lug pattern for after market disc brakes?

[ZenC10]

Soooooo, in my failed attemps of the use of an 83 rearend. i am looking to buy new 5 lug axles to replace the 6 lug on my 70. i've have conflicting feedbback on what years fit what. i messured the lug pattern on my front disc's and it is 5 on 4.75. "which seems odd to me"! i was under the impression that the 71-72 lug patterns were 5 on 5. can anyone tell me what the pattern on their truck is? stock or even better aftermarket?
also i found an eaton posi for sale, it came out of a 12 bolt 79 blazer. it's 30 spline. i just need to know if it the right application for my 70 housing. bearings etc.

[CC69Rat]

The key here is going to be the length of the rear end housing. Yours being a 70, it could either measure 55.5" or 57" if you measure from the inside of the brake backing plate to the other.

1967 thru and including most of the 69 model year were 55.5" long, 'Plate-to-Plate'

In 1970 it was a transition year. Transition year ??

In 71, as you know they started putting discs up front and it widened the front track width. So, they had to compensate on the rear housings too. The 1970 (late) thru 1972 housings will be 57" 'Plate-to-Plate'

It depends on which housing you have. I have a 70 model GMC, 6 lug originally but it's the late rear housing. 70 can get a little goofy. I have seen the narrow housings and wide.. it's a crap shoot really. I think they just used what they had available.

Knowing this, means you will have different size axles. If the original axles were 30.5" long from the FRONT of the mounting plate to the end of the axle.. it's an early axle. The late ones were 31 5/16 using this same measurement.

If you find you have 57" plate to plate .. you need 31 5/16" replacement axles.
If you find you have 55.5" plate to plate .. you need 30 1/2" axles.

the earlier housings are more desirable, because you can run more backspacing on your wheels. The axle will be .75" more narrow per side. Make sense? All 5 lugs I have seen have been 5x5

I do believe they make and sell a 5 x 4.75 conversion rotor for the front, I would think it would open a lot of options for wheels .. but I'm not sure I have ever seen 5 x 4.75 axles for the rear (?) Someone will correct me I'm sure .. I have just personally never seen them available.

I'd get 5x5 pattern rotors before I put 5x4.75 pattern axles if it were me. Just saying.


On the posi, I'm not sure. I'm probably not the best person to answer... but I will listen so I can learn too.


Welcome to the board by the way!

[ZenC10]

awesome feed back! i never thought about it being a split year... i have my heart set on the wheels i want so i guess ill go forward with that in mind. i've seen some 4.75 pattern axles on summits site, but did know the length issues. now i do! thx a million. this has been a headache...

[lolife99]

Are you sure you are measureing the front correctly?
They should be the 5x5 pattern.

[ZenC10]

you just saved me alot of trouble, i was under the impression it was the center of each lug. so i am running a 5x5. thats makes things easier. thanks!
To cc69rat, i meassured plate to plate and it came out to be 58". but the origional axle is 31 5/16. so as long as the axle lenghts are the same then i know what i need as far as the axles go. thx again to both of ya

[CC69Rat]

Yeah.. you have the wider axle housing. Get the axles for 71-72 and they will be 5 lug already. Might be cheaper to do a whole rear housing swap if you can find one with a posi already in it.
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