TH350 to 700 Swappers, Need your help

[rn2fine]

My 72 C10 Burb has a 700R4 where the TH350 used to be. Used the TH350 crossmember but had to move it and drill new holes in frame. Have noticed vibration associated with speed. Using process of elimination I have replaced tires, had them dynamically balanced, replaced shocks, will soon replace all suspension too, and alignment is good as there is no adverse tire wear. The driveshaft was shortened and balanced by a reputable shop. Could the angle / elevation of the 700R4 be wrong by using a TH350 crossmember? Any TH350 to 700R4 swappers out there who can comment?

[1972C10]

Shouldnt matter since where they mount is going to be around the same hieght.

Like where the mount is on a 350 compared to the base line of the engine and where the mount is on a 700 i think is the same.

Weve done the 700-350 swap a few times and put a few 700s in place of 350s in cars and trucks .

Is the Crossmemeber centered where the holes were drilled?


I dont think it would effect it that much but never know.

[BtnkBndt]

Have you checked your motor and trans mounts?

[Bowtie67]

Was the vibration there before the swap? if not look at angles between engine and Trans to make sure they have proper angles, make sure ujoint properly seated in Differential Yoke, check balance on driveshaft. If vibration there before could be, tires, rotors out of round, calipers sticking, shocks, ball joints, idler arm, excessive play in stearing box, bent rims, flat spots on tires..

[MrGoodpliers]

If the vibration wasn't there before the swap, it's probably not tires or brakes. Get them to check the drivesaft again along with the u-joints and support bearing. Also, is the burb lowered? If the problem is not with the driveshaft or pinion angle, my next thought would be (gulp) trans... Good luck, let us know how it turns out.

[1-PU70]

Try clocking the drive shaft 180 It dont sound like much but has helped plenty of times

[Don Smith]

I agree with 1-PU70. I'll bet it is drive shaft related.

[onelow69]

What kind of trans mount did you use. I personal used a polly mount from energy Sup and noticed more vibration with it in, then i switch back to the stock rubber one and the vibration went away. Also it dose not shift as hard either. Could be a cheap fix.

[tom hand]

I'll bet on the driveshaft. Even if the pinion angle is wrong it will ussally just wear out a u-joint with no vibration. If it is a two piece driveshaft it could be out of phase. I would try 1-PU70 advice on moving it 180...I have seen this work many times.

[71Rat]

if the angles were different from the trans drop to the pinion lift, you will get a vibration. the angles should be the same so it counters the others vibration. if they arent the same you will have a vibrtion and you will also wear out ujoints quicker.