I need to replace the front derailleur on my 70s Raleigh Grand Prix. I am a novice but am looking forward to riding again. I just turned 67. so i got my Grand Prix out of the back room shined it up a bit took a quick ride only to find the Simplex front derailleur made of delrin was broken. I have a lot to learn but I first tried to get a vintage replacement Simplex delrin front derailleur and was successful in finding one. great until I attempted to install it. the down tube is 28.6mm the clamp on the derailleur is 27.7mm therefor a press fit. plastic and press fit do not go together. add to that the delrin is old and I was sure it would have been in several pieces in short order.
My question will a nice shimano 105 FD 5700 work? what else do I need to change to make this happen?

thanks for your input!

Welcome to Bike Forums, chipscap!

We are going to move your post from our Introductions forum to our Classic & Vintage forum. The members in there should be able to help you out.

Don't bother with trying to find a Simplex delrin replacement; they're not worth it and will break just as the original did. 28.6mm was the most common diameter for front derailleurs; I'm surprised you have had trouble finding a suitable replacement.

If the clamp size is correct the 105 FD should work, but a Suntour VX would be period correct. Some come with the Raleigh name on them.

Another recommendation for a Suntour front derailleur. This Suntour ARX works perfectly on my 1971 Peugeot PX10;

Is there a cable housing connected to the derailleur? From the bottom?

Or does the cable go over a guide and go to the derailleur as a bare cable?

That will determine the type of derailleur that you can use.