maoy,
I see what you are saying but have held say a red and yellow 2nd gear together? I used a set of dial calipers and they are the same diameter. The teeth are thinner on the 23t compared to 21t. The thing is they have changed the tooth profile to allow more tooth per inch and yet still allow it to work. We’re not talking about going from 48 pitch to 64 pitch, we’re talking about maybe 47 or 49 pitch. Yes they are breaking some engineering laws I am sure, but it works.
I have the yellow 17t and the red 23t on mine now and got in about 7 tanks in today and all is well. REAL WELL! Ho and the lash on 1st and 2nd are very similar.
But the real give me should be the fact that HPI’s engineers have deemed to put it right on the gear chart, “You can use any 1st gear pinion with any 2nd gear pinion”.
I know it sounds wrong but I have been running yellow/red or violet/red combos on my HPI cars forever and it works.

Thanks again for all your help!
John

P.S. I will be calling Kamikazetoysandhobbies tomorrow.