TO-3P vs. TO-247 insulators

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Hi
I am trying to find mica insulators for a TO-3P package and am having a hard time finding any. On the other hand, insulators for TO-247 seem to be much easier to find -

1) What is the difference between these two packages, they seem almost identical in size to me.
2) does it matter if the insualtor I use is slightly bigger than the transistor base that I am trying to insulate from the heatsink? or should I cut it down to size?

thanks in advance
vsr
 
It does not matter if the pad is larger than the transistor. Actually if the pad is a little smaller it won't matter either, because the metal center of the chip is the only part that will conduct the electricity, and the heat dissipated off the plastic of the FET is negligible (although some obviously does). So a slight difference in size won't hurt.
 
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