What the thunk is going on?

[mdbaker42]

My 1970 hi boy has a 360 with a 4 speed manual trans. and for the most part as far as I can tell it's all stock, It has less than 85k and seem's to work well.Of course everything has a few problems, this is one of mine. when I depress the clutch petal I normaly go into 2nd because nobody likes grany in trafic. When I release
the clutch there's a serious thunk, no noise,no grinding, just thunk... Any and all thoughts welcome. thanks, Mike.

[fordman]

u joint?

[BobbyFord]

My bet is that the diff is "wrapping" under braking and unloading under acceleration. Strap in and hang out the door and watch the diff under braking and while letting off the brake, accelerating, or have a passenger do it (pinion will point toward the ground under braking and release under accel). I'm sure you will see the diff rotating. Only fix I've seen is to replace the leaf and shackle bushings.

[1972hiboy]

ill throw in motor/tranmission or maybe a t-case mount....doesnt hurt to check them anyway.

[averagef250]

ill throw in motor/tranmission or maybe a t-case mount....doesnt hurt to check them anyway.

Bad mounts make big thuds. Check driver's side motor mount and t-case mounts. If nothing pops out at you get under there and start pushing things around.

[mdbaker42]

Wow,ok,thanks everyone... Thats a lot of good stuff. My bump is my hobby, so I move slow but,I'll check it all and let you know what I find out. Again, thanks, Mike.