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Necoa #: 1108
Location: 17(S.CA,S.NV)
Joined: Jun 17, 2003
Posts: 296
Year: 1977
Model: El Camino
Posted: Thu Feb 22, 2007 11:04 am Post subject: 4th gen single-hump vs. double-hump crossmembers EXPLAINED
OK, hopefully with some pics and short descriptions we can clear up the confusion with double-hump (DH) transmission crossmembers on 4th gen El Caminos. Of course this all comes to light because of the desire to install a true dual exhaust system on our '77 project. (Many thanks go to Caveman49 (Lee) for his help in solving this puzzle...)
A little history first - from what I've been able to find, all '73 - '75 Elkys were equipped with the provisions for DH crossmembers, whether they had dual exhaust or not (all 454-equipped cars had dual exhaust). Then Uncle Sam stuck his nose in the works and along came the catalytic convertor in late '75. With this was a shift to single exhaust systems with an inline cat, and a redesigned crossmember. Dual exhaust systems were never offered again.
So... why can't those of us with a '76 - '77 Elky who want dual exhaust just take a DH crossmember from a '73 - '75 and bolt it up and run with it..?? Simple answer - GM changed the frames (at least in the area where the transmission crossmember bolts up) in '76 to coincide with the new, wider single-hump crossemember. Let's take a look....
(Note - you can click on any image for a larger, detailed pic)
Below are the two crossmembers laid out - top one is the DH, bottom one is out of our '77. Visually they are quite different. Measurements are very different too.
DH crossmember measures 46-3/4" end-to-end, while the '77's SH version measures 51" end-to-end...
So what's up with the difference in widths? Why, the frame attachment points, of course....
'73 - '75 frames had extra brackets - or "tabs" as I like to call them - welded to the frame rails that served as the attachment points for the DH crossmember. Starting in '76, these tabs were eliminated....
A couple of '74 frames....
And here's a pic of ProStreet Garry's DH setup on his project '75
Here's a '76 frame showing NO TABS...
For the '76-'77 models, the frames were changed to eliminate the mount tabs, therefore a DH crossmember cannot just be "bolted up." The later models' crossmembers were longer and bolted thru holes in the actual frame rails themselves, as you can see in the above pic.
So... how do those of us with either a '76 or '77 get dual exhaust? There are no aftermarket alternatives available, like in the 5th gens. Caveman49 suggested welding-on tabs that'll allow the use of an earlier DH crossmember. Great idea, but this would most easily be accomplished during a frame-off restoration. To me, it would seem like modifying, or "notching," the stock single-hump crossmember would be the simpliest solution, and one that we're probably gonna try.
If anyone has more they can add to this discussion, or if my info is a little off, then chime in here...
Great writeup mayhem. I had to solve a similar issue on my 5th gen. I couldn't wait for G-Force to supply me one so I built my own. I would consider going to the wrecking yard and pulling both type of stock crossmember from fourth Gens and make a jig and build them to sell if there was enough demand.
Heres a pic before I changed exhaust:
and after:
_________________ 78 project car and 87 G-Machine.
Its better to have a bottle in front of me-
than a frontal Lobatomy ...!!!
Mayhem, it's not the cold but the wet with all the snow melting. Weather permitting, I'll get some measurements and pictures underneath in the next couple of days. This will give you an Idea of where the holes in the frame I used are. They are the same as where the single hump member bolted up on the drivers side and used the holes opposite on the passenger side. _________________ "Still Just Truck'in"
"Johnny"
Bradley, Illinois
Necoa #: 1108
Location: 17(S.CA,S.NV)
Joined: Jun 17, 2003
Posts: 296
Year: 1977
Model: El Camino
Posted: Thu Feb 22, 2007 6:13 pm Post subject:
PaPa Johns 77 wrote:
They are the same as where the single hump member bolted up on the drivers side and used the holes opposite on the passenger side.
Damn, PaPa John.... you got icicles hanging off that thing... BRRR...!! You need to find a nice, dry garage for that ol' 77 to sleep in...
I'm really interested in seeing your setup.... since you've got a '77, you won't (or shouldn't) have the frame tabs for mounting the Chevy DH xmember, so that Olds one you have must be WAY different than the Chevy one if it's bolting up to the '77's single-hump frame holes. Very interesting... and this could give us another option by using an Olds DH xmember...
