Hallo friends! Yesterday I while I was replacing the battery, (1989 K75, 212.500 km) I discovered that the battery tray had a free play on the rear part. When I looked a bit closer I noticed that two of the four rubber mounts (the ones towards the rear wheel) although they were secured to the tray they were not fixed to the holes on the gearbox. I dismantled the tray and I realised that for the two front mounts I could properly screw them to their place but for the two rear they were rotating freely like the threads have been striped - no obvious damage though! To be honest I am puzzled what could have caused such a damage. I do not see the reason to take the tray apart when I do the scheduled service to the official BMW dealer, which I do for the last 20 years, except the time, years ago, when I replaced the gear plates and lube the axle. Could have been then? Of course it is not a problem that could live me stranded but I would like to be able to fix it sometime as it puts more tension to the two front mounts and adds up to the vibration. Is it Helicoils the only way, and is it possible to use at the specific location without taking the bike apart? Has it happened to anyone else? Is it me that I don't see something? Thank you!

my guess is the rubbers have failed , the part left in the gearbox can be a bitch to remove , best of luck , replacements are cheap if not BMW origional parts

It is a known issue. I put some thread inserts (helicoils) which are steel or you can enlarge the holes and tap them to 8mm. 3 of the holes are blind so dont drill all the way or you will strike oil. There are a few cars that use those rubber mounts, here in Australia the Holden Commodore is the car which is an Opel so you may get them there.

Amazon has them:

https://www.amazon.com/HOMEKNOBS-Absorbers-Vibration-Harvester-Generator/dp/B08CVGV48L

Thank you all for the replies. The threads on the absorbers are intact - same goes to the rubber part. It's those in the hole where the absorbers supposly fit that seem to have the problem. I guess Helicoil is the way to go. Rick G you say it's common. What on earth could cause it? How many times in the life of a bike you would need to dismantle the tray from the top of the gearbox? And what are the chances that the one time you would have to do it in order to fix something you will overtight them and cause another problem, (specialy if you are an experienced mechanic in a BMW garage)?...And not one, but two of them...symmetricly distributed?

firstle wrote:my guess is the rubbers have failed , the part left in the gearbox can be a bitch to remove , best of luck , replacements are cheap if not BMW origional parts

The rubber doesn't fail. The adhesive between the metal disc and the rubber fails.

The ones used from 94 on have a larger diameter: (34522312621)

Stavros wrote:Thank you all for the replies. The threads on the absorbers are intact - same goes to the rubber part. It's those in the hole where the absorbers supposly fit that seem to have the problem. I guess Helicoil is the way to go. Rick G you say it's common. What on earth could cause it? How many times in the life of a bike you would need to dismantle the tray from the top of the gearbox? And what are the chances that the one time you would have to do it in order to fix something you will overtight them and cause another problem, (specialy if you are an experienced mechanic in a BMW garage)?...And not one, but two of them...symmetricly distributed?

I can't see any reason to remove them from the transmission unless one of them fails.

Stavros wrote:Thank you all for the replies. The threads on the absorbers are intact - same goes to the rubber part. It's those in the hole where the absorbers supposly fit that seem to have the problem. I guess Helicoil is the way to go. Rick G you say it's common. What on earth could cause it? How many times in the life of a bike you would need to dismantle the tray from the top of the gearbox? And what are the chances that the one time you would have to do it in order to fix something you will overtight them and cause another problem, (specialy if you are an experienced mechanic in a BMW garage)?...And not one, but two of them...symmetricly distributed?

From what I see is if the thread becomes lose then due to vibration the thread gradually gets worn away then age catches up with the glue.
I have repaired a few and usually use some blue loctite which prevents them working lose. The problem as I see it is that they cant be made very tight on installation or you risk damage and breaking the bond with the rubber.

Rick G wrote:

Stavros wrote:Thank you all for the replies. The threads on the absorbers are intact - same goes to the rubber part. It's those in the hole where the absorbers supposly fit that seem to have the problem. I guess Helicoil is the way to go. Rick G you say it's common. What on earth could cause it? How many times in the life of a bike you would need to dismantle the tray from the top of the gearbox? And what are the chances that the one time you would have to do it in order to fix something you will overtight them and cause another problem, (specialy if you are an experienced mechanic in a BMW garage)?...And not one, but two of them...symmetricly distributed?

From what I see is if the thread becomes lose then due to vibration the thread gradually gets worn away then age catches up with the glue.
I have repaired a few and usually use some blue loctite which prevents them working lose. The problem as I see it is that they cant be made very tight on installation or you risk damage and breaking the bond with the rubber.

Impressive intuition and experience!
Thank you all so much for your time and thoughts.

