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Old 07-24-2014, 11:14 PM   #1
ts71281
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Is a transmission crossmember neccessary?

Ok, so after 15 years of hibernation, I'm getting to work on my 70 c10 again.

15 years ago, I swapped out the 307 3-speed configuration for a 468 big block and a 4 speed muncie M20.


I used the '70 engine towers and motor mounts, and I used the old style bellhousing (small hole) and attached it to the muncie 4 speed. This allowed me to use the factory transmission crossmember that supports the transmission by the bellhousing, not the tailshaft.

Is this ok to use like this or should I be putting in another type of crossmember? It seems sturdy, but I don't know if this transmission would be better suited to be supported by the rear or if the bellhousing support suffices. My thoughts are, it was good enough for the 3-speed, why would the 4-speed be any different? Maybe my thought process if flawed. Please provide input and correct me if I'm doing things wrong.
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Old 07-24-2014, 11:21 PM   #2
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Re: Is a transmission crossmember neccessary?

Isn't that the way even the SM465's are mounted in the 2wd's in these years? Sounds like it should be fine if you have good mounts able to handle BBC torque.
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Old 07-24-2014, 11:50 PM   #3
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Re: Is a transmission crossmember neccessary?

Hey! Nice to see a fellow minnesotan!

I'm not sure on the sm465, but the original trans was the saginaw 3-speed.

I put about 4,000 or so miles on the truck when I first did the big block swap years ago.

I did a frame off resto on it about 10 years ago, then once it was done, I never drove it.

I'm back in the game now, I just want to tidy up a few things I didn't like during the resto, and get back to driving it. Maybe we'll see you around.
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Old 07-25-2014, 12:04 AM   #4
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Re: Is a transmission crossmember neccessary?

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Originally Posted by ts71281 View Post
Ok, so after 15 years of hibernation, I'm getting to work on my 70 c10 again.

15 years ago, I swapped out the 307 3-speed configuration for a 468 big block and a 4 speed muncie M20.


I used the '70 engine towers and motor mounts, and I used the old style bellhousing (small hole) and attached it to the muncie 4 speed. This allowed me to use the factory transmission crossmember that supports the transmission by the bellhousing, not the tailshaft.

Is this ok to use like this or should I be putting in another type of crossmember? It seems sturdy, but I don't know if this transmission would be better suited to be supported by the rear or if the bellhousing support suffices. My thoughts are, it was good enough for the 3-speed, why would the 4-speed be any different? Maybe my thought process if flawed. Please provide input and correct me if I'm doing things wrong.
I don't know what the length of your 3 speed case was but I'll bet the Muncie is longer. A rear support is cheap insurance against a cracked bell housing.
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Old 07-25-2014, 02:25 AM   #5
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Re: Is a transmission crossmember neccessary?

5 mounts are better than 4.

Right? LOL

I always thought a 5 mount set-up would be cool. With that 468 and M20, I think I'd put in a cross-member/mount there, maybe lighter one than the stocker CM.

Last edited by mechanicalman; 07-25-2014 at 02:26 AM. Reason: grammar
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Old 07-25-2014, 07:48 AM   #6
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Re: Is a transmission crossmember neccessary?

I've been running an M20 behind a 350 with no crossmember for 10 years, with no issues. No racing, but I do some towing with it. I thought of putting a crossmember under it, but it didn't seem worth the trouble. The 3 speed and 4 speed were exactly the same length.
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Old 07-25-2014, 08:02 AM   #7
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Re: Is a transmission crossmember neccessary?

All the car 4spds are designed to only use the transmission crossmember mount at the tailshaft. Just use the automatic transmission crossmember. The mounts are right where you need them. This was good enough for those high torque muscle cars. You'll be fine. You're on the right track
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Old 07-25-2014, 08:57 AM   #8
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Re: Is a transmission crossmember neccessary?

Seems as though you guys are missing something hmm maybe it's the 468 he said he has in the truck I think you would be foolish not to run a cross member with the increase in trq
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Old 07-25-2014, 09:02 AM   #9
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Re: Is a transmission crossmember neccessary?

ran a muncie behind a 350 / 350 on my tow truck /daily driver for about 15 years with no rear trans support and no problems....and i beat on the thing...seem to recall aluminum case trannies had a habit of snapping off mounting ears, but that could have been the result of lazy people only installing them with 3 bolts back then....when i installed the 454 i immediately tore all the teeth off first gear on a rolling start while trying to jet the bog out of the edelbrock....so an extra crossmember may be the least of your worries
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Old 07-25-2014, 11:01 AM   #10
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Re: Is a transmission crossmember neccessary?

My understanding is that you want one or the other, but not both. If you a do a search there are several threads on this subject with a lot of opinions.

I think with a steel or cast iron case trans you are fine with the bellhousing support. For an aluminum trans I'd prefer a tailshaft support.
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Old 07-25-2014, 11:46 AM   #11
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Re: Is a transmission crossmember neccessary?

