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1970 Ford Mustang Convertible
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Discussion Starter · #1 ·
Hi everyone,

So I had the car into the mechanic the other day for some routine maintenance (oil change, tire rotation, brake check, etc) and was told that I need to replace the third member gasket. I have always known that this was going to happen one day and I guess now is the time. I know that in addition to doing the third-member gasket, he will probably be replacing the axel bearings, axel seals, and possibly the pinion seal.

The question I have is if the ring and pinion gears are somehow damaged, what do I replace them with? I am not finding any manufacturers that are making 2.79 gears. I am wanting to keep the open differential and I know the very, very maximum ratio I want to go to is 3.25 (but I don't want to do this), which means that the only ratio that seems available is 3.00.

Am I missing something...should I upgrade to a 3.00 ratio (will there be a difference in acceleration...don't really care about mileage as this is just a beach cruiser...302 2v engine...so no real horsepower to speak of), am I missing a manufacturer that makes a gear set with a 2.79 ratio?

Many thanks
 

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Who's to say there's anything wrong with the gears? If there is a noise not fixed with a new bearing or there are chipped teeth, sure, but these things are TOUGH. Besides, there are tons of donor 2.79, 2.80 and 3.00 rears out there. Don't let the mechanic talk you into an unnecessary expense and delay at getting back on the road.
 

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1970 Ford Mustang Convertible
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Discussion Starter · #5 ·
Who's to say there's anything wrong with the gears? If there is a noise not fixed with a new bearing or there are chipped teeth, sure, but these things are TOUGH. Besides, there are tons of donor 2.79, 2.80 and 3.00 rears out there. Don't let the mechanic talk you into an unnecessary expense and delay at getting back on the road.
Mechanic is not talking me into anything (he is one who is all about keeping the car as original as possible)...I am trying to think of possibilities / opportunities / anticipating problems. I have already resigned myself that this could be a $3k expense...but if it lasts another 50 years...then I don't care, and it gives the mechanic a chance to see if there is anything else going on (condition of axles). This is the only part of the drivetrain that my mother did not have rebuilt back in 1983. Chassis has appx 170k miles on it...so its time for this to be done (engine and transmission probably have around 30k on rebuild).

I must admit it is a little tempting to do a mild upgrade to the gear ratio while the pumpkin is out of the car just so its a little easier to pass others when I want to (2 barrel carb + 3 speed C4 + 2.79 gears do not a quick passing make...really have to get it up to speed to really pass anyone).
 

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I hear ya. I've been satisfied with my 2.80s because I pass more gas pumps that way :ROFLMAO:
I haven't had any issues with my giddy-up but my engine had a nice build and the first two gears on Ye Olde C-Four are steep enough to make it thunder. If I were to upgrade to an OD transmission I would probably get a little steeper out back.
 

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As far as rear gears go, 2.79 and 3.0 are very very close and you'll barely notice a change. That said, I personally would want to get into a more performance gear ratio. I like things sporty so I would probably go 3.73. You could cut the difference and go 3.55. This'll add a bunch of pep but not kill the mileage.
 

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Post a want add,
Many times guys will be swapping out original 8” & 9” open 2.79 & 3.0’s for 3.55, 3.73 or 3.89 posi’s and selling their old units for cheap.
i swapped out stock 3rd member, open 2.79 out of our 66 for a 3.55 posi and gave the complete 3rd member away for free.
It was never going back in and taking. Up space.
I am sure there are lots fo4 5hose to 3 had.
 

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Chances are your gears are just fine. Only reason to replace is if you want to change the ratio. I’d just seal it up and move on.
 

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67 Fastback T5 331 TCI Frt End, Canted 4 link rear susp
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Since the axles need to be pulled, it seem at 170K miles it would make sense to do the wheel bearings and axles seals.
 

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Mechanic is not talking me into anything (he is one who is all about keeping the car as original as possible)...I am trying to think of possibilities / opportunities / anticipating problems. I have already resigned myself that this could be a $3k expense...but if it lasts another 50 years...then I don't care, and it gives the mechanic a chance to see if there is anything else going on (condition of axles). This is the only part of the drivetrain that my mother did not have rebuilt back in 1983. Chassis has appx 170k miles on it...so its time for this to be done (engine and transmission probably have around 30k on rebuild).