Necoa #: 1108
Location: 17(S.CA,S.NV)
Joined: Jun 17, 2003
Posts: 296
Year: 1977
Model: El Camino
Posted: Thu Feb 22, 2007 6:36 pm Post subject:
Heatsoaked wrote:
I would consider going to the wrecking yard and pulling both type of stock crossmember from fourth Gens and make a jig and build them to sell if there was enough demand.
Therein lies our problem... and the reason 4th gens have such poor aftermarket support - not enough demand...
I'm going to take a stab in the dark here,Mayhem. Before I actually get to spend some time doing the DH switch for myself. I think what is leading you to believe that the stock DH won't fit the later cars might be that you are looking at the mount points for the SH version. That xmember goes in at an odd angle whereas the DH installs 90 degrees relative to the frame rail. This difference could easily account for the difference in length that you measured.
I could be completely wrong, but before actually doing the switch that's how things appear to me.
However, if you are correct, that may mean that the "tabs" you mention were a cheap way for GM to use the xmember from the gen 3 cars! If that is true, then we have a much wider choice of donor cars to get them from.
Necoa #: 1155
Location: 2(CT,MA,ME,NH,RI,VT)
Joined: Jun 26, 2003
Posts: 230
Year: 1975
Model: El Camino
Posted: Thu Feb 22, 2007 10:13 pm Post subject: More DH Info
Mayhem...Good job on what you’ve come up with! This is the first time I’ve found out that there is a difference all 73-77’s frames. I’ll add my finding to this post so that down the road other can refer back to it.
prostreetgarry wrote:
Ok more cross member questions on fit. I’m hoping to try to explain what I’ve found out. I’ve tried to post this on my SOS post. My 75 started out with the single cross member and I put in the double cross member and it was a direct bolt on.
The single cross member bolts up to the chassis at an angle un-like the double. (there is a flat section on each side frame rail with different boltholes)
Looking at my pic
See how the cross member runs parallel to the frame rails, The single cross member bolts up to the same two holes on the drivers side and then picks up two other hole farther back on the passengers side, making the cross member bolt up at an angle.
Looking at Lee’s pic
Note the holes on the left hand side and the tab that bolts up to the Tranny. See how the tab is longer on the single cross member? Now think about lining up the holes on each one of the Tranny tabs and rolling the single cross member so that the holes on the left side line up. If you do that you’ll see that they will line up and that the overall length changes so much that you can see that it will work.
Now putting this into words is a tuff thing I’m hopping it makes since?
All I can say is I've done it and it works
(Yes it worked using 73 parts on a 75)
Looks like 76-77 may be differant
Love this site!
Garry _________________ 1975 El Camino Ground up Resto Complete 7-17-07 Scaldinghotelcamino.com
Necoa #: 1108
Location: 17(S.CA,S.NV)
Joined: Jun 17, 2003
Posts: 296
Year: 1977
Model: El Camino
Posted: Fri Feb 23, 2007 2:29 pm Post subject:
Camino LS6 wrote:
I think what is leading you to believe that the stock DH won't fit the later cars might be that you are looking at the mount points for the SH version. That xmember goes in at an odd angle whereas the DH installs 90 degrees relative to the frame rail. This difference could easily account for the difference in length that you measured.
Camino LS6 - the mount points are in different locations on the frame. If you look at the frames, you'll see that each xmember (SH vs. DH) mount at different locations - see this pic:
Now, this brings to light a new question - the frame in the pic is a '74, and as you can see it has the mount holes in the frame just like the later models do, AS WELL AS the tab mount for the DH xmember.
So, maybe we can get Garry to chime in here, as he mentioned his '75 originally was an SH setup that he changed over to a DH setup. Hey Garry - put on your thinkin' cap and climb into the "wayback machine" and see if you can remember which mount holes your SH xmember was bolted to - the frame location, or the tab location.
This could tell us if the SH xmembers are the same for all years of 4th gens. If the SH bolted to the tabs, that would tell me it is shorter than the '76-'77 ones and aren't interchangeable. If it bolted to the wider frame location, then it makes sense that when GM dropped dual exhaust, there was no more need for the DH xmember or its tab mounts, so presto... tabs were eliminated for subsequent years, and the SH just continued on unchanged (and now unloved.... )
Hey Lee - I don't remember for sure, but I think you said both your early cars were DH equipped, so I'm guessing you won't have an early SH xmember laying around....
