I have had a nasty vibration in my car since I got it running. I put about 200 miles on it 2 years ago, the tranny mount was shot so I put a new one in hoping the issue would go away, and it got 20 times worse. I haven't really driven it more than in and out of the garage when I want to work on a DD since. The car came from a guy who built it in 83, and parked it a few years later. I have no idea why it was parked he said his kids were born and that was that, maybe it had other issues so wasn't driven any longer? Beats me, it came with a Mallory dual point set up, but the dist. was gutted. I never heard it run before buying it 7 years ago.

The motor is smooth as can be at idle, it doesn't shake at all with your hand on the valve covers. Around 1500 - 1800RPM it starts to shake on the inside and its not comfortable to even sit in, it rattles your teeth. Here's the front of the flex-plate, and it appears the left ears on the stock plate (looking at from the front of the car) are bent. What does a flex plate failure look like? I have heard they crack, would that cause this issue? Obviously the plate is damaged but could it be the tranny/converter?

The motor is a 9-9-68 casting 340 with the early (thinner) dampener which I already swapped with another one I had, the converter doesn't have any weights welded onto the flat spots. I thought this was all correct maybe I'm wrong? It's a 727, and it vibrates in park, neutral, and all forward/reverse gears. Timing is set at 28 or 30 flat with no vacuum advance, electronic conversion Evan's harness and MP dist, orange box.

Thanks in advance, TC...

Pull the trans because something is very wrong with that fles plate. I'm not sure if it could be installed backwards or not but it has to come apart.

When I finally looked at it this evening I was happy to see the flex plate had visible issues. I don't know if it can be installed backwards but it would make perfect sense. All of the bolts in the plate appear to be intact and tight.

You will want to check the converter snout fit into the crank area. When the converter is fit in the trans all the way it should be about 1" in from the edge of the case. With the trans bolted up you can push the converter flush to the flex plate. The flex plate should not bend to meet the converter pads. Also be sure to line the flex plate and converter bolt holes up and mark one ear on the flex plate and the corosponding pad on the converter they only fit one way. One hole and one pad are offset.

I sold a SB 727 set up out of a 318 car to a Air Force guy going overseas back in 1971 or 1972, he had a 1971 340 Duster 4 speed car that his wife couldn't or wouldn't drive due to the clutch or shifting, anyways he put the flex plate on wrong, it broke that flexplate and I now think it vibrated also. OP, look at the flexplate, the outer converter mounting ears should be further forward to the front of the motor than the center of the flexplate is What that does is allow the converter to spin without the converter hub being centered into the crankshaft hub By rubbing it is telling you that it is not correct, time to figure out what is wrong with it

Thanks for the quick replies. The plate is almost certainly on backwards, it all makes sense because the mounting holes only allow it to line up one way. I'm sure having the holes offset because the plate is on backwards causes the whole deal to be out of round and therefore vibrate. It's amazing the guy I got it from never changed it around?

I'm not doing anything crazy with the car just want a jump in and go cruiser, I'm most likely going to order the B&M 10230 and some new bolts. With any luck the flex plate was the weak link and it didn't rough up the crank or back of the block. Maybe I can even get to drive this thing before the weather breaks.

I wasn't aware of the B&M having issues, I put one between the 72 or 3 340 and 904 in my Dart project that I haven't touched in 7 years. Obviously never started it but I thought that was the way to go at the time. I will look into another brand, I figured if i'm going through the trouble to yank the tranny in my garage I should put a new one in and get one that's SFI approved. I may very well have a used one laying around that I can put in as well. Thanks again for the heads up.

I'm going to start taking it apart this evening, I'm hoping I don't break any bolts and create a mess especially with the headers.

You can't put a flexplate on backwards for several reasons.

Man that had to make a ton of noise from the looks of it.Oh and to the people that say you cant put a flex on backwards.Guess again.Had a guy at the shop that did on the Mittsy 2.6.He didnt think anything was wrong with all the bolts not lining up.But he put it in and had the motor all bolted together.It was a lot of work but he did it.After all these years I still wonder how.Rocky

I'll have more pics as time allows to drop the tranny. If it can't go backwards I don't know what I will find. And not to say it didn't make a boat load of noise back in 83, but it makes zero noise minus a slight resonating vibration around 1800 or 2k that goes away just above that. The inspection cover is loose and I figure causing that sound. It just shakes like crazy, no rattles, bangs, grinding etc.

Not only are the bolt holes offset but the flexplate is dished toward the motor, if you could get it on backwards the converter would be shoved back into the pump and the starter likely wouldn't fit.

How about some pics as you tear it apart. Front of the converter when you drop the trans and a pic of the flex plate before and after you remove it.

One thing you might consider when you change the flexplate is to get some converter bolts with a thinner head. The bolt head in the pic looks pretty thick and they can get very close to the block.

The thing that bothers me is the location of the swipe mark on the flex plate. If I'm thinking correctly the only thing that protrudes off the back of the block in that area is the camshaft plug. With what you described I would have to say the plate is deflecting as the rpm's go up from a very worn fit or broken flex plate.

I've seen this happen on electric motors with plastic fan that don't rub turning by hand but hit the endbell or cover when running at full speed.

It will be interesting to see the pics of the damage

I'm going to call this bad news because the flex plate doesn't appear to be nearly as bad as the pics show. As it was already said the flexplate is installed correctly and everything looks ok. I got the header and starter off to get this far after work today. First is the inspection cover...

Flex plate with basically the rust rubbed off.