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1968 Mustang Fastback, 289, T5-5 speed
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Discussion Starter #1
I put a T5 in my 68 289 with 8” rear diff about a month ago. I had the driveshaft shortened, new u-joint and balanced. Car ran great, nice and smooth.



I then swapped the 2.79 open 3rd member with a 3.55 LS Yukon. I pulled the rear wheels off, rear brake drums, slide the axles out, remove driveshaft, pull 3rd member out. The passenger side axle bearing felt 100% good. The driver side felt 95% good. So I had just the driver side bearing replaced.



Installed the new 3rd member (no gasket, just RTV), 80w90 with the 4oz friction oil, axles, brake drums, driveshaft, wheels.



I now have a slight vibration that starts at 70mph. It’s not terrible but I can feel it in my seat and rattles the car a little. I’m certain it wasn’t there before the 3rd member gear swap. So it has to be something I have just done.



I need to jack up the car and change the speedo gear to the correct one. The first thing I will check is flip the driveshaft 180 degrees. I did NOT mark it when I took it off. I didn’t think it mattered but reading that it can affect it. I’ll also torque the u-joint clamps. Maybe I over tightened them.



What else should I check? Any ideas?



I contacted my diff builder and he said he’s never had a 3rd member cause vibrations.



The car is very drivable. Maybe I’m just being paranoid because I’m driving with the radio off and specifically listening for issues. It just doesn’t seem as smooth as it was with the 2.79 open gears above 70 mph. I don’t think it’s an RPM issue because even with the 3.55 gears I’m at a lower rpm then I was with my 3 speed and 2.79 gears.
 

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I would venture a guess that the Yukon gears just don’t compete with Ford factory gears. I’ve read that Yukon’s can be noisy with certain ratios, but I have no real experience. As for gears creating a vibration, I think your right on with re-indexing the driveshaft. Try that first, it’s easy. Report back with your findings.
 
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Check carefully that the u joint is fully seated in the yoke. If it is you can try removing the driveshaft and rotating it 180*s and bolt it back down. A ring and pinion or carrier will not cause the problem you are having unless the pinion geag itself is bent. This would not be the case on a new ring and pinion.
 
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The way a pinion gear is lapped on a Gleason lapping machine would prevent a "bent" or out of spec pinion gear from getting out. That's why I said "would" instead of could. In my 52 years of doing Ford differentials , I have yet to see a pinion over .0002 "out". In the same amount of years I have only seen one bent 9" pinion and it was from a racecar that crashed. I broke it in two when I tried to straighten it. Lesson learned , throw it away.
 

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eric69 may be onto something there. Did you happen to have the axle straightness checked when the bearing was replaced? If you do take the 3.55 3rd member out to try the 2.79 again, try a LubeLocker gasket. They work very well and no RTV mess.
 

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1968 Mustang Fastback, 289, T5-5 speed
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Discussion Starter #10
eric69 may be onto something there. Did you happen to have the axle straightness checked when the bearing was replaced? If you do take the 3.55 3rd member out to try the 2.79 again, try a LubeLocker gasket. They work very well and no RTV mess.
I flipped the driveshaft 180 degrees. Seems to have helped a little. Did not cure everything but may have it back to the “standard” it was at before.

Before I flipped it the vibration was worse on de-acceleration. Now it seems about the same.

I jacked up the car and spun the wheels at idle in 2nd gear. Both rear tires do not spin 100% true. Both have a slight “wiggle”. Probably been like that for the 25 years I have been driving it.

I will drive it for a bit and see what I think.


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Sounds like a driveline angle issue that has been magnified by the gear change (higher driveline speeds). Swapping in the T5 sometimes drops the tail shaft down a tick more than stock. Tremec has a really handy app you can get that will measure some working angles and give you an idea what's potentially off..
 

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Discussion Starter #12
Have you checked pinion angle?
Everything seemed fine until I changed the gears. So I assumed the angle was okay.


