Calling all "Double Hump" crossmembers.

[THE "10" SILVERADO]

Thinking about upgrading to a double hump crossmember to clean up my exhaust since I'm installing headers but I'm having a bit of trouble.

So far I've found out that these trucks have them,
73-75 1/2 ton
76-79(?) 3/4 or 1 tons

If that information is wrong please correct me

My questions though have to do with installation.

1) So far I have read stories of people's transmissions being at different heights causing problems, will the crossmembers from the above models cause this?

2) The few pictures I have seen show supports bolted to the top of the frame, are these required or can I just skip these and have it bolted on the bottom of the frame on both sides?


Any help would be appreciated. I've probably gone through 40 threads about these and most were created back in like 02 and 03 so the pictures are long gone. Any pictures would be nice too.

Thanks,
Michael

[J Knight]

I converted to this crossmember without using the top braces in my 78 GMC 1/2 SWB 350ci/TH350 with no issues other than having to drill mounting hole on the lower left side frame flange. I do know they are in 73-76 1/2 tons, not sure how long they were used in 3/4 and 1 tons. I plan on using this one on my 82.

[THE "10" SILVERADO]

Originally Posted by J Knight

I converted to this crossmember without using the top braces in my 78 GMC 1/2 SWB 350ci/TH350 with no issues other than having to drill mounting hole on the lower left side frame flange. I do know they are in 73-76 1/2 tons, not sure how long they were used in 3/4 and 1 tons. I plan on using this one on my 82.

Thanks, glad to know I don't have to use those top brackets. 1 more question though, does the crossmember bolt onto the bottom of the frame or like inside above the lower flange?

[KQQL IT]

You can use a 3/4 ton x member on a 1/2 you will need to notch the center to keep the angles right. As 3/4 ton frame is deeper thru that area. Some minor changes to upper bracket.
Drill new holes and some tweaking where its bolts to the upper flange. Also by using a 3/4 ton makes room for 3" pipes.
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[J Knight]

Originally Posted by THE "10" SILVERADO

Thanks, glad to know I don't have to use those top brackets. 1 more question though, does the crossmember bolt onto the bottom of the frame or like inside above the lower flange?

They bolt to the inside of the frame flange on both sides just like your current crossmember does on the right side. I don't know if its advisable or not to go without top braces but like I said, I didn't have any problems when I did it.
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[Jonboy]

I left mine off as well. Don't know if it is the right thing to do, but it seems to work.

[Hertzdaddy]

I have the double hump on my 73 Jimmy and it had the upper braces removed when I bought it and the exhaust run above the x-member. I found some uppers from later model trucks and installed them. You can use uppers from either side, they are reversible.

I felt like I needed them since the Jimmy is a full 'vert and flexes a lot. I know I had to do some fancy spreading on the frame to get all of the x-member holes to line up. I had to drop the exhaust back down to the stock location too. A non-'vert application may not need them though.

[Hertzdaddy]

Originally Posted by J Knight

They bolt to the inside of the frame flange on both sides just like your current crossmember does on the right side. I don't know if its advisable or not to go without top braces but like I said, I didn't have any problems when I did it.
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I suppose you could flip the x-member upside down and bolt it to the bottom of the frame rail, weld the uppers to the underside (now facing up) and maybe have enough room to pass the exhaust between the upper and recessed area on the flipped x-member.

I'd like to see a pic of the 3/4 x-member installed too.

[Corts60]

I know there was just a recent thread about this not too long ago. I thought we determined that any truck with factory dual exhaust came with them. I know my '78 1/2 ton diesel has the dual hump because it had factory dual exhaust.

[superwade57]

hey jo(H)n (knight) is it possible for you to get me an overall length on that crossmember and a measurement from the outside edge to the center of the exhaust hump ... on both sides if they differ? im looking at taking two regular crossmembers and doing a little cut and paste to make my own to accomodate my headers and hold up my t56

[KQQL IT]

Like so.


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[J Knight]

Originally Posted by superwade57

hey jo(H)n (knight) is it possible for you to get me an overall length on that crossmember and a measurement from the outside edge to the center of the exhaust hump ... on both sides if they differ? im looking at taking two regular crossmembers and doing a little cut and paste to make my own to accomodate my headers and hold up my t56

Here you go Wade,
Overall width is........................................33in
Outer edge to center exhaust hump.............4.5in (both sides)
Exhaust hump rise from bottom of xmember...1.25in

These are exact measurements not appoximate, I was suprised it didnt have a lot of oddball fractions in the measurements.

BTW Why did you spell my name that way?

[bigblock73]

I am guessing he assumed your first name was John, but didn't know if it had an H or not.

[J Knight]

Originally Posted by bigblock73

I am guessing he assumed your first name was John, but didn't know if it had an H or not.

LOL, I hadn't thought of that Jeff, when I first saw it I thought Wade had to do that to get it past the word filters or something.
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[gchemist]

Someone on this board made me one.


I have pictures of it installed. I just can't find them on my web photo album. The higher hump allows the pipes to be tucked up under the frame.

Only the driveside header is seen from the side.


The Chevelle headerare designed one lower than the other. My guess is to fit the starter on the other shorter header side.

[superwade57]

Originally Posted by bigblock73

I am guessing he assumed your first name was John, but didn't know if it had an H or not.

correct, i forgot which it was but if i remember correctly it does have an "H" having been called Wayne to often in my life i assume the with or with out h thing is a major pet peeve of yours and i wanted to be sure if i couldnt do it right, that i didnt do it wrong. thanks for the measurements, do the humps line up pretty well with headers?