After seeing a set of wide pegs on ebay that were made specially for an Elsinore and a direct bolt on, I just had to laugh, but at the same time I figure I would pass along my pretty vast experience with wide pegs on vintage bikes.
Wide pegs can be a great thing, but they can also cause a lot of problems on a vintage bike. The original small pegs that came on the 73-78 CR250's and CR125's hurt plenty when you land off a big jump or hit a really nasty hole in the track, but for the most part they did not cause much of a problem. When the small pegs were on the bike your foot would take a lot of the impact and colapse around the peg, thus absorbing a lot of the impact and not transmitting it to the frame. As soon as you put that wide peg on you will notice all sorts of problems, the 250 with the right side peg that is bolted to the frame will bend the bolt and you can't keep it tight. You will also start noticing more sag in the pegs if you ride hard or are a heavy rider, two problems cause this, the standard 8mm pin will no longer hold up to the higher stress of the wide peg, another is twisting the lower tubes from the force, or even worse yet, tearing the mount completely loose from the frame. I have experienced all of this first hand. I also know of people that were seriously hurt when they landed off of big jumps and sheared the pegs right off the bike.
So in order to make a big peg work on a vintage bike without causing a lot of problems, here are a few things you should know and some helpful info.
1) Don't put too wide of a peg on the bike, it shortens the distance from the front of the peg to the shifter and makes it harder to get your foot under the shifter.
2) When mounting wider pegs it is a perfect time to fix those worn out peg holes and replace the pin with a 10mm one, like the pin on a modern bike with wide pegs. Don't just drill the hole out, it will get thin on the outer edge, take a grinder and work the material out towards the frame, when you get it close, then do the final drill to size, that way the pin location is back in the original position, not sagged outward. Grind the stop pads to put the peg at the correct position, I always leave them just a tad high so when they wear in they sit slightly up from level.
3) On the 73/74 CR250 you should make sure the right side peg mount that bolts on is fitted properly and weld it to the frame, don't just weld it up in the sagged position. If you are a serious rider and really punish the bike you should have the frame braced in the peg areas, prefferably by a proffesional that has experience.
4) The 74/75 CR125 is bad about tearing the peg mounts loose from the frame, it should be braced up, again, by a proffesional with experience.
Hope this helps any of you guys that are considering putting wide pegs on your vintage bike, or you guys that have already done it and are having problems.
