Modifying Mugen Rails to Fit non-Mugen Bucket Seats
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From: Bay Area, CA, USA
Modifying Mugen Rails to Fit non-Mugen Bucket Seats
The Mugen rails are really only designed to work with Mugen S1 seats. The S1 has two mounting bolts on the side of each seat. The side-to-side distance between these mounting holes is 390mm front and rear. The front-to-back distance between the holes is 290mm on each side, which appears to be an industry standard (Momo, Sparco and Recaro at least use this distance on their side-mount seats). The Mugen rails are designed work with these dimensions only.
The problem is that there seems to be no industry standard specification for the side-to-side distance between the mounting bolts. Hence, if you buy another bucket seat, there is no guarantee that it will work with the Mugen rails. On a Momo Cup-GT, for example, the mounting holes are 415mm apart front and rear; Sparco Evo bolts are 410mm apart in the front and 400mm apart in the rear. Recaro SP-G and SP-A seats, however, have the same side-to-side dimensions apparently and are a direct bolt on. Spoon bucket seats are also a direct bolt-on.
As far as I can tell, however, most buckets seats tend to have longer side-to-side dimensions than the Mugen S1s. Hence, the base mounts will not line up with the threaded holes in the floor of the car when the rails are mounted on these seats - the holes on the mounts will be too far apart. If the seats have a smaller side-to-side mounting distance, spacers will be needed between the side mounts and the seats, a solution that will not look very pretty. (I have yet to see seats with narrower mounting dimensions than the S1s, though.)
Others have reported that some seats will work with the Mugen rails if the aluminum side mounts are reversed on the base mounts. Reversing the side mounts will allow a wider seat to work with the rails. Momo Top Nascar and 1000 Lakes seats, for example, are reported to work with this trick. This side mount reversal trick will only work if the center of the side mounting holes on the seats are 25mm or less from the bottom of the seat. If they are more than this distance, the side mounts cannot be reversed because the angled part of the mount will interfere with the side of the seats.
The upshot of all this verbiage is that the chance of a random bucket seat working with the Mugen rails are slim. And having bought one of said seats and not wanting to return the seats or the sliders, I decided to work on a way of making them fit. The seats in question are Momo Cup-GTs. The side mount reversal trick does not work with these seats because the center of the side mounting holes is further than 25mm from the base of the seats. With the unmodified Mugen rails mounted on these seats, the mounting holes on the base are 25mm too far apart because the side-to-side distance between the bolts on the GTs is 25mm longer than the 390mm expected by the rails.
Hence, I needed to find a way of moving the base mounts 25mm inwards relative to the side mounts. 12.5mm spacers on each side between the base and side mounts are an obvious solution but I was not sure how secure they would be. However, while poking aroud at my local Orchard Supply I had an idea - why not take a nylon spacer with an inner diameter smaller than the threaded bolts used to secure the side mounts to base mounts and tap a thread in the spacer? The spacer could then be threaded onto a longer bolt between the base and side mount and there would be next to no play in the system. Obviously, one could also drill a 6mm hole in the center of the spacer, but I think this would allow more play than the threaded hole solution. You would also need a fairly sturdy jig to hold everything in position for accuracy.
The stock Mugen rails come with twelve M6x1.0, 25mm, grade 10.9 bolts (six per side). 6mm washers and nylon self-locking nuts are used with each bolt. 37.5mm or longer bolts will be required to fit the 12.5mm spacers. Fortunately, my local Orchard Supply carries M6x1.0mm, 40mm bolts. They are grade 12.9, which is overkill, but they are 50c each so I am not going to lose any sleep over it. The 6mm washers and self-locking nuts from the Mugen rails can be reused.
They also carry 1/2x0.194x1/2" spacers, which are close to ideal. A 0.194" (4.9mm) hole is perfect for tappinfg a 6mm threaded hole and length of 1/2" (12.7mm) is pretty close to 12.5mm.
