I'm in the process of making a tow strap setup from the GT4CS strap P/N 981 722 695 8A to fit a 997 GT3/RS.

I know that there are a variety of tow hooks available for the 997 and I just don't like them in comparison to the factory Tow hook. I found a company that does make a tow strap version, but it doesn't have the detail of the OMP strap with the hook and loop velcro and I wasn't a fan of the design of their threaded stud. So I decided I will purchase the GT4CS tow strap and see if I can create my own. I understand that this won't be a cheaper alternative, but the project goal is to create something that is equivalent or better to the factory CS strap.







I acquired the CS tow strap from rennsport-one, it was about $150 including shipping and taxes.

First order of business was to disassemble the tow strap without damaging the strap itself. I was a little concerned about this step, because if I couldn't get this OMP assembly apart this project is over. I did a little digging and found tools specific for torqueing eyebolts for load bearing purposes. These specific tools were extremely expensive and I really didn't want to make my own tool, because I'm too lazy for that (think cutting a slot in bar of steel), so I bought a 1-1/2" universal fit socket. I used a 17mm wrench on flats of the OMP stud, the universal-fit socket with a breaker bar and I was able to loosen the eye bolt with strap from the OMP stud. Success!






The eyebolt is a 8.8 grade high tensile steel, finished with a bright zinc plating. It is a standard size used for a variety of safety anchors in motorsports. OMP has it listed under P/N OMP DB420. The thread is a 7/16-20, the thread length is approximately .85in. If Porsche Motorsports is using it, I think it will be just fine for my HDPE use.

The next step in the process is to design a threaded stud/adapter that fits the 997 and this eyebolt, but I didn't know what the thread profile is for the 997. I did some digging, reading and measuring and found that the thread profile for the 997 and other older P-cars is a vintage not commonly used thread profile. It is called a knuckle thread, primary use being able to engage threads without cross threading in dirty/harsh environments, which makes sense for a tow point. More specifically it is Rd 20 x 1/8 (20mm major diameter with a .125in thread per inch pitch). In order to cut this thread I will need some more tooling. Oddly enough none of the tow hook manufacturer's list their thread profile, some even say proprietary, which is just BS and some don't even look correct (perhaps using an ACME thread, which is very similar but not the same and it might even engage/fit).


Prior to committing to a design I mocked up a test piece to check fitment relative to the front bumper, trying to determine how long should the stud be? The factory tow hook doesn't have any thread relief or a flange, the threads just end and you tighten against that material interference. Most other aftermarket or the OMP tow strap have a shoulder that you can tighten against, I believe that is a more conventional solution. Some tow hook manufactures place spacers between the shoulder and the threaded stud to achieve a specific alignment or clocking of the tow hook relative to the car. The shoulder of the stud needs to tighten against the silver/aluminum part shown below, which is about Ø1.0in.



I machined a little PVC plug that fit into the threaded hole, so that I could mark the distance of where I thought I want the tow strap.


After this fitment check I took it one step further and drilled and tapped the hole for the eyebolt 7/16-20. You can see the overall length difference below in the different components and also some markings on the OEM tow hook relative to the bumper.


Fit check. Overall not too bad, but a little long IMHO. I'd like to have the eyebolt recessed a bit further. You never want to be towed at angles exceeding 45° do to the reduction in load rating on everything, but you also don't want to tear of your bumper if being towed at an angle. Based on the images for the CS tow strap, I think I could get it in tighter, but I had design issue.



Design issue continued. Due to the threaded stud length on the eyebolt, I need to drill a hole deep enough in the adapter that I can tap and I want to maintain a minimum wall thickness in the custom adapter. Concept 01, shown below.




In order to reduce the amount of exposed Eyebolt, I would need to reduce the depth of the 7/16-20 threaded hole and reduce the length of the stud on the eye-bolt. I removed approximately .25in from the stud (hacksaw, cleaned up the threads with grinder and a file) and I reduced the length of the shoulder on the adapter, which effectively reduced the hole depth by .25in. This is as compact as I could make it within reason. As a rule of thumb you always want to have a minimum of six threads fully engaged in any threaded connection (I've got eight). Technically anything beyond three is almost overkill/safety factor.




Fit check. I like it. I think this is as good as I can get it without compromising the structural integrity of this adapter. In the image below, it's hanging down a bit, because there are no threads engaged.



Design revision. This is a very short and compact adapter. Which is better than something long which could have more leverage.



