70 GMC Disc/5 Lug Conversion

[tallboyD]

I'm sorry if this topic has been beat to death, but I just want to make sure i'm going in the right direction. I have a 1970 GMC with drums all the way around. My question is what is the most affordable way to convert to disc and 5 lug...i was planning on buying a front disc conversion with drop spindles and booster, but the i'm a little unsure about the rear......can i directly swap 73-87 axles? what options do i have?

thanks for the help

[cooltx]

hey tallboy i dont think the 73 to 87 axles will bolt in but Speedway sells a 5 lug axle kit that will bolt into your original housing.

[mgchevyparts]

or buy the drop kit with the 6 lug rotors and keep it that way.

[tallboyD]

Thanks for the responses. I like the wheel choices with 5 lug. There is a guy near me parting out a 71 & 72, gonna see if he has a 5 lug rear with discs

[67 cst swb]

A 71-72 truck will have 5 lug rear axle, but no factory discs.... That is the way to go when swapping to 5 lug... or the speedway kit (Early Classic has axles kits also I think)

[5025speed]

You could just change the axles and drums from a 71 or 72.

[RenoKeene]

Keep in mind that 71-72 axles are wider than your 70. Do some research on here, lots of good posts on the subject.

[Mike C]

71-72 5 lug axles are the same width as 70. The 67-69 axles are narrower. 70 is the easier conversion since you don't have to change housings. But you need to measure to make sure your truck still has the original rear.

The 71-72 will have discs in the front, which honestly, is all you need. That is the easier swap. If you do a 73 and up you need the newer truck ball joints as well. I did a 73 and newer swap on a C20 but you need to research on a C10 as it may not be the same parts.

[tallboyD]

the more i look into it, i'm considering just keeping the 6 lug. I plan on bagging the truck and running 22s or 24s in all corners. I have found a few 6 lug wheels i like....any suggestions/opinions?
thanks everyone, love to hear from anyone who loves these trucks

[BADPURPLE70CHEVY]

Originally Posted by RenoKeene

Keep in mind that 71-72 axles are wider than your 70. Do some research on here, lots of good posts on the subject.

As stated measure your rear axle width. I lucked up and my 70 gmc had the wider rear just like the 71-72. I found a 72 donor truck and swapped axles.

[boostedc10]

Last I checked the 70 axle can be either narrow or wide one. I would measure it before doing anything.

[CC69Rat]

I have seen narrow and wide axle housing's in the 70 model year. I think it just depends on when the truck was built. (early builds would have possibly gotten the more narrow rear housing)

Lay under the truck from under the back bumper area. Have the wife / a buddy hold a tape measure on the inside edge of one rear brake backing plate. Measure the total length of the axle housing from one backing plate to the other (a.k.a. Plate-to-plate) it will be 57" for the later housing, and 55.5" for the early housing.

At least this way you'll know whick one you have in the truck (who knows, it could have been swapped over the years) 70 model could be 6 lug and the wide housing (I had one once upon a time) but I have seen the narrow ones too.

When you buy your 5 lug axles, the narrow housing (early) will be 30.5" long if you measure from the mounting surface of the wheel to the end of the splines. The later ones will be 31 5/16.

For example, say you find some for sale and you know you have the narrow housing.. if they come from the 71-72 model year and are 31 5/16 surface to splines.. they wont work. A lot of times you'll find original 5 lug for sale, but chances are they are the wide ones. If you had the wide housing.. youre in luck.

Go ahead and change your bearings and seals too while you're at it. It's pretty easy. If the pin bolt breaks off when you replace the axles, PM me, I know the fix for it.

IMO, I'd just get what you wanted from Moser that way youd know they're new and in good shape. If you find good GM ones, of course they'll be 40+yrs old. Just something to consider.

Welcome to the board by the way!

[ERASER5]

The measurement is from Wheel Mount Surface to WMS. Not the back of the brake backing plate. Different brakes can have different depths and contours. The WMS to WMS will always work.

I got some cheap angle iron stuff from the hardware store. Get the kind with lots of hole ans slots all ready in to. Bolt a piece to each drum and use a tape measure between.

[CC69Rat]

Regardless of where you measure from, the later rear end housing is 1.5" longer than the early housing. The plate to plate option just keeps you from having to take the wheels off to know which housing is in the truck. 6 of one 1/2 dozen of the other.

So, .. just for the sake of knowing, what's the WMS measurement for both the early and late housings?

[67 cst swb]

If I remember correctly...
71-72 is 65"
65-69 is 63-1/2"
70 could be either one

[lolife99]

When you are looking at rearends installed in a truck,... The quick and dirty measurement is backing plate to backing plate.
Super fast.
Super easy.
I've had three 1970 trucks and they all had the wider rearend.
But for 1970 you have to measure to be sure.

[tallboyD]

awesome that's a huge help...looks like mine is the wider rearend as well. Not to change the course of this post, but what wheel sizing advice if any do you have. My ultimate goal is an airbag setup and i'd like to be able to lay as low as possible. Looking at 22s now, i'd prefer to go with 24s, but have read it get really expensive to make those tuck. thanks again guys, love this forum

[Mike C]

I bought a set of Z-71 6 lug wheels to put on my '69 and that won't work. Drum brakes have the same pilot hole diameter as the 4 wheel drive trucks so the newer 6 lug wheels won't work without boring the center.