Forester STI – How to Repair Brembo Caliper Threads

As I showed in my last post, when I replaced a rear wheel stud, Brembo calipers can cause problems when it comes to removing them. This is because they are an aluminium alloy and they are bolted with steel bolts. Galvanic corrosion happens between the two different metals which weakens the aluminium and it sort of fuses to the steel bolt. So when you try to remove the bolt the weakened aluminium thread stays stuck in the bolt thread and pulls away from the caliper. This might not have effected the whole thread but is more likely to effect the threads at the end of the bolt as they are exposed to moisture and road salts. This means when you undo the bolt if the end 2 or 3 threads are stuck to the bolt they then get pulled out through the rest of the thread in the caliper as the bolt is unscrewed which destroys the whole thread. Or worse the bolt gets so bound in that the bolt head just breaks off. So you end up with this:


Rest of the bolt stuck in the caliper.


This is a bolt removed from one of the front calipers which is a perfect example of only the end 3 threads having fused to the bolt but they obviously destroyed the rest of the thread as they were screwed out through it.


The resulting destroyed thread in the caliper.


The best way of preventing this happening in the first place is to grease the threads when they are assembled but obviously that’s not done in the factory. The next best thing you can do before attempting to remove the bolts is to use some penetrating oil like the WD40 branded one I used, applying it repeatedly to the bolts at both ends a few days before if you can. I did this and I’m sure it helped though it clearly didn’t completely prevent the problem. Also applying some heat can really help. I used a heat gun concentrating the heat on the bolt head for few minutes before trying to release the bolt.

If you end up with the same ruined threads in the calipers as many others do then you’ll need to repair them with a thread repair kit usually known as a Helicoil though that is just one of the brand names of repair kits.

With something like this I would always choose to use a kit with a good name and/or from a good supplier. I don’t think this is the time to go for that super cheap eBay kit!

I will also say that a well made repair to the threads with a quality insert, using the right tools is totally safe. This is not a ‘bodge’ fix and steel thread inserts just like these repair kits use are one of the ways threads are made in aluminium if just tapping a thread in the aluminium alone isn’t strong enough. Having said that this does need to be done properly, if the incorrect tap or drill size is used, not drilled correctly or generally performed badly then that could reduce the effectiveness of the repair in how well the steel insert holds. So if you are looking at doing this then make sure you take your time and get it right.

The repair kits I chose were V-Coil. These are a quality kit at a reasonable price.

The thread sizes on the front and rear calipers are different so you’ll need to get whichever size you require or both.

The original threads on the calipers are:
Front M12 x 1.5 x 18mm
Rear M10 x 1.5 x 15mm

The repair kits come with the specific size drill bit required to drill the hole the right size for the tap for the given steel insert. Inserts can be got in various lengths for any particular thread and this length is given as a value compared to the thread diameter. So a repair kit sold as an M10 x 1.5 x 1.0d means the repaired thread will have a 10mm diameter with a 1.5mm pitch at 10mm length. An M10 x 1.5 x 2.0d would be the same but with a 20mm length.

So to repair the Brembo caliper threads I got two kits
Front M12 x 1.5 x 1.5d                              (12 x 1.5 = 18mm)
Rear M10 x 1.5 x 1.5d                               (10 x 1.5 = 15mm)

I purchased them from South Essex Fasteners. They sell through eBay which is how I purchased my kits and the guy I spoke to from SEF was very helpful.

Their website:

Their eBay shop: SEF eBay

Here they are


and this is how I used them.

First the original threads need drilling out to the size required for the tap, using the drill bit supplied. It’s important to drill the hole as straight and in line with the original hole as possible. I chose to fix the caliper to a block of wood making it easier to line up in a pillar drill. Obviously using a pillar drill is best but not entirely necessary. If I didn’t have access to one I’d be happy drilling the holes with a hand drill though it would take some extra care to be sure the hole is drilled correctly.




Nice and square



I used a smaller drill bit that just fitted through the thread to line caliper up first.


Then I switched it for the larger drill bit supplied and drilled the hole


Next I switched the drill bit for the supplied tap. This ensures the tap stays in line as I turned the chuck by hand to begin cutting the thread. Don’t forget to lubricate the tap.



With the thread started I could finish the rest as normal.




With the thread now cut into the caliper the stainless steel insert could then be fitted by screwing it in with the tool provided.





Next the tang just needed to be broken off with a light hit.



That’s one done.


Same again for the other thread in that caliper.



And one of the front calipers.







Sorted. Now they are ready to go back on the car, not forgetting a new set of genuine Subaru caliper bolts and some grease on the threads.



Bolt tightening torque values:

Brembo Caliper Front       75 lb-ft
Brembo Caliper Rear        49 lb-ft
Front Strut Bolts               129 lb-ft
(I found it necessary to remove the lower strut bolt to get access to the upper caliper bolt with a socket)