So this is where I am at today. The next steps will be to order the cutter for the weird knuckle thread and then machine a test piece or just go all out and finish this thing. I have put some thought in to the material selection of the threaded stud/adapter and I've decided to make it from Grade 5 Titanium, primarily because I don't want use Steel and have to heat treat it to achieve the high tensile strength. Stainless steel would probably be fine, but if I'm making it, why not go all out. Grade 5 Titanium is used all the time for high load/stress fasteners and it's light weight. The one thing that I think about is the clocking of eyebolt relative to the car/bumper, maybe I'll get lucky and it will line up perfectly. If not, I'll figure something out.

I welcome any positive or negative feedback, comments, etc. Sorry for the long intro thread, I wanted to start this thread earlier.

 

Oh man. I typed up a long reply and Rennlist went down. annoying. Here's the short version:

1. Nice work, I appreciate your attention to detail! If it were me, I would have just bought the off the shelf OMP tow strap and bolted it to the bumper support.
2. Your design is not very mfg friendly, there are some changes you could make that would make manufacturing much easier
3. Ti is cool and has great strength to weight but you can find other alloys that are stronger (if that is what you desire). It's really not that strong. I think Ti in this application is overkill, unless you want it for bragging rights.
4. Remember, you are bolting this tow hook to an Al bumper support...so whatever you're doing on the other end may be necessary and overkill
5. Before you mfg, 3d print your design. No need to model the threads, you can just reduce the side that threads into your bumper and open up the side the tow strap threads into. Online printers are a rip off, so if you can't find somebody locally, I could probably print it for you

Good luck!

 

Subscribed, really like this, bought the GT4 strap before realising the threads were different.

 

Ahh, the benefits of being a design engineer with access to the proper tools, SW and machine.
Thanks for the informative post, I'll read again in detail and offer feedback.

When I saw your first photo I thought, whoa, nice upgrade.

 

Ngng, thanks for the feedback. Sorry your response got deleted, I hate when that happens.
1. OMP does not sell an aftermarket/OEM tow strap that is threaded in to the bumper support.
2. To me it seems like a straight forward part that I plan on machining (no CNC). The trickiest part is machining the threads, but again no big deal.
3. Good point. I might just make it out of Stainless Steel, since its way cheaper than Ti.
4. Agreed.
5. I'll probably just go for it and either machine it in plastic first and then metal.

 

That was my issue as well. I couldn't confirm the thread information anywhere online. Just on some older model P-cars and then based on measurements of the OEM tow hook.

Hopefully, this post captures all of the information needed for anyone trying to attach to the factory tow point.

 

LOLs, just the tow strap.

 

c'mon porsche....really?

 

Well done on this project. I too exhausted all the options for a nylon tow strap and knew long ago the threading was different between the models. If you get this done count me in for a purchase. If you are going to go all the way you might want to think about different colored straps. No doubt people will want to match their tow strap to their choice of matching color to their car or brake caliper. You think I'm kidding Maybe black, red, yellow and silver..

 

The red nylon tow strap comes directly from OMP already sewn to the eyebolt and it only comes in red. I've seen a lot of chatter on the 991GT3/RS message board about these straps. Apparently, the GT4 sizing doesn't fit the US spec GT3/RS (it's too short). Interestingly enough these tow straps do appear on EU spec GT cars from Manthey Racing. I'm not sure if they are making their own adapter like I am or if they have some factory pull at OMP.
https://rennlist.com/forums/991-gt3-...ennline-4.html

Part of my design process was to avoid sewing my own strap (even though I could) and to utilize the factory red OMP strap. I don't really have any intentions on selling these, but I'll keep it in mind if there is sufficient interest. I could have some of the custom adapters CNC'ed.

 

I'd be in for 1 at least.

 

I too would also be interested in one. This is slick work.

 

I will keep this in mind. I believe the tow straps can be ordered from the EU at a lower price than the retail cost here in the US. There still is labor associated with disassembly and modification of the eyebolt on the tow strap, reassembly of the two components, material costs and fabrication (labor) costs. Basically, the worlds most expensive tow hook. I'm not sure if that is a good thing. I'll reach out to some of my buddy machinists to see if they are interested helping out. I need to get this thread cutting tool on order, so that we can start fabrication.

And I still need to figure out/test the clocking relative to the bumper. I'd really prefer not use any spacers for clocking. But in order to accomplish that consistently both the internal and external threads must be CNC'd. Also assuming that the factory threads in the support bar don't change from car to car. Again, worst case spacers or you get what you get with regards to clocking.

 

Finally got the special cutter for this specific knuckle thread. Just comparing the thread profiles to the OEM tow hook, looks like a perfect fit.

Carmex P/Ns:
Cutter: 16 ER 8 RD BMA
Tool Holder: SER 0500 F16

Next step, start machining. Unfortunately, super busy with work, so it might be another couple of weeks, before I cut some metal or plastic.

 

Nice work man.. looks great