Is the truck a coil sprung truck? Tire spin could really cause a problem if you had a shock that had leaked a little [witch is highly unlikely I know most are on top of maintenance better than that] but if tire spin were to occur the trans case could break. My dad used to drive GMC back in the early 60's and with the coil sprung rear ends and the 3 oot they would get in a little bind on the turn row on the farm and then pop the case was gone. So much so that the dealer kept a few cases in stock in the parts room. If it were me I would remove the center and put one at the rear. Jim
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Old 07-25-2014, 12:54 PM   #12
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Re: Is a transmission crossmember neccessary?

What Lattimer said. Use the two bell housing mounts or the single tailshaft crossmember mount, never both. This is not a case of "more is better." Reason is that when the engine provides twisting torque, you want things moving correctly together as designed by GM. An additional mounting location, while it might sound like a good idea, can cause unwanted torqueing within the eng/trans driveline combo - possible bind, etc. Mimic how GM did it - one or the other way, not both. Either is fine.
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Old 07-25-2014, 02:07 PM   #13
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Re: Is a transmission crossmember neccessary?

I understand that running 2 crossmembers would be bad

I was wondering if it was best to support the trans by the bellhousing or eliminate that crossmember and install a tailshaft mounted one?

From the sounds of it, I should save myself the hassle and just keep the bellhousing mounted one.
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Old 07-26-2014, 01:24 AM   #14
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Re: Is a transmission crossmember neccessary?

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Originally Posted by jocko View Post
What Lattimer said. Use the two bell housing mounts or the single tailshaft crossmember mount, never both. This is not a case of "more is better." Reason is that when the engine provides twisting torque, you want things moving correctly together as designed by GM. An additional mounting location, while it might sound like a good idea, can cause unwanted torqueing within the eng/trans driveline combo - possible bind, etc. Mimic how GM did it - one or the other way, not both. Either is fine.
Ive read a few threads on this and I think your right on...If it was designed by GM that way, Im good with it...Ill be running a T5 5 speed overdrive and was wondering what to do regarding this issue,,my transmission guy said the same thing...possible binding would be a issue..
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Old 07-26-2014, 11:22 AM   #15
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Re: Is a transmission crossmember neccessary?

sixt8lou - I ran a WC T5 behind a mild 283 in my 66 with just the bell mounts, worked great.

ts71281 - good point, I was mostly replying to some of the info provided that mentioned using both, as in more is better. Yours is still a good question, and I'm not certain we've really answered it. I'm not certain which is the stronger setup - but Tim probably has a point here, on the higher hp arrangements, GM used the tailshaft crossmember. With a 468, hmmm.... that's kinda high hp! If it were me, and I wasn't sure HOW MUCH better the tailshaft mounting position was regarding handling hp - then I'd just stick with what I had. If you're planning any massive burnouts, perhaps might want to reconsider and move to the tailshaft approach(?)
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Old 07-27-2014, 11:57 AM   #16
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Re: Is a transmission crossmember neccessary?

Massive burnouts and bang shifting can and eventually will break things no matter how you mount them. One or the other,and I go by how that particular transmission was mounted in what it came in.
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Old 10-26-2017, 12:11 AM   #17
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Re: Is a transmission crossmember neccessary?

I don't mean to resurrect an old thread but 'old" is relative. I have been considering running two crossmembers also and I don't see where the forward crossmember (at the bell housing) does much more than provide a extra set of motor mounts. From my perspective, the bell housing is dowel pinned and bolted such that it is nearly an integral part of the block.

The transmission (in my case) has no support other than the bolts to the BH. My sm465 just hung off the BH. The trans I am putting in (NV4500) has provisions for a tail crossmember mount.

I suspect that supporting the tail of the transmission might be beneficial and if there is concern over introducing unwanted torque, just don't bolt the tail to the mount. Let the trans tail rest on a buffer pad.

It may not be unnecessary, but for me, the additional support would bring a little peace of mind.
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Old 10-26-2017, 12:24 AM   #18
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Re: Is a transmission crossmember neccessary?

-
I don't have a cross member under the back of my NV4500. All 200 pounds of it are hanging off of the bell housing. I have about 2500 miles on it with no problems so far. If you do put a cross member under the tail stock I would not bolt it down. The guys are right about the 3 point mounting. If you bolt it down in three places there is more chance of breaking something.

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Old 10-26-2017, 10:42 AM   #19
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Re: Is a transmission crossmember neccessary?

My Dad has a T-10 4 speed in his 1970 CST/10 with the stock transmission mounts at the 'housing. The T-10 is just hanging from the bell housing by 4 bolts. Considering how many times that truck has been brought up to RPM and launched by side stepping the clutch and the T10 is still just fine... I'd say that a second crossmember isn't needed at all.

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Old 10-26-2017, 11:35 AM   #20
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Talking Re: Is a transmission crossmember neccessary?

The GM thinking back in the day was for a stock truck with stock HP rating!

To put this in a modern day truck I think you would be fine using both. Most of these trucks aren't stock motors any more. If it was stock then why would you ask to begin with so I assume it's a modified truck...correct.

Using both would be to me a good choice. Do you need both maybe not but it couldn't hurt. Torqueing of the motor and trans as stated before shouldn't effect the mounting points that is what they are for to begin with. The GM engineer's said it wasn't needed for the HP of the truck.
If you go with the longer 5 speeds and over drives I would think twice about adding it for the added weight. Again GM engineer didn't know these type transmissions where going to be installed in these truck.
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