I6 3spd Transmission Mount

[69ShortStep]

Hello,

I have been investigating a vibration under cab when exceeding 50mph. Have replaced wheels/tires and it made no difference. Took a look under the truck and found that the transmission did not have a crossmember/mount. Is that normal for an I6/3spd?

Thanks in advance.

[dznucks]

yes. mine is the same way. seems odd that the tranny is cantilevered off the back of the motor. i was thinking of fabing one myself. Fatigue wise, that cant be good for the motor mounts or bellhousing/tranny and bellhousing/engine interface.

[sjarrett71]

I have a 6cylinder 3 speed combo and I have a crossmember although it's almost rusted in half!

[69ShortStep]

Thanks for the replies.

Does anyone have a picture of a stock or fabricated crossmember/mount they have made?

If anyone has recently purchased one, please let me know what you got.

[bhoeger70]

the 6cyls manual trans trucks that I have had and have all had the two front motor mounts and the rear mounts were on the bellhousing. The trans is just left hanging there. The vibration you might what to check could be the driveshaft (u-joints or hanger bearing).

[68gmsee]

Originally Posted by bhoeger70

.......... The vibration you might what to check could be the driveshaft.............

I agree.

I forgot to mention that I have a slight vibration on mine at about the same speed. I know it's an out of balance driveshaft since I've had tires rebalanced, replaced u-joints, yoke asm, driveshaft bearings and checked mounts, etc.

The drive-line shop said about $50 to balance it a few years ago. I just haven't gotten around to doing it.

[LOW DOUGH]

Originally Posted by 68gmsee

I agree.

I forgot to mention that I have a slight vibration on mine at about the same speed. I know it's an out of balance driveshaft since I've had tires rebalanced, replaced u-joints, yoke asm, driveshaft bearings and checked mounts, etc.

The drive-line shop said about $50 to balance it a few years ago. I just haven't gotten around to doing it.

Try spinning the the rear u-joint 180* in the rear yoke. It worked for me after i had mine out and put it back in it vibrated so I figured i didn't put the u-joint back in the same way it came out.

[nicks05z71]

do you have a 2 peice drive shaft? If so you might look at your carrier bearing.

I have a 3 speed w/overdrive (longer), with only the crossmember on the bell housing, with no issues.

You might also want to check to make sure all your bell housing bolts are tight and rivets on the cossmember are not broke.

[Tx Firefighter]

No need in second guessing GMs engineering. No matter that it looks crazy, the trans hanging off there is sturdy enough that you don't need to supplement it with other mounts. Those engineers were pretty smart.

You think it looks weird having a 3 speed hanging in mid air, you should see a granny 4 speed truck. That's a lot of iron.

[68gmsee]

If you use your truck like a work truck and/or off-road, a crossmember on the back of the 6 cylinder 3 speed may cause your bellhousing or transmission to crack. This is because of the length of the engine to the rear of the transmission and the twisting or flexing of the drive train or frame when going over uneven terrain. The rigidity of the three mounting areas prevent enough flexing.

[69ShortStep]

My truck has a 1 piece driveshaft. U-joints seem tight ( No clunking when shifting into reverse ). I will check that the bell housing bolts are tight and rivets on the crossmember are not broke.

Thanks for the info/advise.

[Shaky]

Another thing to check is your harmonic balancer. Mine slipped the outer ring and it had a vibration at certain rpm. Of course, it was impossible to get the timing right too.

[Tx Firefighter]

I'm cheap. I always put a big hose clamp around the driveshaft and rotate it a bit at a time until the vibration goes away. It takes a few test drives and crawling under the truck to get it clocked right, but free is free.

[LOW DOUGH]

^^^^ yep, you could do that too. Fine tuneing LOL

[68gmsee]

Low Dough, Tx Firefighter... I haven't tried the 180 turn on my 68 but I did use the large hose clamp on my 69. Worked pretty good and might be worth a try.

[Legolas894]

I'm not sure I'd notice the vibrations in my truck with the mirrors whistling, 4-speed howling, wind coming in the cowl, back bed clunking, metal dash and doors etc.

I don't think these trucks were meant to be quiet.

[69ShortStep]

Thanks for all of the great suggestions.

I am definately going to try spinning the the rear u-joint 180* in the rear yoke and the large hose clamp if necessary.

Will report back with results.

[dznucks]

as far as the clocking u joints go. you want to make sure that the grease fittings are clocked 180* out. if you have both grease fittings on the same side this will cause a vibration.