Prop shaft vibes – maybe solved?

Last week I had some success in reducing prop shaft vibrations. If you read this blog you know I have been chasing this problem up one tree and down the other. One thing to finally clear up before I go on, is the question of the presence or not of a plastic spacer on the front mount of the front diff. I think I can conclude, for sure, that on my diff at least, there is no plastic spacer(s). The hole in the diff mounting ear is too small in diameter to allow the spacers found on the other mounts to fit.

Here is a pic of one rubber mount (they are installed in pairs), a stock plastic spacer, and a home made spacer when I was still thinking my van was missing them.

As is, this still leaves the front diff flange pointing down at 3.7 degrees which is not close to the transmission flange angle of 4.6 degrees. By the book, these angles should be within 0.5 degrees. When I say by the book, I am not referring to the Bentley manual which has no information on this subject. Instead, I get this fact from a drive shaft makers publication, here;

SpicerDriveline

I don’t like the fact that the transmission angles down that much, but I can’t really see how to raise front of trans. easily, and besides , it looks like there is little room above transmission to do this.

Out of frustration more than anything else, I decided to try adding weight to the drive shaft, thinking that perhaps the shaft was out of balance (I had replaced U joints, and who knows, it may have been out of balance before I got the van). I attached a gear clamp 3 ” from the front end of the shaft, right where there is a factory weight. test drive revealed no change. I rotated clamp 90 degrees and, by god, test drive showed a big reduction in vibrations, in fact the vibrations were almost gone.

Next step is to refine position and try additional weight. Local drive line repair shop does not have mandrels to do syncro shafts, but the tech. said he would be willing to make mandrels if supplied with measurements. I may be on to something.

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