I am replacing the heads of my 1985 GT with a pair of 1987.

I am going to replace the valve stem seals in the “new” heads, I removed 4 of the springs with a lot of trouble and the overhead valve spring compressor broke. I got a Lisle 16750 valve spring compressor, but the springs feel like hard and that they won’t compress at all...

Any advice? Change the tool?

I used whatever tool advance auto gave me. It clamps onto the valve on the bottom and reached over to the top of the valve to compress the spring. Work for me.

I agree with the last post. The large c clamp type compressor works the best with the heads off. However most will work.
make sure you have something under the head on the valve to keep it from "falling down" while trying to compress the spring.
Also grime, varnish, dried out oil tends to make the retainers stick to the valve. When first starting to compress the spring, take a small hammer and lightly tap the cap. This will help loosen the "keepers"retainers. A pocket screwdriver with a magnet, works great for removing the keepers.

For those who do this in car. Use the "chevy" type handle compressor that uses the rocker bolt. Remove all the spark plugs. Remove all of the rockers. Then move the cylinder down on the cylinder you are working on. Through the spark plug hole, fill the cylinder with soft cloth line. Then bring the cylinder back up. The cloths line will help keep the valves up when compressing the spring.

I've used one of these to compress the valve springs. Easier to crank than to squeeze.

https://www.amazon.com/OTC-...s=automotive&sr=1-10

Spoon

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