As good a place to mention this as any
Many years ago, Dave Webster told me that the conventional tool for engine mounts - a large tube with cut-outs to suit & M16 studding/nuts - worked a lot better by overcoming the inherent friction in the device, thus transferring the majority of the force to the engine mount itself.
The way he did it was just with the simple inclusion of a roller thrust bearing between the M16 nut/washer & the tool. It needn't be a roller thrust bearing as a deep groove ball bearing - even a worn one - will serve the same purpose. You just need the addition of a large washer so that the outer element of the bearing is taking the force direct to the tool cap face. Or just have a bearing & washer between the mount & the M16 nut/washer. Better still, combine both. I'm sure you'll get the gist
Since then, with a number of varying size deep groove ball bearings, I have always pulled in or out engine mounts without ever resorting to heat. In fact, my tube is just that. No welded on cap with a hole to suit M16. I just use a deep groove ball bearing that the outer fits onto the face of the tube, then use a Christmas tree stack of smaller ones to get down to the M16 clearance hole size.
Why would anybody want to risk heating up the rubber & change it's properties or risk distorting/enlarging the precise fits of the mount to the case? The steel used in the mounts is 'swaged' smaller after the moulding process to add desirable, further properties to the rubber, so that steel is a grade 'very friendly' toward being shrunk in diameter yet again
(& please. Don't think that the application of heat will not encourage it to shrink smaller) That appears to be what has happened here
I have literally changed dozens in my time & the only extra applied might be a good penetrating oil.