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Discussion Starter #1
Hello Folks

Dead original 66 Fastback, V8 car, Whole underside is original and has not had rails or pans or anything

4 speed car, had a modern driveline kit fitted to the car. 5 speed T5, New everything, Had driveshaft balanced. Had rear axle assembly sent off to "the guys" in the UK for doing these things, new ltd slip, new bearings, new gears installed. Axle housing straightened at the same time

Had a vibration over 60mph from the initial install over 60mph. This led to me having the driveshaft balanced and the rear axle looked at. Still there

I had it in a garage who felt it was the rear axle so the car went back to the specialists who dissembled the rear axle, pulled the axle shafts, pulled the diff. All perfect/as new.

They said it was a driveshaft alignment issue. The tail of the trans needs to go up. Unfortunately there is no clearance to do this. They also tried angling the rear diff a bit to check if it helped, it did not.

Anyone have this issue? If you did sort, how did you do so?

Their suggestion was to alter the trans tunnel which I won't be doing on this car.

Many thanks, Peter
 

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Are you using a stock rubber trans mount or a poly mount? You could also try and lower the engine with the Ron Morris mounts.
 

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Discussion Starter #3
Stock rubber mount

When we are released from lockdown here in the UK I will post look at it on a car ramp to see if lowering the motor will help.

Thinking about it you could lower trans mount to give more pivot clearance when you lower the engine..Cheers Peter
 

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I'll be curious to see what you find as I've got the same issue. Mine's a '65 fb, 351W, T5, Currie 9" rear end- vibration feels cyclical at 55+; all new components, new dshaft rebalanced twice, new tires balanced twice, new trans rubber mounts, everything triple checked and supposedly fine shape. I do have the Ron Morris engine mounts and had to slightly raise the engine and bring is forward for clearance. When I checked the angles, they were all good- the engine and rear axle were at the same angle and the shaft was something like 3-4 degrees. I assume you've already dug into yours and determined it still vibrates when at speed and with the clutch pedal depressed?
 

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I have a very similar setup in my '66 fastback. MDL supplied all the conversion components including the trans crossmember and mount. I installed new driveshaft, U-joints and Currie diff. Up front, I used Ron Morris mounts with 1/4" spacers to raise the front of the engine slightly for header clearance. I have experienced no vibration at any speed. I did use my 4-speed bellhousing and the adapter supplied by MDL.
 

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I cut the the tunnel support on my coupe and re-shaped it to allow for more room to shim up the tail shaft of the transmission. This allowed me to get the angle at 3 degrees.
748145
 

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Discussion Starter #8
Car is a 66 GT350 so I cant cut the tunnel area :)

I import and sell Mustangs here in the UK and have had several with T5s including a near race example in stock now. None have had this issue

The axle specialist said my driveshaft was a touch short.
 

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Discussion Starter #9
Hello GT350sr.

Mechanics words. Engine and trans mounts all look stock and in good condition. Propshaft looks good, Vibration is speed related through the drivetrain. Putting the car in neutral/holding the clutch down doesn't change the vibration at all.

Cheers. Peter
 

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You need to check the driveline angles. Tremec has a need app to do so. Also, have you tried ruling out the wheels and tires?
 

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Did you check your total operating angle of the driveshaft yet? If not correct it can give you exactly what you have, a speed related vibration that doesn't go away when you push in the clutch. Might not be it but if you don't check it you won't know.
 

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M.2.0 nailed it. You need to relief cut the trans tunnel brace. I did my T5 mock up over the winter and could not achieve the necessary driveshaft angle needed for a mild drop in a 65-66 without that cut. A stock height 65 66 will work, but the second you do a 1” drop or reverse eye, or blocks, forget it.
 

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Car is a 66 GT350 so I cant cut the tunnel area :)

I import and sell Mustangs here in the UK and have had several with T5s including a near race example in stock now. None have had this issue

The axle specialist said my driveshaft was a touch short.
My guess (prior to reading the post I'm replying to) is that the shaft is a bit too long, pressing the slip yoke too far into the tranny tail.
I was told by many when I swapped mine from C4 to T5 that I might have to shorten the drive shaft. I was one of the lucky ones. Everything fit nicely.
 

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I could be mistaken, when I was reading up on driveline geometry I believe I read it is desirable to have a small difference between the trans angle and pinion angles. Only like 1/4 degree. For example if the trans is 3 degrees down the pinion would want to be 3.25 degrees up.
 

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I went through my car front to back.. chasing a driveline vibration with the T5 install. There are many things that can cause it, but most are driveline angle. Some have bad u joints, some have bent or out of balance driveshafts. Some are broken trans mounts. I've even seen one where the bell housing wasn't aligned. But that trans was intended for a Fox body and not a '66! Sure they 'fit'.. but it usually results in the tail pointing downward..

You know what solved it? we got the trans angle up by using Ron Morris lowering mounts.

I then identified another vibration that turned out to be a bent axle housing!! Took forever to find.. until I dropped it out and measured front to back...
 

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M.2.0 nailed it. You need to relief cut the trans tunnel brace. I did my T5 mock up over the winter and could not achieve the necessary driveshaft angle needed for a mild drop in a 65-66 without that cut. A stock height 65 66 will work, but the second you do a 1” drop or reverse eye, or blocks, forget it.
Then mine should vibrate like a personal toy ! My car went from stock T10 with an 8 inch to T5 ,Versailles 9 inch with the factory rubber isolated mounts to springs. Non modified driveshaft.
 

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Discussion Starter #18
Wheels and tyres are new and balanced

I need to wait til I can get back to work (we are currently in lockdown here in the UK) and take readings of the angles from the using the website information mentioned above. Many thanks, Peter
 

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Discussion Starter #19
Hello Folks, Trying to find Ron Morris adjustable motor mounts. Can anyone tell me where I can purchase? I have looked at Summt Racing and strangely they don't have them.
 
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