Williams Heat Wave.
I'm an EM convert and have several 1970's wedgeheads that I enjoy... but I always avoided anything older like the plague (especially with 2" flippers) . I got a chance to play Heat Wave at Pinfest in Allentown earlier this year and it totally reversed my view of short flippers.
It has a thermometer animation in the backbox (like Centigrade 37) and you're trying to advance it towards a special via 7 mushroom rollovers and two swinging targets. The game also features a drop target with a value that advances along with the thermometer. Gameplay is lively and challenging and the artwork is just classic.

I have since added a Heat Wave to my lineup and also a GTB Melody to round out my 1960's short flipper lineup. These are both addictive "just one more game" pins.

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_{Post edited by Drano: posted in wrong EM thread}