Hardened Isolastic Rubber

Checked my '71 isolastics setup. The tall 20 year old rubbers, particularly in the back, were rather hard. Short rubbers seemed OK. I replaced the tall rear ones with much softer Andover replacements. I probably should have replaced the short ones also but they were not as bad.

Do the tall rear ones do most of the work and is that the reason why they harden before the others?

Is it important to have matching rubber all around?

I just changed my front and rear iso rubbers and buffers. The rubbers seem more important than the buffers. I know this from making the mistake of initially changing out all rubbers and buffers with new items including rubbers that were hard. The bike vibrated uncomfortably throughout the rev range.

The vibration was so bad that I bought AN rubbers and buffers and replaced everything again. I did notice that the AN rubbers were soft like the original items I replaced. Both the AN and other new buffers were made of more hardish plastic type consistency (rather than rubber) by comparison to the originals.

The softer AN rubbers seemed to be the key to returning the bike to a rideable machine.

I cant answer your q about changing out the front or rear since I replaced both the front and rear at the same time, but if you suspect that that your front rubbers are hard, its easy to change those. Changing the front isos is a pretty easy job.

The small donuts are merely bump stops to control the outer limit of movement, so I would place more importance on the large ones, but why not just replace them all while you're in there? Also, everything is supposed to move in unison (although we all suspect the front mount catches the most hell) so replacing either the front or the rear without replacing the other doesn't make sense either.

If you don't have the spring from the engine head to the frame, the front isos will take nearly all the engine weight.