Flexplate Runout At Counterweight

Jeff Massey · Jun 20, 2016

Hello All, I have a 66 hardtop with a 68 302 that I rebuilt. Machine shop balanced the aftermarket balancer, crank, rods and flexplate but I now have rhythmic vibration at idle and when driving. I checked the flexplate for runout and it runs true all the way around EXCEPT at the counterweight, where it jumps to .030" runout. My first thought was warpage from the heat of the weld, but they would have caught that when balancing. I've since read the description of a Scott Drake flexplate (mine is not) and it says it has counterweights on both sides. My question is; could the counterweight on the other side be against the TC and thus causing the runout? Is there a recess in the TC for this reason? Any help is much appreciated, thanks.

tos · Jun 21, 2016

Well youre right, the weld will cause some warpage but I wouldn't bet 100% that they would catch that or maybe they checked runout before welding and balancing but not after? although they should have. It also may be that the runout was good and only shows up after you bolt it to the TC? Yes it could possibly be that the flex plate is laying on some weld from the counter weight or against the counterweight. I would check the fit between the flex plate and TC to see if there are any gaps first. The flex plate really should kind of flatten out when you bolt it up and eliminate that runout unless something is under it (maybe a washer on one of the TC studs)?
I can tell you from experience that I bolted up a TC once and didn't align the TC drain plug with the hole in the flex plate and was able to get all the nuts tightened down but it had a hell of a lot more than .030 runout. Maybe you have a smaller drain plug but the same misalignment?

Jeff Massey · Jun 21, 2016

tos said:
Well youre right, the weld will cause some warpage but I wouldn't bet 100% that they would catch that or maybe they checked runout before welding and balancing but not after? although they should have. It also may be that the runout was good and only shows up after you bolt it to the TC? Yes it could possibly be that the flex plate is laying on some weld from the counter weight or against the counterweight. I would check the fit between the flex plate and TC to see if there are any gaps first. The flex plate really should kind of flatten out when you bolt it up and eliminate that runout unless something is under it (maybe a washer on one of the TC studs)?
I can tell you from experience that I bolted up a TC once and didn't align the TC drain plug with the hole in the flex plate and was able to get all the nuts tightened down but it had a hell of a lot more than .030 runout. Maybe you have a smaller drain plug but the same misalignment?

Yes I think I saw your mention of the misaligned drain plug, or maybe even 2 drain plugs on your particular TC? Anyway that is what I'm going to check for, along with the flexplate being pushed out by the TC due to the rearward counterweight. Thanks

Jeff Massey · Jun 22, 2016

No dice, I measured with a depth gauge through the small holes in the flexplate all the way around and the depth to the TC was the same. I did discover the transmission crossmember mount bolts were loose and thought surely that was the problem, but after tightening them it was the same result. I think it may have even gotten worse! Any other ideas guys?

Jeff Massey · Jul 5, 2016

After reading several other threads I discovered that rubber engine mounts and polyurethane transmission mounts don't play well together, and sure enough that was the problem. The .030" runout on the flywheel is just minor weld warpage that would not cause vibration. Thanks tos

Flexplate Runout At Counterweight

Jeff Massey

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