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Flexplate question?


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I have a cracked flexplate for my 64 Riv and just got a used replacement one. It's the same but different in that it has what appears to be random holes in different places than my original one? With all the talk about external balancing, etc., it seems unlikely that the balance would be the same?

Any thoughts on this gentlemen

Thanks

DSC04856.jpg

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I'm guessing the one on the right is the replacement. My guess is the holes on the perimiter are for balancing. The other three are for a larger diameter torque converter.

I was thinking the same thing regarding balancing but it seems like a rather odd way to bring it into balance? Also after I posted the pic I noticed the center hub, or whatever you want to call it, is somewhat different as well?

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The other holes are for the smaller torque converter when the flexplate is used with a nailhead in a Skylark GS. That would have had a 401 with an ST300 trans in it. Both are OE flexplates and if the alignment holes are lined up correctly, it's balanced. I don't think the series of holes in the the plate on the left are OE.

Ed

Edited by RivNut (see edit history)
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The other holes are for the smaller torque converter when the flexplate is used with a nailhead in a Skylark GS. That would have had a 401 with an ST300 trans in it. Both are OE flexplates and if the alignment holes are lined up correctly, it's balanced. I don't think the series of holes in the the plate on the left are OE.

Ed

Interesting, the one on the left is my original cracked one (not sure how original but the one I just took off). I'll be putting it all back together soon enough so I guess we'll see.

Thanks

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This may sound a little crazy, but take both to your machine shop. Spin and check the old one for balance. They should be able to tell you how much it is out of balance and where. Based on what we have read here it should be. Then spin the new one and compare. Add or remove weight to it to match the old one. Crack in the old one might make a difference, but you should be close. I haven't talked to my machinist, so I may be way out in left field on this, but that is how I would approach this if it were mine.

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Flexplates between engines are likely not going to be balanced the same and this appears to be a good example. The question is how much difference will it make? I have read at least 2 or 3 instance where vibration was tied back to the flexplate index hole not being aligned when an engine was rebuilt so the potential is there. If you look at the design of the plate you can see the plate itself is intentionally out of balance from a rotating standpoint with the two large openings on two sides and mostly solid on the third side. Undoubtedly this is why the index hole alignment is so important. Because of this I think the drilled holes will have less effect on vibration vs a misalignment of the indexing hole.

The series of 4 holes at 10 o'clock on the left plate and the two holes at 1 o'clock on the right one are for balancing. To balance a rotating mass you can add weight or remove weight. Here they chose to remove weight in the form of drilling holes. The process of drilling the plate doesn't necessarily balance the plate itself, but rather the whole rotating assembly. This is based on information I have obtained over the years and is of course subject to error.

The plates appear to have variation in offset between center hub and ring gear as well as the TC mounting holes. Place both plates on the engine to be sure the differences are only superficial and the ring gear ends up in the same location from the block as well as the converter mounting holes.

You can try the replacement plate and see what happens. Definitely still line up the index hole when you install. Maybe it will be fine as-is. I'd think if changing plates caused a problem in every case we would hear other complaints because you aren't the first to replace a plate. Worst case if its not right, you'll have to take it back apart and attempt to have it weighted the same or have your old one weld repaired.

Its a good suggestion to check with a automotive machine shop and see what they suggest.

Edited by JZRIV (see edit history)
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Thanks guys.

As usual even simple things end up being complicated. I don't have a lot of faith in the machine shops in regards to them having a clue about how it should be balanced. Also up here everyone charges about $90/hr plus our modest 12% tax. They said $150 to weld and check for balance.

I think I'll just install the new one assuming it's offset is the same and then see what happens. If I have to pull the trans again so be it.

I assume the alignment hole in the fleplate is the extra hole where you bolt it to the crank and the crank has some sort of corresponding dimple I have to look for.

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Thanks Rob! That's as clear as it could possibly be. I haven't really looked at the crank side of it since it's still on the engine stand. I'm painting it this weekend.

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You Bet Man,

Ive been prepping and painting all my engine and accessory parts today, plus my evaporator box metal top. Next I will get back in the engine compartment and get that prepped out for some paint. My block and heads are still at the Machine shop, but I went in to visit them yesterday and went over some stuff with the machinist.

I even got all new stainless sheet metal screws of the proper sizes to attach all the stuff that goes back on the fender wells and firewall. I must be hanging around this forum too much! :D

Better get back to it...........

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You Bet Man,

Ive been prepping and painting all my engine and accessory parts today, plus my evaporator box metal top. Next I will get back in the engine compartment and get that prepped out for some paint. My block and heads are still at the Machine shop, but I went in to visit them yesterday and went over some stuff with the machinist.

