M-21 3rd gear!

I've been using Muncie trannys for over 25 years and haven't had that much trouble with them. I've recently put some decent power in my car and now I've blown out third gear three times. Has anyone else had any problems resembling this? I could use any input out there. Thanks for any help!

that would be because you are exceeding the power capacity of that transmission. muncies breaking 3rd gear is nothing new. i have blown m-21 and m-22's before. its just the fact that 40 year old alluminum isnt as strong as it once was. and guys were brekaing them way back when.

if your building serious power look into a jerico, or g-force, or even a richmond. or if you want OD a tremec/tko setup.