55.2-59 56 Crossmember Mod For Tranny Fit

[thumper56]

My 56 C3100 has a 350/TH350 combo in it. For a few years I have dealt with the crossmember under the firewall being so close to the flex plate and torque converter that I can't get a torque converter dust cover installed. Been just running it open- watch out squirrels! Also, front bolts for tranny pan are about 5/8" above the rear of the crossmember, PIA!

I now have to replace my warped flex plate. While the engine and tranny are separated seems to be a good time to address this annoyance.

I have thought about removing it and replacing with something like a universal motor mount tube, or doing some creative fabricating on the original crossmember. Wonder what others have done?

BTW, my motor and tranny mounts are supported by the aftermarket tube type crossmembers, and seem to be placed where they need to be.

Any ideas appreciated!

[dennislbrooks]

Can you trim the lip off the crossmember to get enough clearance to put on dust cover?

Can you trim off even more of the crossmember to get clearance if needed? Sawzall or whatever. If you can leave any of it, 2/3 or more it probably will not effect anything. Trim as little as needed.

My 59 is now a 4wd and I removed that crossmember altogether and I added support at my rear trans crossmember. Mainly just because it is a 4wd. 350/350TH/NP208

I take you do have an added crossmember at the rear of the tranny.

[dsraven]

are you running side engine mounts or front engine mounts? if front engine mounts and rear trans mount then you are possibly putting a lot of stress on the aluminum bellhousing at the engine to trans connection.
the cross member under the rear of the engine on these trucks helps support the frame so it doesn't spread apart and cause the front cab mounts to flex which can cause more headaches. I have suggested to a few guys before here to possibly make that cross member removeable or modify it so it is lower. possibly a new u shaped member that bolts to the outside of the frame would work? easily enough removed when needed and then bolted right back in place when done whatever service work was required. some larger trucks used this concept over the years. if using the front and rear mount concept you could use the heavier aluminum torque converter shield from a 4x4 model of truck which used the support rods from that shield to the engine block for some added support in the middle of the long span between engine and trans mounts. just spit balling.

[thumper56]

Thanks for your thoughts guys!

To be more specific... When I originally installed the motor and tranny, they were bolted up to each other and only the cab sat on the frame. I slid in to where I felt I had a good relationship between the firewall and bell housing, and proper thrust line. Then I installed a universal tube type motor cross member, to use side motor mounts. I then installed a universal tube type transmission crossmember and mount to support the tail of the tranny- so added two crossmembers all in all. (cheap tubular type- bolted in place)

What I didn't do was take this original crossmember into account!!! After seeing that I only had about 1/4" clearance to the torque converter, I did whittle out a small chunk from the rear center of it, maybe 1" by 6" long. This made me feel better about converter clearance, but still prevented tranny pan front bolt exposure and dust shield installation.

I was leery of going further, not knowing how much I would weaken the frame structure. The thought of allowing the frame to spread is one I had not thought of!

I am thinking I may try a combination of your ideas. If I can get the original crossmember out in one piece, I may cut a 12" by 2" deep section out of the top center of it and have a good welder (not me) box it on three sides and bolt it back in.

Any good ideas on drilling out, or grinding out the member to frame rivets while the cab is still on? My shop is limited- no lift or torches. Are these rivets fairly soft and drill-able, starting small and working up?

[dennislbrooks]

The front and rear cab mounts still hold the frame apart.

Any bolt in would have to be as rigid if you want to duplicate that crossmember now.

I would not remove the mounting portions at the frame. I would be more inclined to sawzall - long blade if needed -- the crossmember on both sides strategically near the frame, missing the exhaust and anything else. Then fabricate your bolt in/drop off of the stubs left. Is your exhaust above or below the crossmember?

The 63's up do not have that crossmember at all. I realize they were front coil not leaf but neither did the 4wds.

A 63-87 rear trans crossmember can be used --drill new holes --- if you do not like your existing rear tran crossmember.


This is my 59 GMC. Your side engine mount and trans crossmembers may be slightly diff measurements. I did not replace the removed crossmember at all.

Talk with someone who is capable of making the changes you decide upon.

Been there.

