I have them rebuilt whenever possible. That isn't always possible but keep your core until you're sure theirs is working well. You don't want to buy garbage and get screwed out of your core.

All of the shop manuals used to have rebuild instructions - even books like Haynes. Parts are cheap if you find a source.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.