Different front rotors?

[kustomkyle]

I always assumed these cars used the same rotors 1978-1988, which were shared with the 2WD S-10s for a while too.

However, after a quick glance, for a 1980, 1981, and 1986 Monte Carlo there were three different part#s (!)? There were also different part#s for Grand Prixs all to themselves.

What was physically different about these? The part# for the 1986 models and up seem to be when everything was standardized (possibly 1984, I didn't look), which is the rotor shared with later S10s, like a 1994.

Assuming, if anything, the earlier ones were smaller in diameter?

 

[V8 Rumble]

Outer bearing od is larger for the some years. The overall rotor diameter is the same.

 

[ssn696]

78 only had the different bearing size. 78-81 used 7/16-20 wheel studs and 82-up used M12-1.5 lug studs.

 

[kustomkyle]

78 only had the different bearing size. 78-81 used 7/16-20 wheel studs and 82-up used M12-1.5 lug studs.

Ok, thanks that makes sense. I didn't think about them using the standard wheel studs at first then switching to metric.

Thankfully mine is a 1986.

 

[455'ed80Regal]

My car has the larger A3 outer wheel bearing. I may be wrong, but I think when the 3rd gen f-bodies and s series trucks came out that is when everything went metric on the studs as well as being downsized to the smaller A34 outer wheel bearing.

 

[ssn696]

I used 1986 rotors on my 1980 wagon to make all the lug nuts match the rear axle I had swapped in.

 

[Longroof79]

I replaced the front wheel studs with the metric M12 x 1.50 after changing the rear axle. I wanted the lug nut sizes to be uniform.

 

[Geo's66]

Ok guys, I'm doing a 78 Monte now... If I upgrade to blazer brakes, what are you saying is different? I thought it was a direct replacement bolt-in? Thanks. Thought I had the info on so many previous posts here that I needed

 

[clean8485]

Are you doing the 1998-2005 2WD Blazer front brake upgrade? If so, and you're going to retain the original 1978 Monte rear axle, then the front and rear wheel studs will not be the same. From 1978 to 1981, GM used SAE thread wheel studs in the A body cars, Starting in 1982, when GM changed the body style designation to the G body, they also changed the wheel studs to metric threads.

 

[ssn696]

When you change to the Blazer knuckles, the bearing issue solves itself (not the problem you posted, I know). I surfed RockAuto and came up with this data:
DORMAN 610025 7/16-20 Serrated Wheel Stud; .564 In. Knurl; 1-5/16 In. Length
DORMAN 610254 M12-1.50 Serrated Wheel Stud; 12.73mm Knurl; 41.5mm Length [.501" knurl]

It looks like replacing SAE studs with metric does not work (shank is smaller). You can redrill yours (between the existing holes) or find another used or new pair of axle shafts for an '82-up G-body. They are all the same for the 7-1/2" corporate axle. If you are doing any power upgrades, consider dropping $250 on an aftermarket pair of stronger axle shafts - you are currently relying on 37-year-old C-clips / grooves to keep the wheels on...

I did find a source of aftermarket 7/16-20 lug nuts with the secondary external threads that duplicate the GM metrics used on all the F-bodies and trucks in the 80s and 90s - has a black or sliver plastic cap that threads on over the nut - if the lugs will be exposed. This way you can't tell the difference, and they work better with aluminum wheels.