Home Depot or Lowes should also carry equivalent parts in their fasteners section. Finding 24 spacers of the correct size in stock at either of these places may be a challenge though.
Parts
2 Mugen seat rails.
2 Momo Cup-GT seats.
24 Bolts, M6x1.0, 40m, grade 12.9.
24 Nylon spacers, 1/2x0.194x1/2" (metric spacers were not to be found).
8 Bolts, M8x1.25, 20mm, grade 12.9.
8 8mm washers.
Tools
1 5mm hex key.
1 6mm hex key.
1 6x1.0mm tap.
1 T-handle tap wrench.
1 10mm socket.
1 12mm socket.
1 Ratchet.
1 T-handle wrench.
1 Bench-mount vise.
1 Hand grip.
Directions
1. Using a bench-mount vise, secure a spacer. Don't over tighten or you will deform it. Tap a thread into the spacer using the 6x1.0mm tap and T-handle tap wrench. Repeat with the twenty-three other spacers.
2. Using a 10mm wrench and a 5mm hex key, remove the M6x1.0mm, 30mm bolts from the the Mugen rails, thus separating the aluminum side mounts from the base mounts.
3. Reusing the washers, insert the M6x1.0, 40mm bolts into the holes in the side mounts, and thread a nylon spacer onto each bolt.
4. Using a hand grip and a 5mm hex key, tighten the bolt until the spacer abuts the rail. (The very top of the 40mm bolt that I used was not threaded so hand tightening was not sufficient.) Do not over tighten or you will strip the threads on the spacer.
5. Re-attach the base mount to the side mounts and tighten the self-locking nuts using a 5mm hex key, and a 10mm socket and a ratchet. Secure using T-handle wrench .Make sure to tighten by rotating the nut and not the bolt - otherwise you will strip the threads on the spacers.
6. Attach the side mounts to the seats using the 8x1.25mm bolts and 8mm washers. Use a 6mm hex key to tighten the bolt. Do not tighten fully yet.
7. Attach the metal wire between the slider mechanisms on each side of the both seat rails.
8. Mount seats in car with the four stock bolts and secure using a 12mm socket. Now tighten the side mount bolts.
9. Test that the sliders operate smoothly.
Obviously, different length spacers can be used to work with different seats. The spacers themselves could also be shortened with a grinder or a hacksaw. I do think I would be happy going any wider than 12.5mm or so on each side. In any case, 12.5mm is the maximum spacer size possible on the inboard side of each seat because the the seat belt bolt would hit the shifter tunnel.
There are a bunch of other alternatives to Mugen rails. Sparco make custom mounts and Recaros in the US are usually mounted with Wedge Engineering brackets. Wedge brackets are of high quality but tend to make the seats sit a little high. Sparco brackets are of very poor quality and often require drilling to work properly. They also make the seat sit a little high, particularly with sliders. The Mugen rails are of very high quality, look good, and allow the seat to sit very low in the car. Even though they are expensive ($300 per side), the price is not as bad as it first seems - Sparco base mounts with seats belt mounts ($100), sliders ($40) and aluminum side mounts ($100) are only $60 cheaper per side.
Disclaimer
Perform these modifications to the Mugen rails at your own risk. If I didn't think this modification was incredibly secure, I would not use it. However, I make no claims as to its safety in your car. Use metric grade 10.9 or better bolts. There are twelve bolts per seat so any force experienced by the seat will be divided between the bolts. The chances of twelve grade 12.9 bolts failing are slim.
Also, for you track ******, I have no idea if this modification would pass technical inspection. Though, the inspector would have to be pretty eagle-eyed to spot the spacers because they are inside the rails.
This modification is completely reversible.
[Modified by norice, 9:49 AM 9/3/2002]
The problem is that there seems to be no industry standard specification for the side-to-side distance between the mounting bolts. Hence, if you buy another bucket seat, there is no guarantee that it will work with the Mugen rails. On a Momo Cup-GT, for example, the mounting holes are 415mm apart front and rear; Sparco Evo bolts are 410mm apart in the front and 400mm apart in the rear. Recaro SP-G and SP-A seats, however, have the same side-to-side dimensions apparently and are a direct bolt on. Spoon bucket seats are also a direct bolt-on.