I must admit it is a little tempting to do a mild upgrade to the gear ratio while the pumpkin is out of the car just so its a little easier to pass others when I want to (2 barrel carb + 3 speed C4 + 2.79 gears do not a quick passing make...really have to get it up to speed to really pass anyone).
Resealing your rear end after 50 years would be a good thing, if you didnt here any strange noises like a clunking or cracking noise you are probably golden. A bad bearing makes a much different noise, if thats the case you might have a bad wheel bearing and galled axle. A leaking rearend is not that much to worry about. I have a complete 3rd member for sale as im sure there are others on here that could help you out if necessary. Good Luck
 

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1970 Ford Mustang Convertible
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Discussion Starter · #12 ·
Wow...did not realize this would blow up this quick :). I guess the thing to do is to wait and hear back from the mechanic. The reason I am worried about the rear gears condition is that this leak has been going on for years...and honestly I am so used to what the car sounds like on the front end that I am not sure what to expect. I am excited about this (not happy about the cost...but it will be worth it).

I guess I will wait to hear from the mechanic on the condition of everything once he gets inside (goes in next Thursday). I will update the thread when the project is finished. But yes I will make sure that the wheel bearings are changed out since "while we are in there" mentality.
 

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My motto when doing car work is "once it's apart, do what needs to be done". Bearings, seals, gasket, etc. makes total sense to do while the third member is out. It's also a good time to check the driveshaft U-Joints and have it balanced if it's of similar vintage. Also, the rubber brake hose from the axle to the body if it's got a lot of years on it.

I assume the reason you don't want to go to a 3.25 (or lower) ratio is you do some highway cruising or don't want to turn too many revs @ cruise? With your mostly stock engine, gears will have the biggest seat-of-the-pants improvement in acceleration. In my old Fox Body, I went from a 2.73 to a 3.27 (8.8" rear) and it felt like I added 50 HP. Yes it turned more revs but it was totally worth it for the around-town performance improvement.

I think a 3.25 would be an awesome choice and a good compromise between picking up extra acceleration and still having a reasonable cruise RPM.
 

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So far, only the 10 dollar gasket is leaking. I would just ask your mechanic to replace the gasket and inspect the rest he has to remove for the job. No need to replace things that are not broken.
 

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Wow...did not realize this would blow up this quick :). I guess the thing to do is to wait and hear back from the mechanic. The reason I am worried about the rear gears condition is that this leak has been going on for years...and honestly I am so used to what the car sounds like on the front end that I am not sure what to expect. I am excited about this (not happy about the cost...but it will be worth it).

I guess I will wait to hear from the mechanic on the condition of everything once he gets inside (goes in next Thursday). I will update the thread when the project is finished. But yes I will make sure that the wheel bearings are changed out since "while we are in there" mentality.
Good thinking, check everything twice, install once.;)
 

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1970 Ford Mustang Convertible
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Discussion Starter · #16 ·
My motto when doing car work is "once it's apart, do what needs to be done". Bearings, seals, gasket, etc. makes total sense to do while the third member is out. It's also a good time to check the driveshaft U-Joints and have it balanced if it's of similar vintage. Also, the rubber brake hose from the axle to the body if it's got a lot of years on it.

I assume the reason you don't want to go to a 3.25 (or lower) ratio is you do some highway cruising or don't want to turn too many revs @ cruise? With your mostly stock engine, gears will have the biggest seat-of-the-pants improvement in acceleration. In my old Fox Body, I went from a 2.73 to a 3.27 (8.8" rear) and it felt like I added 50 HP. Yes it turned more revs but it was totally worth it for the around-town performance improvement.

I think a 3.25 would be an awesome choice and a good compromise between picking up extra acceleration and still having a reasonable cruise RPM.
U joints were replaced recently...do not thing driveshaft needs to be re-balanced since I do not have any real vibrations upon acceleration (though I do get a small vibration while at idle in drive...shift to neutral and the vibration goes away...any ideas on this)
 

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U joints were replaced recently...do not thing driveshaft needs to be re-balanced since I do not have any real vibrations upon acceleration (though I do get a small vibration while at idle in drive...shift to neutral and the vibration goes away...any ideas on this)
Motor mounts , trans mount?
 

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1965 Ford Mustang fastback T5 Ncas 9in Locker
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