So I'm checking the undercarriage during an oil change and I see the subframe bushings are starting to look ragged. I went to loosen the large bolt going through the center of the bushing to see how hard/easy it would be to do the replacement and the thing won't budge. I didn't lean into it as hard as I could for fear of breaking something if I was missing a step or doing something wrong. Are these things usually that tough to loosen or is there a secret I'm not privy to?

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1999 E320 (gave to daughter) 250K
1994 F150 (may be the end of its road) 240K
1989 560 SL (Reassembly after paint job) 160K
1986 560 SL (deceased., gave it's life protecting my wife)
1988 300 TE (departed)
1994 945T (still running strong with an upgraded cam) 349K
1986 242Ti (deceased)
1968 GT500KR (under restoration)