As far as I can tell, however, most buckets seats tend to have longer side-to-side dimensions than the Mugen S1s. Hence, the base mounts will not line up with the threaded holes in the floor of the car when the rails are mounted on these seats - the holes on the mounts will be too far apart. If the seats have a smaller side-to-side mounting distance, spacers will be needed between the side mounts and the seats, a solution that will not look very pretty. (I have yet to see seats with narrower mounting dimensions than the S1s, though.)
Others have reported that some seats will work with the Mugen rails if the aluminum side mounts are reversed on the base mounts. Reversing the side mounts will allow a wider seat to work with the rails. Momo Top Nascar and 1000 Lakes seats, for example, are reported to work with this trick. This side mount reversal trick will only work if the center of the side mounting holes on the seats are 25mm or less from the bottom of the seat. If they are more than this distance, the side mounts cannot be reversed because the angled part of the mount will interfere with the side of the seats.
The upshot of all this verbiage is that the chance of a random bucket seat working with the Mugen rails are slim. And having bought one of said seats and not wanting to return the seats or the sliders, I decided to work on a way of making them fit. The seats in question are Momo Cup-GTs. The side mount reversal trick does not work with these seats because the center of the side mounting holes is further than 25mm from the base of the seats. With the unmodified Mugen rails mounted on these seats, the mounting holes on the base are 25mm too far apart because the side-to-side distance between the bolts on the GTs is 25mm longer than the 390mm expected by the rails.
Hence, I needed to find a way of moving the base mounts 25mm inwards relative to the side mounts. 12.5mm spacers on each side between the base and side mounts are an obvious solution but I was not sure how secure they would be. However, while poking aroud at my local Orchard Supply I had an idea - why not take a nylon spacer with an inner diameter smaller than the threaded bolts used to secure the side mounts to base mounts and tap a thread in the spacer? The spacer could then be threaded onto a longer bolt between the base and side mount and there would be next to no play in the system. Obviously, one could also drill a 6mm hole in the center of the spacer, but I think this would allow more play than the threaded hole solution. You would also need a fairly sturdy jig to hold everything in position for accuracy.
The stock Mugen rails come with twelve M6x1.0, 25mm, grade 10.9 bolts (six per side). 6mm washers and nylon self-locking nuts are used with each bolt. 37.5mm or longer bolts will be required to fit the 12.5mm spacers. Fortunately, my local Orchard Supply carries M6x1.0mm, 40mm bolts. They are grade 12.9, which is overkill, but they are 50c each so I am not going to lose any sleep over it. The 6mm washers and self-locking nuts from the Mugen rails can be reused.
They also carry 1/2x0.194x1/2" spacers, which are close to ideal. A 0.194" (4.9mm) hole is perfect for tappinfg a 6mm threaded hole and length of 1/2" (12.7mm) is pretty close to 12.5mm.
Home Depot or Lowes should also carry equivalent parts in their fasteners section. Finding 24 spacers of the correct size in stock at either of these places may be a challenge though.
Parts
2 Mugen seat rails.
2 Momo Cup-GT seats.
24 Bolts, M6x1.0, 40m, grade 12.9.
24 Nylon spacers, 1/2x0.194x1/2" (metric spacers were not to be found).
8 Bolts, M8x1.25, 20mm, grade 12.9.
8 8mm washers.
Tools
1 5mm hex key.
1 6mm hex key.
1 6x1.0mm tap.
1 T-handle tap wrench.
1 10mm socket.
1 12mm socket.
1 Ratchet.
1 T-handle wrench.
1 Bench-mount vise.
1 Hand grip.
Directions
1. Using a bench-mount vise, secure a spacer. Don't over tighten or you will deform it. Tap a thread into the spacer using the 6x1.0mm tap and T-handle tap wrench. Repeat with the twenty-three other spacers.
