This may be a dumb question, but just putting in a new cam chain, and wondering how to mushroom the heads of the master link?

you need a chain staking TOOL Or 2 hammers a cold chisel & 3 hands.

unfortunately those are the "dies" for a $135.00 chain tool. BUT: You could use them in conjunction with a slightly modified $5 C-Clamp from the hardware store too. (Take a large c-clamp, amputate the swivelly (is that a word?) bit on the end of the screw. File screw flat on end and drill into it for one of the bits. File a flat on the back of the opposite side of the clamp to start your drill and drill through the "pad" for the other bit. You don't need the extra strength of the commercial kit for just a camchain.

Talked to an old timer M/C mechanic, and he told me all he has been doing for years was hammer and chissel /drifts and hammer method.
Worked great.

Tanks for the input!