XRAY - Model racing cars

Recently acquired a lightly used FK05...
When I start to tighten up to diff, it is a lot tighter than on my FK04.
Is this normal for the FK05?
I have installed new balls and new plates.
Thrust bearing had been replaced to the 2sided copper coloured one.
Everything seems to be in order but it's still very tight.

Anything else I should be checking or replacing?

Thanks for the replies!
Diff was rebuilt conscientiously, taking special care on the washer positions. I think it was properly done.

I didn't expect it to be THAT tight!
Will run it a little loose on the track to break it in.
Let's see how that goes...