What size bolts to use for side mounts on a Momo seat?

Noel · 11-30-2005, 07:23 PM

I was just curious if anyone knows what size bolts I need to mount the side mount adapters into the threaded holes in the side of a MOMO Cup Seat? I imagine that all momo seats take the same size bolt.

I was hoping I wouldn't have to take the seat to the hardware store.

Thanks,

MotoSook · 11-30-2005, 07:27 PM

Bring the HW store to you

Go get some M8 and M10 screws. You might even be able to "borrow" a screw off the car. IIRC, my MOMO seat takes M8 screws.

Wil Ferch · 12-01-2005, 04:59 AM

I'm *almost* certain they're 8mm for all types of shell buckets..... 6mm recessed hex heads, I believe....

Anyone confirm for sure ???

- Wil

Jeff · 12-01-2005, 07:12 AM

Noel, On my 87 they are the same thread as the bolts that hold the rail and slider together. They will need to be longer and spacers will need to be made if you are using Porsche sliders. Use at least a grade 8 bolt.

Wil Ferch · 12-01-2005, 08:24 AM

Jeff.... try finding a Grade 8 * METRIC * bolt...... Grade classification only applies to non-metric threads...

Get property class 8.8 ( equiv to our Grade 5)...or property class 10.9 ( equivalent to our Grade 8). Seeing the OEM stuff is 8.8.... that should be good enough.

- Wil

Jeff · 12-01-2005, 11:40 AM

Thanks for the clarification Wil, That's what I was trying to say. I think I need to drink more caffine BEFORE I start typing.

Noel, Here are the spacers I made to mount the seat. They are 1/8" walled tubing with a washer welded on each end. They will get purtied up after saturday's track day. Hope this helps.

Noel · 12-02-2005, 03:53 AM

THanks guys. It was indeed a M8 1.25 pitch bolt.

Jeff, interesting solution to the issue of fitting the seat into our cars. Last night I mounted one side rail with the mounting/floor tab going under the seat and the other going outside the seat. This way the rails lined up to the stock rails perfectly, however the seat was offset to the left and I wasn't sure if I would like that.
The only thing I'm curious about with your solition is the potential for the bolt to break in the event of an accident. Any thoughts?

Jeff · 12-02-2005, 07:01 AM

Noel, I am not following. Do you have pictures of your set up?

Noel · 12-02-2005, 09:26 AM

I don't but will take one this weekend. More simply clearly stated, I have one side mount mounted to the side of the seat correctly and one mounted backwards. From the back of the seat the side mounts look like this:
| |
| ------Seat---------- |
|___ |___
------------Floor-------------
Instead of how I think you have your side mounts mounted to the seat which from the rear of the seat would look like this:

| |
| ------Seat---------- |
|___ ___|
--------Floor--------------
I hope this is more clear.

Noel

Noel · 12-02-2005, 09:42 AM

Ok, let's try that again with an actual photo (I was never very good at sketching anyway):

THis is how I originally had my side mounts on. This way the seats fits perfectly on the Porsche seat rails (Floor). However, the seat is ofset slightly to one side this way.

I do like your solution, but was worried about the stress on the bolt in the event of an accident.

I may end up getting a plate of steel and fabbing a foundation on which the seat will be bolted. THe foundation will be bolted to the factory seat rails (Floor). This way, I could also cut a hole in the steel foudation to attach my crotch belt.

Jeff · 12-02-2005, 11:56 AM

Loud and clear. I thought about doing that but then had the need to weld something. After tomorrow's track day, I am going to make some mounting brackets similar to the Momos but bent in more to accept the seat on our sliders. I will re-enforce it with small triangular pieces at the bottom bends to keep it from collapsing.

Walt Fricke · 12-02-2005, 01:03 PM

Here are some thoughts on these two seat mountings:

If (as appears to be the case) you can use the Momo L bracket mounts either way without screwing up the seat height/slope, then there should be zero strength implications for Noel for mounting this way. He may or may not be less centered on the steering wheel than stock, but it took me years to realize that the stock seat position wasn't square on to the steering wheel so maybe a little more of that doesn't matter much.

I'd be concerned about Jeff's spacers. Not enough not do do one DE weekend with them, but concerned enough to see how an engineer would view this. I have no question (both from theoretical calculations and from observing the results of some horrific track wrecks) that 4 M8 8.8 bolts holding the standard L bracket flush with whatever is epoxied inside a composite seat are up to the job asked of the seat in a wreck. But that's mostly in shear (or tension) with not much bending. The spacer, despite being of a generous OD, has to introduce some more bending moment, and that's beyond my untutored ability to calculate. Maybe it is trivial.

What Noel might be concerned about with his SC is the mounting to the car chassis. My SC, alas, is an ersatz SC - a '77 Euro 2.7 with flares and a 3.0. So I don't know just when Porsche improved the strength of these mountings up to what is found on an '84 and later. But on mine, and on earlier 911s generally, the chassis rails are not really up to racing strength standards. You can grab one with a gloved hand and flex it. Not a lot, but enough that you know it moved, and that you are not putting anywhere near the load on it that a really big rear end collision will do (like sliding backward into a wall or barrier at over 100 mph). A good idea here is to weld in some simple reinforcements so that these pieces are not just cantilevered out over the floor but supported (tension in the front, compression in the rear) better. This is particularly valuable if you have a roll bar cross bar your seat can hit if the seat mounts flex or fail in a heavy rear end crash. The 6mm bolts holding the stock slider to this do not inspire confidence either, though in theory they seem to be strong enough (steel is really strong stuff) and if you are using a seat slider upgrading here would be difficult. If you are just bolting the L brackets in a fixed location with no adjuster, then maybe you could upgrade to 8mm bolts and nuts at the same time.

Walt Fricke

Jeff · 12-02-2005, 01:16 PM

Good thoughts Walt. I have been rethinking my set up since Noel had questions about it and I think I am going to change it to Noels type set up before the track event. After that I will make custom brackets that bolt directly to the seat (No spacers).

Noel · 12-03-2005, 11:19 AM

Walt, Thanks for your comments. You got me thinking about being offcenter with the stock seats, so I switched the side mounts on the Momo Cup seat (Left to right and right to left) and now the seat is perfectly centered with the steering wheel. I couldn't be more pleased. Now I don't have to fab anything. The seat will bolt right up, as soon as I drill some holes in the stock rails. BTW, I'll check the integrity of the rails out too. Thanks for the heads up.

Also, Jeff, I plan to get a second seat for the passenger side and since I was planning to buy a second set of side mounts with it, I think the seat will be easier to adjust if I use two left side mounts on one seat and two right side mounts on the other seat. You might want to consider this also if you also have a second seat. I'll post photos when my installation is complete to help others out who may read this thread later.

Jeff · 12-03-2005, 02:15 PM

Good idea Noel, but would that put the passenger too close to you?

Noel · 04:26 AM

Jeff, I considered that and thought that if it did, no problem, just swap the side mounts on the passenger seat too and move it closer to the door. Since the Momo seat is narrower than the stock seat it really isn't an issue. However, if you are refering to mounting a Momo cup and keeping the stock passenger seat, then the answer is "No it is not too close." I still have the stock passenger seat in my car until I figure out how I want to mount the Momo Cup on the drver's side, then I'll mount the passenger seat.