2. Using a 10mm wrench and a 5mm hex key, remove the M6x1.0mm, 30mm bolts from the the Mugen rails, thus separating the aluminum side mounts from the base mounts.
3. Reusing the washers, insert the M6x1.0, 40mm bolts into the holes in the side mounts, and thread a nylon spacer onto each bolt.
4. Using a hand grip and a 5mm hex key, tighten the bolt until the spacer abuts the rail. (The very top of the 40mm bolt that I used was not threaded so hand tightening was not sufficient.) Do not over tighten or you will strip the threads on the spacer.
5. Re-attach the base mount to the side mounts and tighten the self-locking nuts using a 5mm hex key, and a 10mm socket and a ratchet. Secure using T-handle wrench .Make sure to tighten by rotating the nut and not the bolt - otherwise you will strip the threads on the spacers.
6. Attach the side mounts to the seats using the 8x1.25mm bolts and 8mm washers. Use a 6mm hex key to tighten the bolt. Do not tighten fully yet.
7. Attach the metal wire between the slider mechanisms on each side of the both seat rails.
8. Mount seats in car with the four stock bolts and secure using a 12mm socket. Now tighten the side mount bolts.
9. Test that the sliders operate smoothly.
Obviously, different length spacers can be used to work with different seats. The spacers themselves could also be shortened with a grinder or a hacksaw. I do think I would be happy going any wider than 12.5mm or so on each side. In any case, 12.5mm is the maximum spacer size possible on the inboard side of each seat because the the seat belt bolt would hit the shifter tunnel.
There are a bunch of other alternatives to Mugen rails. Sparco make custom mounts and Recaros in the US are usually mounted with Wedge Engineering brackets. Wedge brackets are of high quality but tend to make the seats sit a little high. Sparco brackets are of very poor quality and often require drilling to work properly. They also make the seat sit a little high, particularly with sliders. The Mugen rails are of very high quality, look good, and allow the seat to sit very low in the car. Even though they are expensive ($300 per side), the price is not as bad as it first seems - Sparco base mounts with seats belt mounts ($100), sliders ($40) and aluminum side mounts ($100) are only $60 cheaper per side.
Disclaimer
Perform these modifications to the Mugen rails at your own risk. If I didn't think this modification was incredibly secure, I would not use it. However, I make no claims as to its safety in your car. Use metric grade 10.9 or better bolts. There are twelve bolts per seat so any force experienced by the seat will be divided between the bolts. The chances of twelve grade 12.9 bolts failing are slim.
Also, for you track ******, I have no idea if this modification would pass technical inspection. Though, the inspector would have to be pretty eagle-eyed to spot the spacers because they are inside the rails.
This modification is completely reversible.
[Modified by norice, 9:49 AM 9/3/2002]
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Joined: May 2001
Posts: 6,402
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From: Fairbanks
Re: Modifying Mugen Rails to Fit non-Mugen Bucket Seats (Big Phat R)
Very well written! 
An alternative method is to have a machine shop make a custom billet aluminum spacer instead of the nylon washers. Nylon washers has tendency to compress after a little while and your seat starts to move a bit. Martin, please check the seats after every event or every 2-3 autoX, etc.
-Sean
An alternative method is to have a machine shop make a custom billet aluminum spacer instead of the nylon washers. Nylon washers has tendency to compress after a little while and your seat starts to move a bit. Martin, please check the seats after every event or every 2-3 autoX, etc.
-Sean
Enjoy your seats.
Thread Starter
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Joined: Jan 2000
Posts: 1,919
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From: Bay Area, CA, USA
Re: Modifying Mugen Rails to Fit non-Mugen Bucket Seats (Cosworth)
An alternative method is to have a machine shop make a custom billet aluminum spacer instead of the nylon washers. Nylon washers has tendency to compress after a little while and your seat starts to move a bit.
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