drivetrain slack

After three years in my driveway, hundreds of hours and some blood, sweat and tears- the M725 is finally drivable again. Thats been a great achievement for me, but I'm observing something when I drive that feels and sounds like excessive driveline slack and was wondering if anyone has any experience with this and might be able to help ID the likely causes and fixes.

My build:

• Stock 350 SBC with Fitech fuel injection, SM465 four speed connected via a very short custom Spicer slip-jointed shaft to the stock NP200 that drives the rear axle through a rear driveline relocation kit.
• SBC front motor mounts are new rubber Anchor mounts and the same for the rear transmission mount. Factory rubber TC mounts are old, but tightened down with new hardware. SBC motor mounts don't appear to have any free play or issues that I can see.
• Both stock driveline U-joints are also new and both stock axles have been regeard with 4.56 gears by the local Six States/Spicer place. They said that the stock spider gears were cleaned and reused.
• The TC and SM465 both seem to have some wear, but seem to work okay and I see that the TC has gotten hot in the past. Both TC and SM465 came to me with pretty major leaks and had been run low on oil in the past. (The SM465 is on my winter rebuild list and the NP200 is going to be swapped for a divorced rebuilt NP205. The clutch and TO bearing will get changed too.)
• All transmission and transfer case fluids were changed and so were all the shaft seals that were accessible from below the vehicle -and the TC and SM465 are both holding oil now.
• I didn't touch the clutch other than adjusting the pedal mechanical linkage free play. Clutch holds okay and is much smoother than I thought it would be.
• All leaf springs were cleaned and painted and spring eyes/bushings replaced with new.
• Drum brakes are also all new parts.

The clunk:
In 2wd high, when I let off the gas and then push the clutch pedal down, there is a clunk or metallic thud that is louder between 2-3 shift than the 3-4 shift. It can be felt through the floor and heard loudly in the cab. I can lessen the clunk somewhat by being more delicate/slower with the clutch, but its still there. Also, the exact same noise can be duplicated in any gear by getting off the gas and than mashing back down on it quickly.

I know this is difficult to diagnose via the forum, but if you've read this far, I'd appreciate any thoughts/theories or ideas on what driveline components may be responsible for this noise, any tests I should conduct to narrow the "suspect list" and if the TC swap and SM465 rebuild is likely to fix this or if it's likely something else?

Thanks,
Bryce

My completely stock truck has a bad clunk as well:confused:

I have terrible slop in the NP200 input shaft.
While overhauling the transfer case , I found that the gear splines were loose on the input shaft.

The service manual didn't have any guidelines on this, and my logic was "heck the truck only has 17,000 miles on it, so it's probably normal".

Now, I kinda worry about the shock to the other parts every time it clunks and clangs.:oops:

I drove mine to work this morning. Someone started from a green light and then stopped in front of me. Off power and clutch down caused a drive train clunk. It always does.

I have found when shifting with the clutch at speed/rpm/load combinations that would let me shift without the clutch. There is no clunk from the drive train. Basically, that is the happy speed for all the rotating parts and they don't protest. Each gear change on every drive will call for a different throttle/speed combination to find that happy speed. It makes the drive more of a challenge and when it is gotten correct. More fun too.

That make sense at all?

SM465 is synchro'd in 2nd 3rd and 4th...

sloppy synchros showing their age potentially?

Mine does the same thing. Ford 390/T18/divorced np205, lockers front and rear. I can minimize it like Barrman did by adjusting clutch/throttle habits, but it's still there.

I think in my case it's the transmission. I pulled it from a 71 F250, old farm truck and probably ready for a rebuild. At 3500 RPM and 60ish MPH the transmission protests... loudly

While I was swapping stuff out I noticed the slip joints on both my driveshafts were pretty sloppy. I swapped the ends out with the ones on the F250 donor truck. Same spline, same Ujoints.
Not saying your slip joints might be causing your thunk, that was causing a vibration at speed. Just something else to check when you're doing your swap.

I'm going to keep driving mine as is till I get the new windows on the house paid off. Then rebuild the trans and regear the axles... maybe some 38 inch XMLs.

Glad to hear that I'm not alone in the clunk club.
My slip joints still look like new
I suspect that the PO running the TC and transmission low on oil may account for a lot of the wear and slack. The transmission rear mount connections were so poorly executed (thats the white angle iron "spider-armed thing-ee" in the pic that used to connect the rear of the transmission to the factory cross-member) that when I got the truck that it you could measure what it leaked from its bent/damaged bolts in cups -not drips.
So far I'm happy with my revised tranny mount, torque arm and short slip joint shaft to the TC

That looks much better. Nice work.