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What Stud Size?

10K views 66 replies 10 participants last post by  sudden_impulse 
#1 ·
My uncle is currently going through his 1974 Nova and replacing/upgrading parts. Not the best pics but this is the car.

He wants to go from an open diff 10 bolt to 12 bolt "Posi" rear end. I told him from what I understand the Truetrac is better. The car is just a cruiser, no drag racing or anything. Currently has a 350 in it but may upgrade to a 383 stroker.

Anyway, any idea what the correct stud size is for his stock drums? He doesn't have a caliper and I am helping him remotely. I assume the first bolt pattern is correct for his car?

Also on gear ratio any recommendations? He doesn't drive it very far so doubt it will be going 70 on the highway for very long at all but he doesn't want too aggressive of a gear ratio. Maybe stock or a notch or two more aggressive. Any suggestions? He said all he cares that it's a 12 bolt and has "positraction" or equivalent. Any help is appreciated as obviously we are not rear gear dudes. Lol

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#50 ·
If the number on the edge of the ring gear were visible we could tell you what ratio they are. I cannot tell just by looking at them. You can count the pinion rotations to one rotation of the wheel if you jack up the rear end. A 3.42 will be 3.42 pinion rotations to one rotatation of the wheel.

I could be wrong but the tire size you have given (325/60/15) don’t seem likely to be the correct size as pictured on the car. The new 3.42 gears are going to be a pretty big difference from the 2.73 ratio. The car will accelerate faster and will be at a higher RPM when at highway speed. I don’t recall if we discussed the transmission that is in his 74 but I will assume it is a non-overdrive automatic most likely a TH350.

Another thing to keep in mind is that the rear gear change will also effect the speedometer reading. The speedometer cable is connected to the transmission and the reading will be off and appear as a higher rate then the actual speed being driven. This can be corrected by switching the speedometer gear in the transmission. You can also use a app on your cell phone to check the actual speed through the GPS system and see how far off tha speedometer is.

I have posted a transmission gear ratio chart so you can run some scenarios and calculations as accurately as possible.
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#51 · (Edited)
If the number on the edge of the ring gear were visible we could tell you what ratio they are. I cannot tell just by looking at them. You can count the pinion rotations to one rotation of the wheel if you jack up the rear end. A 3.42 will be 3.42 pinion rotations to one rotatation of the wheel.

I could be wrong but the tire size you have given (325/60/15) don’t seem likely to be the correct size as pictured on the car. The new 3.42 gears are going to be a pretty big difference from the 2.73 ratio. The car will accelerate faster and will be at a higher RPM when at highway speed. I don’t recall if we discussed the transmission that is in his 74 but I will assume it is a non-overdrive automatic most likely a TH350.

Another thing to keep in mind is that the rear gear change will also effect the speedometer reading. The speedometer cable is connected to the transmission and the reading will be off and appear as a higher rate then the actual speed being driven. This can be corrected by switching the speedometer gear in the transmission. You can also use a app on your cell phone to check the actual speed through the GPS system and see how far off tha speedometer is.

I have posted a transmission gear ratio chart so you can run some scenarios and calculations as accurately as possible. View attachment 413275
On the rear tire size I meant to put 235/60R15, must have fat fingered it.

Also yes, he has a non-overdrive 3 speed. I can't say for sure but pretty sure it is a TH350 also.

So my uncle just called me and he said when the speedometer reads 60MPH, his tach (aftermarket), is reading 5,000 RPM and at around 70/75 it is reading 6,000RPM!!!

Good point on switching the speedometer gear, but even with that...these RPM's seem alarmingly high no?

I may be able to go for a ride with him tomorrow and use a GPS app on my phone to verify his actual speed, as my uncle is not technology inclined.

What would be the most likely scenario for having 5 to 6K RPM on the highway? He also says he feels the car shift into 3rd...Has to be something wonky with the rear end? I am thinking maybe there is a Nova at the drag strip as we speak going 55 down the strip, while we have a drag strip setup under ours o_O.

Any input is appreciated.
 
#55 ·
Dunno, he's had it for years and no idea if it was ever set up correctly. Based on his description of what the car is doing I believe they are right. He says the car is revved up high on the highway and he barely moving. We are going to jack it up tomorrow and see how many times the drive-shaft spins through one tire revolution. Our guess at the moment is the diff guy put the biggest gear he could in it.
 
#59 ·
The 1/2 turn free play between one side and the other is just gear lash take-up in all the workings of the helical diff.
Sounds like a gear ratio close to the low to mid 3's to me.

You need to be looking elsewhere....either in your uncle's head(LOL), wrong tach setting,..... or trans not upshifting if rpm really is overly high.
Verify the tach is set for the correct number of engine cylinders, and not on the 4 or 6 setting.
Then take it out on the road, use your GPS and tell us what the tach reads for rpm @ GPS indicated 60 mph.
 
#60 ·
The 1/2 turn free play between one side and the other is just gear lash take-up in all the workings of the helical diff.
Sounds like a gear ratio close to the low to mid 3's to me.

You need to be looking elsewhere....either in your uncle's head(LOL), wrong tach setting,..... or trans not upshifting if rpm really is overly high.
Verify the tach is set for the correct number of engine cylinders, and not on the 4 or 6 setting.
Then take it out on the road, use your GPS and tell us what the tach reads for rpm @ GPS indicated 60 mph.
Ok, my uncle is now saying (before I read the message) that the issue is trans related. He doesn't believe it is shifting into 3rd. May be a while before we verify but wanted to say thanks for the tips and info. Good to know the backlash is normal. I'll update this post once the car has a new transmission and what not. I think he said he wants a TH750 or something like that.
 
#62 ·
@Ericnova and @Nova Thug

Well my uncle is convinced it is the tranny... I still haven't been able to see the problem in person myself and haven't talked to him much myself. I believe it is tranny or torque converter myself just based on the symptoms but I always leave room for my guess to be wrong...

Anyway, my uncle wanted to put a new tranny in the car anyway. He is saying he wants a 700r "performance" automatic tranny. Any input on this? I will start a new post but figured you guys may know a thing or two as if we are headed in the right direction as far as the tranny goes?
 
#64 ·
Well he isn't really interested in troubleshooting. Rip and replace I suppose haha. He wants to put a 700r tranmssion in it, with new torque converter, etc... maybe eventually a 383 stroker also. So figured not much left to do except let the tranmy guy do his thing lol. Will update when this happens. Likely will be a few months or so.
 
#66 ·
With the 3.42 gear ratio in the differential and the 3.06 1st gear in the 700r4, that car should take off from a stop noticeably stronger:. These two swaps represent pretty big mechanical advantages over the original differential gear and trans ratios. The rpms will be higher at first but will not be too obnoxious. The shift into 2nd gear is a pretty big drop in rpms.

A shift kit is a modification to the valve body in the transmission that makes the shifts between 1st, 2nd, 3rd, & 4th quicker and more defined. Basically, it minimizes the slippage and heat that takes place as a result of shifting. A shift kit is an enhancement that makes for better towing and performance minded usage. Less friction and heat shoulf make for a longer service life of the transmission.
 
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