1977 C20 Heavy duty with 11.15 drums?

[klunker797]

Hey guys,

I have a 1977 c20 sierra with a 10.5” 14. Bolt FF differential. I want to replace the master cylinder but I need to know if the drums are 11.15 or 13”. I want to avoid removing the drums to measure internally.
From everything I read, they should be a 13” drum ( especially being that it’s a 10.5 FF) but it’s GVWR is only 6400 and by measuring from underneath the truck, the internal drum seemed to be approximately 12”. There is also no mention of JB6 or JB7 on the RPO code sheet.Seems simple, but I don’t want to install the wrong MC.

Thanks for the help!

 

[HotWheelsBurban]

That drum does seem a bit small....my one ton has that rear axle but hydro boost instead of vacuum booster, and mine definitely has the 13" drums. Got a full floater out of a one ton square body in the shop, and I'm 99% sure it has the 13" too.

 

[legopnuematic]

especially being that it’s a 10.5 FF

Not really a factor in this case, until the introduction of the 9.5 14b SF all 3/4 tons were 10.5 full floats.

J55 is the RPO code for HD brakes.

You have the JB6 brakes and 11" drums. JB6 master reservoir walls are straight up and down, JB7 has a bulge in-between the bails.

 

[Shorty81]

Wow, that is a sharp truck!!!

 

[Old Guy Bill]

My ‘78 K20 has the 11 5/32” drums on a 14 bolt FF.
You need to pull a drum to verify…

 

[AuroraGirl]

My ‘78 K20 has the 11 5/32” drums on a 14 bolt FF.
You need to pull a drum to verify…

no you dont, if you have a 14 bolt you simply need to look at the back of the wheel where the drum is. the 11 inch drum will have lots of room

The 13 will fill up the gap

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[bucket]

Yep, it's a very obvious size difference between the two of them. Anybody that has seen both drum sizes can tell by looking at them. And now thanks to @AuroraGirl posting pictures, anyone that has read this thread can now know what size they have, without finding a tape measure.

 

[AuroraGirl]

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Heres some more differences in the 1973-1987 or so 14 bolt FF brakes (11 to 13)
I have no idea what the slip on era (88-01?) look like 11vs 13 or if they even have both. figured this would be good reference too kinda like bucket said

 

[klunker797]

Thanks everyone. Very helpful, that’s why I love this group!