Vilter 440VMC Block heater info needed

Hi,

I need to replace the block heaters on my Vilter 440 VMC 4 cylinder ammonia compressors.

I sourced a Vulcan model through Grainger, 1000 watt thermostatic control 100 thru 190 degrees farenhiet.

Can anyone tell me the thread size on the cover where the unit screws in?

Is the Vulcan unit mentioned above a viable replacement?

The boat is at sea and I would like to order this part asap.

The ammonia system is on a fishing vessel to chill salt water, I have three Vilter 440vmc compressors ammonia system, two chillers with two water condensors. I am in the north atlantic in winter and the pump room ambient temperature can get very cold, down around freezing at times.

any help appreciated!

Thanks,

Vin