We just got my sons 1976 Ford Explorer back on the road. It is a 1976 F100 Explorer extended cab that we replaced a 360 with a 300 six backed up by an FX automatic transmission. Between 45 and 50 mph the whole truch has a really bad vibration but above and below those mph it drives smooth. Besides the engine and transmission swap we had the front driveshaft tube replaced by a guy that has done driveshaft extending and shortening for rockcrawlers. I also Replaced the rearend seal. The "U" joints all felt good. Any ideas?

 

You may not have the right converter flexplate to get the balance correct. Did you use one that was made for the 300 engine ? Wrong one will beat the bearings out in short time if it's vibrating a lot.

 

It could be anything from a missing wheel weight to a nut missing from a u-joint u-bolt .. There are so many possibilities there .. How was it before drive shaft work ? Is it at the tires or in the drive train ? Also agree with Jim's idea

 

The 300 six and transmission we got were used before we got them as a unit so nothing was changed.

 

Okay I believe it is something else then. Any time i had one with the wrong flywheel or flexplate they would shake all of the time. I believe there is the possibility of the drive shaft you had work done to is out of balance or has a bad U-joint in it. Maybe even they joint where the couple together .

 

Maybe the driveline is out of phase? DRIVE LINE PHASING

make sure the u joints are on the same plane.

 

The guy that did the driveshaft made sure everything lined up. He had me bring all of the driveshaft so there wouldn't be any problem with the driveshaft.

 

Remove the front drive shaft then drive it and post back ..

 

I got the impression its a 2wd and hes talking about the short shaft, I maybe wrong....

 

Oops your right been working on too many Ford 4x4s today

 

In your first post you say it is a F-100 , so is it a short box ,wheelbase , or long box ? I have a 73 F-100 with the short box and it has a one piece drive shaft , I had a 79 F-100 and it was the same . They had a 302s , one with a 4 sp. and the other with 3 sp , that i changed to a C/4 later , both 2 wd. All three super cabs with two wd I had came with the two piece shaft.

 

Lets assume there was no vibration before the engine swap. Can you feel a vibration in the truck with your foot on the brake, truck in gear while raising the RPM? I don't know if you have a tach hooked up but 45 -50 mph would be about 2100-2300 RPM?
If you can feel it vibrate in this situation than you can probably rule out the drive train and start looking at the engine. I had a very similiar situation at about 2200 rpm going down the road, felt like a wheel was going to fall off. It turned out my 302 didn't like the richporter distributor I put in. I can't explain it, don't know if the gear was wrong for the after market cam that the PO put in but it's gone now.

 

Im having brake dragging issues on my 76 F-100...and when it does it feels like the truck is coming apart...perhaps this could also be happening?

 

I think we have it figured out. Comparing the middle carrier bearing of the 1976 F100 to my F250 middle carrier bearing the 1976 seems to have more side to side play. We're going to replace the carrier bearing this week and I do have one question. How does the old carrier bearing come off and how does the new go on?

 

Here you go ..