Correct Socket for '91 K100 Cylinder Head Bolts?

[ivangt]

Hi everyone,

I'm currently working on my 1991 K100LT and I’m removing the cylinder head to replace the head gasket, clean everything up, and lap the valves.

I’ve run into a bit of a snag trying to remove the head bolts. I tried using a T50 Torx socket with a Milwaukee mid-torque impact wrench (rated at 600Nm breakaway), but the bolts didn’t budge. The T50 fits pretty snug, but I’m hesitant to apply more force since I don’t want to risk rounding the heads if it’s not the correct tool.

From the shape of the bolt, it looks more like a Polydrive/Ribe, maybe an M10, since the splines are more rounded than a Torx. I’ve searched online, in both the Clymer and BMW manuals, but haven’t been able to find a clear answer.
Does anyone know for sure what the correct socket is for these head bolts? I’d really appreciate any confirmation or tips on how you’ve removed them safely.

Photo of the bolt head attached for reference.

Thanks in advance!

— Ivan

bolt.jpg (33.56 kB . 432x576 - viewed 551 times)

[daveson]

If you get a closer photo in focus, that might help, but I think it says ribe on the bolt head.

[ivangt]

WT..! You’re absolutely right! How on earth did I miss that? You’ve got a sharp eye, Daveson — seriously, thanks heaps! Turns out it is a Ribe after all. I still can’t believe I couldn’t find that info anywhere, even after digging through manuals and forums.

Here’s a photo with better focus — the inscription stands out clearly now.

Anyway, cheers Daveson— really appreciate the help!


bolt2.jpg (26.51 kB . 432x576 - viewed 605 times)

[daveson]

My eyesight is a bit crappy.

I used the force.

And I used the zoom, which is better than 20 20 vision, I think.

[frankenduck]

It's a regular Torx head. I've removed a few K1100 heads and those are REALLY stubborn bolts.

[Laitch]

It's a regular Torx head.

It's likely a Torque Plus. Mighty Gryphon may have written here about his encounter with this variant. Here's a excellent photo essay showing differences encountered by somebody on a Bimmer forum.

[The Mighty Gryphon]

The best driver is a Torx Plus which has different geometry to get a better contact with the screw's head.  The drive contact angles were changed from 15 degrees to 0 degrees to reduce rounding off and camming out.

I doubt that the screws on our engines are Torx Plus since the Plus pattern wasn't introduced until 1990 when the original patent ran out.  However, the Torx Plus drivers are backward compatible with earlier fasteners.

I suspect that almost any large Torx driver made in the past 25-30 years is a Plus by default.  To be sure, look at the size marking which should have a P suffix.  I just checked mine and it is marked Torx 50 IP.  I suspect the "I" designates it is suitable for an impact driver as it is also Parkerized instead of chromed and was advertised as for impact use.

Hope this helps.

[Laitch]

Here is a short course about Torx for anybody who is interested.

[ivangt]

I doubt that the screws on our engines are Torx Plus since the Plus pattern wasn't introduced until 1990 when the original patent ran out.

Thank you very much, Mighty Gryphon. Exactly! That’s why I ruled out the Torx and Torx Plus options. This thing is messing with my head. Plus, the fact that it says Ribe on the bolt head, it has to be a Ribe socket. No way around it.

Here is a short course about Torx for anybody who is interested.

That is great! Thanks Laitch

It's a regular Torx head. I've removed a few K1100 heads and those are REALLY stubborn bolts.

Thanks frankenduck No issues with deforming the head when you took them off?

[The Mighty Gryphon]

Just watched the video.  Boy, do I have some bad information! 

So it appears my '92 4 valve K100RS had Torx Plus screws in the head.

[daveson]

Plus, the fact that it says Ribe on the bolt head, it has to be a Ribe socket. No way around it.

Why do they always make simple things complicated. There's the possibility that Ribe is a company name and that they also make bolts for other fastening system's as well. And they wonder why we need to take a ride every now and then.

Good video, thanks Laitch.

[ivangt]

Why do they always make simple things complicated. There's the possibility that Ribe is a company name and that they also make bolts for other fastening system's as well. And they wonder why we need to take a ride every now and then.