I even got all new stainless sheet metal screws of the proper sizes to attach all the stuff that goes back on the fender wells and firewall. I must be hanging around this forum too much! :D

Better get back to it...........

You must post pics when you're done. The stainless sounds like it would set things off quite nicely.

I just got my motor back from my builder, he had fits with parts, wrong cam shipped, wrong frost plugs 8 times!!!!! wrong oil plugs, obsolete main seal, etc.

I'm not going overboard on the engine compartment at the moment, or the rest of the car, just the drivetrain until I finish the other resto I'm in the midst of.

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Hi, Rob. Just a friendly reminder: don't forget to put flat washers under the flexplate mounting bolt heads. Lock washers being in direct contact with the surface of the flexplate can be the source of future plate cracking. Best, John

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Did you mark the flex plate to the crank before you took it off???? I would put the replacement in the same position as the cracked one was. Based on looking at the pictures. Your old one should have an outline on the crank side as to how it sat.The plate probebly was balanced by bubble not spin thats why it should go back as the original. Hope this helps.

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Thanks for the tip John, I'll make sure to use the flat washers.

I didn't remove the flexplate so no I didn't check it. I pulled the motor and took it to the builder with the plate on. I'll check for alignment marks but I thought they all went on the same way, with the alignment hole matching up with the one on the crank? I don't know what balanced by bubble not spin means?

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Not old enough to remember balancing tires by setting them on a "bubble balancer?" You'd place the tire on a machine that had a bubble in the middle; if the tire was too heavy on one side, it would tip that direction. When you added the correct weight, the bubble would be in the middle of a circle on the machine.

I don't think the flex plate by itself cannot be balanced! It has to be balanced as part of the reciprocatiing assembly - harmonic balancer, crank, all rods and pistons, and the flexplate.

I can't recall seeing any OE flexplates for the nailhead that had but two mounting holes for the torque converter(s). Most I've seen have a small weight tack welded to it somewhere though.

Ed

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Not old enough to remember balancing tires by setting them on a "bubble balancer?" You'd place the tire on a machine that had a bubble in the middle; if the tire was too heavy on one side, it would tip that direction. When you added the correct weight, the bubble would be in the middle of a circle on the machine.

I don't think the flex plate by itself cannot be balanced! It has to be balanced as part of the reciprocatiing assembly - harmonic balancer, crank, all rods and pistons, and the flexplate.

I can't recall seeing any OE flexplates for the nailhead that had but two mounting holes for the torque converter(s). Most I've seen have a small weight tack welded to it somewhere though.

Ed

Ah that makes sense about the bubble balancer, thanks for the explanation.

I guess I'm going to find out how this balance business all turns out soon enough. Still have to paint the motor and misc parts before putting it back in but should be finished in the next month or so.

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Well I got it all figured out I think, the new flex plate while different in many ways has the same offset, etc. so I'm not worried. The old one was aligned with the hole and dimple so no change there.

Mine also didn't have washers of any kind so I'm wondering if that's the norm or even if it isn't it would be worth putting some flat ones on? Just odd that it didn't in the first place?

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Mine also didn't have washers of any kind so I'm wondering if that's the norm or even if it isn't it would be worth putting some flat ones on? Just odd that it didn't in the first place?

Nope, no washers from the factory. I personally would not add them.

Take a closer look at the bolts, they have a built-in washer in the form of a larger than normal head. This serves the same purpose. Torque them to spec and they won't come loose.

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Good point about not using flat washers in this nailhead flexplate application, Jason. I have worked on other Buick engine flexplates (mostly 350's) that did not have the larger mounting bolt head you mentioned. We added flat washers, so a crack would not be prone to start at any of the flexplate mounting bolt holes. Thanks for the "education" regarding the nailhead. John

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  • 5 years later...
On ‎4‎/‎3‎/‎2012 at 6:08 PM, 64R said:

I have a cracked flexplate for my 64 Riv and just got a used replacement one. It's the same but different in that it has what appears to be random holes in different places than my original one? With all the talk about external balancing, etc., it seems unlikely that the balance would be the same?

Any thoughts on this gentlemen

Thanks

DSC04856.jpg

 

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  • 2 years later...

I have six used OEM bolts IF needed.  Be careful on the Mr Gasket bolts.  Most are for a SBC & are normally too long.

 

Tom t.

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I've run into this before also. ARP are made for SBC & too long like my post above.  Not a problem IF you know what to look for. Just cut off the excess.

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My chassis manual states that the "crankshaft to flywheel" bolts are 7/16" x 20. No length given.

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Let's keep comparing apples to apples and leave the oranges out of it to avoid confusion.  This thread is about the 6 bolts used to mount the flex plate to the crank.  The previous post is refering to the 3 bolts used to bolt the converter to the flex plate . 😁

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