The problem is due to the inertia of a heavy battery firmly attached to the battery plate that is attached the rubber vibration mounts. 

When the bike hits a bump the battery pulls on the plate and the mounts.  The constant motion of the rubber mount threads wears away the softer aluminum threads of the transmission.  If the bump is big enough, and the mount is firmly attached to the transmission, the rubber mount is torn apart. 

The only solution is to use a smaller battery that weighs less but is still able to start the engine.  There are two battery sizes I have seen used on the bricks, one is about twice the size(I think it's 23AH) and weight of the smaller one(about 19AH).  I use the smaller one in my bikes because it's lighter and easier to handle.  A bonus is that it's also a couple bucks cheaper. 

They both have enough cranking amps, it's just that the bigger one will crank an engine that won't start longer.

Rick G wrote:

From what I see is if the thread becomes lose then due to vibration the thread gradually gets worn away then age catches up with the glue.
I have repaired a few and usually use some blue loctite which prevents them working lose. The problem as I see it is that they cant be made very tight on installation or you risk damage and breaking the bond with the rubber.

I've dealt with lots of K trannies (probably 50+) and never seen that, only the issue of the rubber breaking off and having to get the metal disc and threaded part out. I'd use anti-seize over Loctite but that's just me. YMMV.

Because of the dissimilar metals, and the location where water can pool and penetrate the threaded hole, I always use anti-seize and wave washers on those vibration dampers.

Point-Seven-five wrote:The problem is due to the inertia of a heavy battery firmly attached to the battery plate that is attached the rubber vibration mounts. 

When the bike hits a bump the battery pulls on the plate and the mounts.  The constant motion of the rubber mount threads wears away the softer aluminum threads of the transmission.  If the bump is big enough, and the mount is firmly attached to the transmission, the rubber mount is torn apart. 

The only solution is to use a smaller battery that weighs less but is still able to start the engine.  There are two battery sizes I have seen used on the bricks, one is about twice the size(I think it's 23AH) and weight of the smaller one(about 19AH).  I use the smaller one in my bikes because it's lighter and easier to handle.  A bonus is that it's also a couple bucks cheaper. 

They both have enough cranking amps, it's just that the bigger one will crank an engine that won't start longer.

I think the battery trays used on the early K models were because the original design was done in the very early 80s and you needed a battery that big to get the required CCAs.  When they did the redesign of the K for the 94+ K1100s BMW went to a "half" sized battery because battery technology had advanced quite a bit during the 80s so a larger, heavier battery was not necessary.

The only advantage I can see that a larger battery has now is that you can let it sit longer as the LCD clock S-L-O-W-L-Y drains it.

Point-Seven-five wrote:Because of the dissimilar metals, and the location where water can pool and penetrate the threaded hole, I always use anti-seize and wave washers on those vibration dampers.

And if the rubber does break off then a wave washer would probably make it easier to get a grip on the disc with Vise-Grips.

Ducks Post 6 has a photo of the mount showing its component parts. 
I had to remove and replace 2 broken battery tray mounts where the rubber had broken away from the bottom washer with its attached M6 thread screw post. 40 years of steel threaded into aluminium leads to corrosion and thread seizure unless anti-seize was applied to the threads when installed. Sharp tapping, heat and free-up lubrication should be the first methods tried to free up a seized thread. This failed for me. The problem of removing the bottom screw post is that the attached washer has a very low profile. Pliers and visegrips kept slipping off. Do not use Easi-out screw removers, you will probable damage the aluminium around the thread in the transmission. I used a 3mm drill to make a parallel slot in the washer drilling from the edge to the centre. Another option would to be to carefully use a Dremel tool with a cutting wheel to cut a slot. I then used a flat blade screwdriver held into the slot to tap the edge of the washer counterclockwise. This enabled me to free the seized post screw and remove it. I've fitted new anti-vibration machinery mounts with M6 thread posts instead of the BMW originals. A huge saving on cost and they are easy to find. If you can only find mounts of a lower height they can be shimmed to height using washers. Apply anti-seize grease to the threads and use a wave washer. If your top threaded post attaching the battery tray is securely held in with a locknut, there is no way your bottom post will come loose. Hope this helps

redrockmania wrote:The problem of removing the bottom screw post is that the attached washer has a very low profile. Pliers and visegrips kept slipping off.

I've always been able to get them off using Vise-Grips. I've only done it with the transmission off-bike though which allows better, easier positioning of the Vise-Grips so that I can get them absolutely parallel to the surface.

I use quality, Vise-Grip brand because they have better steel and teeth that are sharp all the way to the side. I press down on them REALLY hard with my other hand when "locking" and then turning. Sometimes it takes two or three tries but it's always worked for me.