Yeah, it’s ridiculous. I did some research and as far as I can tell, Ribe only made bolts using their own fastening system. The puzzling part is that the shape of the fastener head shown on their site looks exactly like the one on the BMW bolts, but when I looked at photos of various Ribe socket sets, the profiles didn’t seem to match. I bought one anyway just to try it out. Today I received a Polydrive M10 and it doesn’t fit - it's too big. I’m starting to think it might actually be an M9. I’ve got a Ribe socket set arriving tomorrow that includes an M9, and I’ll try that next. I'm determined to figure out exactly which tool BMW actually used for this, the specific socket they used at the factory on these bolts.

[frankenduck]

Just watched the video.  Boy, do I have some bad information! 

So it appears my '92 4 valve K100RS had Torx Plus screws in the head.

I've removed 4VRS head bolts with Torx bits. It just took a boatload of torque (I used a cheater bar) and a bit of swearing.

[The Mighty Gryphon]

A very big cheater. 

I broke one of the head bolts.  Holy Mackerel, was that a pain in the butt.  If I ever had to do it again I would definitely use an impact wrench.

[ivangt]

If I ever had to do it again I would definitely use an impact wrench.

That’s actually what I tried. I used a T50 impact-rated socket with an impact wrench rated at 600Nm, and the bolts didn’t budge at all. That’s a lot of torque, so it made me stop and rethink the whole approach. It also made me question whether I was even using the correct socket. I didn’t want to push beyond that with more torque on an impact. If 600Nm isn’t enough, something’s not right.

After that, I decided to dig deeper and wrote an email to BMW Group Classic in Munich. For those who don’t know, they’re the division within BMW that manages heritage vehicles, including classic motorcycles like the K-series. They maintain the historical archives, technical documentation, and original service data for older models. I reached out to ask if they could confirm which exact socket is required for the K100 cylinder head bolts, so we can finally clear up this long-standing question once and for all.

We really need an answer from an official source. There’s no mention of the correct tool in any of the service manuals, and nothing definitive in the forums either. It’s time to get a confirmed answer straight from the people who built the engine.

[daveson]

I spose you applied penetrant, and some good taps on the bolt head, and torque clockwise and stuff like that.

I would be looking for water type leaking marks on the head beside the gasket, or signs of it swelling. I've removed a few bolts from our brick alloy engines and thought for sure I'll have alloy threads come out with the bolt, it's insanely tight. But after applying penetrant for a few days, and a short, sharp, hit, it was all good. From this thread, and others, I'm confident that torx or torx plus will work.

In these situations, what I often like to do, is nothing. If you haven't got a leaking head gasket or other symptoms of problems, I'd just leave it.

[ivangt]

Just wanted to thank everyone here for all the help and input. It really made a difference while I was chasing this down.

I just received a full Ribe socket set and tested each size carefully. The Ribe M8 fits the cylinder head bolt perfectly. I haven’t tried with the breaker bar yet (only with the Milwaukee mid-torque impact on max setting, and the bolts still didn’t move), but the moment I seated the M8 in the bolt, I knew it was the right one. There’s zero play. It fits like a glove.

In my opinion, this is strongly a Ribe M8 head. I’m 99.9% confident it’s the correct socket for the K100 cylinder head bolts. The only reason I’m not saying 100% is because I’d still like to hear official confirmation from BMW Group Classic, who I’ve contacted directly.

At first, I hesitated because the lobes inside the bolt looked unusually rounded (especially around the outer edge), and the socket seemed to have a slightly squarer profile. But after looking into it, that shape is actually part of the Ribe design philosophy. RIBE, just like Camcar Textron did with Torx, engineered these fasteners for use in harsh automotive environments where oil, dirt, and debris are unavoidable.

The slightly rounded outer area of the lobes is intentional. It creates space to allow debris or grime to sit without interfering with the contact surfaces. The actual torque transmission happens on the inner flanks of the lobes, where the tool seats securely. So even though it looks like the socket isn’t matching every contour, it’s only gripping where it’s designed to, and the outer roundness actually helps with self-cleaning and tool engagement, not hinders it.

