both a vac and an air line to ensure all traces of debris were removed.

Now you need to screw in the insert and this procedure is more that adequately covered with Helicoil kit instructions.

We now come to the contentious parts - can you reuse the cap screws and can you put some lube on them to prevent further dissimilar metal corrosion?

Search the internet and draw your own conclusions.

Every time the car is serviced I will be ensuring that these bolts can be easily removed. I really do not fancy having to remove a seized bolt, in a Helicoil insert, inside an aluminium housing.

The cap screws were tightened to the required torque and all was well in my world.
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Joined: 14 Apr 2013
Posts: 3715
Location: North East England

PostPosted: Tue Oct 31, 2017 3:09 pm    Post subject: Reply with quote

Lovely little DIY...

996 turbo - slightly modified....
986 S - usually in pieces: http://911uk.com/viewtopic.php?t=112626
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Joined: 16 Oct 2017
Posts: 1

PostPosted: Tue Oct 31, 2017 5:51 pm    Post subject: Reply with quote

Another tip for initial removal:
Soak overnight with WD40 & as mentioned use a breaker bar but once it starts to turn:- Turn half a turn, then reverse a quarter turn. Continue like this slowly until released

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