Heat is your friend! Wipe as much penetrating oil off as you can and hit it.

Another thing to try is to cool the shaft after you heat the arm. I have some "gum and wax" remover which sprays out a super cold substance. The cold shrinks the shaft (I know) while the heat expands the arm. Those air cans for your keyboard work too but you have to turn them upside down. (Caution - flammable!)

When your gear puller is good and tight, smack the end of it with a hammer while there is tension. Many times that will be enough to break it loose.

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1981 300TD "The Green Lantern"
1980 300TD
1983 300D Euro "China Cat"