Here's a photo of the M8 Ribe socket fully seated in the bolt. Again, thanks to everyone here. This forum has been incredibly helpful, and I’ll be sure to follow up once I hear back from BMW Group Classic.


bolt 3.jpg (37.28 kB . 432x576 - viewed 578 times)

I spose you applied penetrant, and some good taps on the bolt head, and torque clockwise and stuff like that.

I would be looking for water type leaking marks on the head beside the gasket, or signs of it swelling. I've removed a few bolts from our brick alloy engines and thought for sure I'll have alloy threads come out with the bolt, it's insanely tight. But after applying penetrant for a few days, and a short, sharp, hit, it was all good. From this thread, and others, I'm confident that torx or torx plus will work.

In these situations, what I often like to do, is nothing. If you haven't got a leaking head gasket or other symptoms of problems, I'd just leave it.

Thank you very much for the recommendations, Daveson. I've been applying Kroil three times a day and tapping those stubborn bolts for the past three days! I'd probably leave it alone too if it weren’t for the fact that I really want to clean the head, replace the gasket, lap the valves, and just give everything a proper refresh. Honestly, at this point I’m even tempted to try the K75-to-K100 piston swap. It’s become a bit of a showdown between me and the engine block now.

[daveson]

Ivan, thanks for this thread, and for your persistence. You have made a good addition to the knowledge base of the forum.

[ivangt]

Ivan, thanks for this thread, and for your persistence. You have made a good addition to the knowledge base of the forum.

Thank you daveson. I’ve learned a ton by reading through the forum over the past few months, so I’m glad I could finally contribute something useful back. I’ll keep updating as things progress!

[Laitch]

There’s no mention of the correct tool in any of the service manuals, and nothing definitive in the forums either. It’s time to get a confirmed answer straight from the people who built the engine.

Don't hold your breath waiting for Corporate to send you encouragement; you need that oxygen to build brain cells and muscle.
Nobody needs to explain what a bolt is when it says Ribe on it. The only thing necessary is to find a wrench in your tool kit that fits it or else buy it, which you seem to have done.

Quit tapping and start hammering. I've used heat, penetrating oil, tapping, impact wrenches, gnashing of teeth and wringing of hands to remove bolts without breaking them Sometime though, just growing a pair and pounding on the bolt is what's necessary.

If you're 99.9% confident, you're not confident, you're analytical. Confidence is certainty. Get on with it!

[ivangt]

Don't hold your breath waiting for Corporate to send you encouragement; you need that oxygen to build brain cells and muscle.
Nobody needs to explain what a bolt is when it says Ribe on it. The only thing necessary is to find a wrench in your tool kit that fits it or else buy it, which you seem to have done.

Quit tapping and start hammering. I've used heat, penetrating oil, tapping, impact wrenches, gnashing of teeth and wringing of hands to remove bolts without breaking them Sometime though, just growing a pair and pounding on the bolt is what's necessary.

If you're 99.9% confident, you're not confident, you're analytical. Confidence is certainty. Get on with it!

Haha, you're absolutely right! I'm definitely more analytical than confident at this stage! I started going at the bolts and a few of them gave that satisfying crack sound. Just a few degrees of movement. But then my Ribe M8 socket snapped. Not entirely unexpected though… it was a cheap set I got just to confirm the size.

Now that I know it's M8 for sure, I picked up a proper Stahlwille socket that should be up for the job. It'll be here on the 23rd.
Really appreciate your recommendations. They’ve been both helpful and motivating (and gave me a good laugh too). That video you shared is gold. That’s one of the best I’ve seen on the subject. Thanks a lot for that!

[Laitch]

Keep in mind that once that cracking sound is heard it could be necessary to turn that bolt one small step tighter and twos small steps looser for a few more times as you unscrew it before giving it the full extent of your mighty power. Don't be shy with the hammer. Destroying the wrenches, while impressive to onlookers, shouldn't be necessary. Of course, if the wrenches have been wrought from reclaimed soda cans, don't expect too much.

[frankenduck]

Another useful tool is a manual impact driver with a small sledge hammer. I may be wrong about using a cheater bar and think that the impact driver ended up working for me.

https://www.amazon.com/dp/B07R96M9VW/