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1968 Mustang Fastback, 289, T5-5 speed
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Discussion Starter #13
Sounds like a driveline angle issue that has been magnified by the gear change (higher driveline speeds). Swapping in the T5 sometimes drops the tail shaft down a tick more than stock. Tremec has a really handy app you can get that will measure some working angles and give you an idea what's potentially off..
I’ll look into that.


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1966 GT Fastback, 289, TKO 5-spd, EFI, 4-discs, TCP coilovers
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I put a T5 in my 68 289 with 8” rear diff about a month ago. I had the driveshaft shortened, new u-joint and balanced. Car ran great, nice and smooth.



I then swapped the 2.79 open 3rd member with a 3.55 LS Yukon. I pulled the rear wheels off, rear brake drums, slide the axles out, remove driveshaft, pull 3rd member out. The passenger side axle bearing felt 100% good. The driver side felt 95% good. So I had just the driver side bearing replaced.



Installed the new 3rd member (no gasket, just RTV), 80w90 with the 4oz friction oil, axles, brake drums, driveshaft, wheels.



I now have a slight vibration that starts at 70mph. It’s not terrible but I can feel it in my seat and rattles the car a little. I’m certain it wasn’t there before the 3rd member gear swap. So it has to be something I have just done.



I need to jack up the car and change the speedo gear to the correct one. The first thing I will check is flip the driveshaft 180 degrees. I did NOT mark it when I took it off. I didn’t think it mattered but reading that it can affect it. I’ll also torque the u-joint clamps. Maybe I over tightened them.



What else should I check? Any ideas?



I contacted my diff builder and he said he’s never had a 3rd member cause vibrations.



The car is very drivable. Maybe I’m just being paranoid because I’m driving with the radio off and specifically listening for issues. It just doesn’t seem as smooth as it was with the 2.79 open gears above 70 mph. I don’t think it’s an RPM issue because even with the 3.55 gears I’m at a lower rpm then I was with my 3 speed and 2.79 gears.
I did the exact same thing on my '66 FB, and got the exact same result you did. I rehabed my 8" housing, put in new axle bearings and seals, had the axles checked for runout, and went from an open 3.00 stock gears to 3.55 trac-lok, but I also put in new leaf springs. Besides whining gears (still), I had a terrible vibration at 55 mph, so I pulled the third member and had a second ring and pinion set put in (US Steel brand). During this session, I learned that I had the wrong u-joint on the pinion side (3 1/16", instead of 3 1/8"), and you could see that the old u-joint had been moving in the carrier. The correct "conversion" u-joint helped with the vibration, but did not fix it entirely (but like yours, it is driveable), so now I need to check the pinion angle. I also have not completely solved the rear gear noise issue, and the comment about "modern" gear sets not being as good as original Ford steel rings true for me. Unfortunately, you can't get 8" ring and pinion sets from Ford Racing any more, just 9". Now I'm thinking about a full-on 9" replacement - the whole shebang.

Nothing is really straightforward ...
 

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1968 Mustang Fastback, 289, T5-5 speed
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Discussion Starter #15
I did the exact same thing on my '66 FB, and got the exact same result you did. I rehabed my 8" housing, put in new axle bearings and seals, had the axles checked for runout, and went from an open 3.00 stock gears to 3.55 trac-lok, but I also put in new leaf springs. Besides whining gears (still), I had a terrible vibration at 55 mph, so I pulled the third member and had a second ring and pinion set put in (US Steel brand). During this session, I learned that I had the wrong u-joint on the pinion side (3 1/16", instead of 3 1/8"), and you could see that the old u-joint had been moving in the carrier. The correct "conversion" u-joint helped with the vibration, but did not fix it entirely (but like yours, it is driveable), so now I need to check the pinion angle. I also have not completely solved the rear gear noise issue, and the comment about "modern" gear sets not being as good as original Ford steel rings true for me. Unfortunately, you can't get 8" ring and pinion sets from Ford Racing any more, just 9". Now I'm thinking about a full-on 9" replacement - the whole shebang.

Nothing is really straightforward ...
I don't really have any noise. Just a slight vibration that starts around 70mph.

I'll try checking the driveline angle tonight and go from there.
 
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