Camino LS6 wrote:
However, if you are correct, that may mean that the "tabs" you mention were a cheap way for GM to use the xmember from the gen 3 cars!
That could be very true. I've never had the opportunity to compare the two gen's DH xmembers, but early on in our build we did go to the boneyard and pull a DH from a 3rd gen, thinking it may work for us, only to find it didn't - so I think I still have that DH laying around in the garage - I'll try and see if I can grab some measurements from it to compare with those of a 4th gen DH that Lee got me dimensions on...
Camino LS6 wrote:
If that is true, then we have a much wider choice of donor cars to get them from.
Yes, but only if you have an early model 4th gen frame with the DH mount tabs...
Necoa #: 1155
Location: 2(CT,MA,ME,NH,RI,VT)
Joined: Jun 26, 2003
Posts: 230
Year: 1975
Model: El Camino
Posted: Sat Feb 24, 2007 3:20 am Post subject:
Pro Street Garry wrote:
Mayhem...Good job on what you’ve come up with! This is the first time I’ve found out that there is a difference all 73-77’s frames.
mayhem wrote:
So, maybe we can get Garry to chime in here, as he mentioned his '75 originally was an SH setup that he changed over to a DH setup. Hey Garry - put on your thinkin' cap and climb into the "wayback machine" and see if you can remember which mount holes your SH xmember was bolted to - the frame location, or the tab location.
I think said that in my last post?
_________________ 1975 El Camino Ground up Resto Complete 7-17-07 Scaldinghotelcamino.com
In capsule, the 73-75 will take a Double hump from the third generation Elky. I have a 3rd Generation DH and recognize the length, that DH that is shown may not have actually came out of a 3rd gen but it's the same none the less.
From 76 to 77 that frame extension was left off the frame and you can not use the Double hump unless you make that extension. Which makes me believe that having one fabricated is still better. I like Heatsoaked (Mike's) offer.
Mike, how much $$$ would you charge? Frankly, I'll take you up on your offer, I like the one you made and one like that for the 76 I have would be better than what I have. _________________ Home Site: http://www.elcaminoplace.com
1972 El Camino Custom 400
1976 El Camino Classic
1980 El Camino Super Sport
E-Mail: engstrvr@sbcglobal.net
Necoa #: 9112
Location: 22Canada (E.of Man)
Joined: Feb 25, 2007
Posts: 7
Year: 1975
Model: El Camino
Posted: Sun Feb 25, 2007 1:41 pm Post subject: Great Info
Will be getting car out of storage soon and was getting set up to put dual exhaust on my 75,It has single hump crossmember. Will dual exhaust still be achievable with single hump or will I have to change to dual hump?Finding a dual hump crossmember might be a feat in itself.
Necoa #: 1108
Location: 17(S.CA,S.NV)
Joined: Jun 17, 2003
Posts: 296
Year: 1977
Model: El Camino
Posted: Sun Feb 25, 2007 2:04 pm Post subject: Re: Great Info
herk wrote:
Will dual exhaust still be achievable with single hump or will I have to change to dual hump?Finding a dual hump crossmember might be a feat in itself.
You'll either need to mod the SH or obtain a DH to do dual exhaust. As far as finding a DH... they're out there, just takes some digging.... You're lucky in that you have a '75 and a DH should bolt right up....
I did some measurements on the 3rd gen DH I have yesterday, and it appears the 3rd vs 4th gen DH's are different, both dimensionally and physically. I took a pic and will post the differences once I get it all together.
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With this was a shift to single exhaust systems with an inline cat, and a redesigned crossmember. Dual exhaust systems were never offered again.
Caveman49 suggested welding-on tabs that'll allow the use of an earlier DH crossmember. Great idea, but this would most easily be accomplished during a frame-off restoration. To me, it would seem like modifying, or "notching," the stock single-hump crossmember would be the simpliest solution, and one that we're probably gonna try.
Mayhem, it's not the cold but the wet with all the snow melting. Weather permitting, I'll get some measurements and pictures underneath in the next couple of days. This will give you an Idea of where the holes in the frame I used are. They are the same as where the single hump member bolted up on the drivers side and used the holes opposite on the passenger side. 
BRRR...!! You need to find a nice, dry garage for that ol' 77 to sleep in... 