[dsraven]

The cross member will likely not come out in one piece even if the rivets are drilled out. I had to cut mine in half at that point.

[dsraven]

Some of the newer trucks with no trans cross member under the bellhousing used a beefier cross member at the rear of the trans. It bolted to the lower frame flange but also had supports that bolted to the upper frame flange. That's just from memory so google it for examples. Just saying that if everything is supported off the lower flanges as you go front to rear on the frame then some unwanted flex may be the result

[thumper56]

Oh boy, both you guys really have me thinking! This is a project I was planning (along with the flex plate replacement) for the spring. I think tomorrow I may take a photo or two. Curious what you think as to "beefiness" of the current aftermarket cross members.

You both understand completely what I am talking about, but a picture is worth a thousand words! Only problem is garage is not heated and single digits, high wind and snow in VT tonight and tomorrow- good for an old fat boy! Hopefully I can figure out how to upload a photo.

To address some questions/ concerns... Exhaust runs under cross members- simple "hugger" headers, pipes straight down to an L bend and straight back under rear bumper (hump over rear end). I do have grinders, cutoff tool and sawsall. Can weld sheet metal well but not confident with heavier, structural welds. (Could tack then take to a pro)

More to follow, and again thank you for ideas.

[dennislbrooks]

Just an example, remember mine is a 4wd. In on the 2nd pic you can see the cut. Your solution may be different.

I do not procrastinate --- I ponder. 8-)

[dsraven]

google the transmission cross member for a square body. they have some that are W shaped to allow exhaust etc to run under them but not protrude below the frame and some that are fairly flat accross the bottom where the exhaust would run above them. they start by bolting on to the outside frame web on one side, thats the vertical part of the frame in a cross section, then bend under and bolt onto the lower frame flange before it goes accross to the other side and bolts onto the lower frame flange, this side also has a bracket that bolts on to connect it to the upper frame flange. this bracket is kind of the same idea as the original cross members found ahead of the rear axle on these trucks. it stiffens or braces the frame so it has less tendency to flex.
some large trucks I have worked onuse this idea as well and I seem to remember some that use a 3 piece unit that basically consists of left and right side pieces that bolt to the web of the frame as mentioned above but they don't go all the way across to the other side. instead they make the bend to horizontal and then end after a long enough part is left there to bolt a center section to. the center section is a glove fit and the contour there is basically U shaped like a C channel with a flange on each side, similar to a regular cross member. I guess they do that 3 pc unit so the center piece can be one of several options depending on the power units installed in the truck. they are strong and the center section is easily removed for transission or clutch work etc.
I have replaced cross members in trucks before (heavy duty mechanic before proffessional firefighter) and the rivet heads can be ground flat then drilled through with a drill smaller than the rivet. this welens the rivet so it can be simply punched through with a drift punch. then bolt the new cross member in with grade 8 bolts that are not threaded all the way to the head of the bolt. this keeps the bolt tight in the hole insted of just being the threads against the hole. ensure to get a bolt that fits tight even if you need to drill the hole slightly, doing one hole at a time, inserting a bolt a snugging it, then going to the next hole so as not to allow the parts to move. when done the bolts can be cut off to the correct length with a zip disc or they could be cut prior to installation. use hardened washers next to the frame or cross member under the head of the bolt and nut or use flange bolts and nuts with red loctite and torque to their recommended torque when all done
just some thoughts, hopefully they keep you thinking.

[dsraven]

sorry, spelling error, shouldda said:
this weakens the rivet

[nvrdone]

not sure about the 55-56 mount, but the mount dennisbrooks has is just like the one I have in my '49. Its been in there for about 40 + years and I have no cab mount or frame flex issues. Might be worth a look see. I did have to cut my cross member in half to get it out but its easy to weld back together.
also, that crossmember supports the trans at the tail shaft

[nvrdone]

Now that I am awake I re read what I posted and its wrong. My crossmember under the bell housing is like dennidbrooks - its removeable but doesnt support anything. Just keeps the frame from spreading and the cab mounts from moving. The 3rd cross member supports the trans at the tail shaft. sorry for the bad info.

[_Ogre]

Look at my build thread to see what I did for the cab mount support, built into the rear trans cross member