I have a 66-4 that has a trigger that "stacks up" at the end of the double action pull just prior to the hammer drop. I was a S and W armorer, but haven't done any revolver work since my PD switched to autos 15 years go. Any help in refreshing my memory is appreciated. Ken

Check the mainspring screw. If it has been backed out the top of the spring may be "knuckling" on the bottom of the hammer.

I just went over my 66-3 tonight.
My safety has been rattling a bit the last few days, so it was time.

Stripped it down, saw about 20 years worth of crud here and there.
I ended up taking it down to bare frame.

It had a light cleaning, oil with some leftover Castrol 5w-30.
I'm out of Hoppe's, I was going to try Ballistol sometime this week.

So the simple things first and see how it goes.

Do you have a copy of Jerry Kuhnhausen's book on the S&W revolvers? If not, that book is a great reference, and will answer most of your questions. It may be available at your local library.

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Ret. LE, FA Instr, S&W Armorer

Check the surfaces of your trigger and hammer.
The plated parts on the 66's just don't wear as nice as case hardened parts.

The plated hammer and trigger on older stainless models ARE case hardened.

S&W simply took standard COLOR case hardened parts and gave them a flash hard chrome plated coating to make them look like the rest of the stainless parts and to help in rust resistance.
They could have given the parts a non-color case hardened finish, but it was cheaper to just use the same parts as all other S&W's and give them the flash plating.

It was the very early Model 60 and 66 revolvers that had actual stainless steel hammers and trigger that didn't wear like S&W wanted.

Thank you for clarifying that.
My point is that on my 66-3 and his 66-4, wear thru the plating can induce unpleasant trigger feel.

Checking the surfaces is advised.

I'm also a fan of detail strip and clean, and possibly spring swap before going into anything more involved if there wasn't an issue previously.