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Thread: 425 flexplate issues

  1. #1

    425 flexplate issues

    Hi all!

    Well the saga of getting the 425 dual quad Nailhead and SP400 trans into the '65 Skylark continues - slowly ,... I've just had the engine and trans built by a buddy.

    I was about to put it in when I found the Postons torque converter I have doesnt fit the 425 flexplate. So I'm digging around for a flexplate at the moment. I have the PE701103 12" 3000 stall converter and I think the stock 425 flexplate must be made for the stock 13" converters.

    I'm told a Buick 350 or 455 flexplate should fit. However I've just found a 350 one on ebay and it is only 13.5 inches across whereas mine is 14 inches across.

    So the hunt continues. There are 455 racing flexplates out there for about $45 so maybe that is the way to go but they need externally balancing and ideally I'd want one that is already balanced. Like this one

    http://cgi.ebay.com/ebaymotors/SFI-P...spagenameZWDVW

    and this one

    http://cgi.ebay.com/ebaymotors/SFI-F...spagenameZWDVW

    They seem way over spec for what I need but are relatively cheap! And look like they have the 2 sets of holes for 2 converter sizes - probably 11" and 10" !!

    What is the issue with externally vs internally balanced please? Does it refer to the harmonic balancer on the front of the crank.

    Also I need to mount the rear of the trans. I remember from previous posts about 8 years ago that I can reuse the trans cross member from the V6/ Powerglide combo, just move it back a few inches - but exactly how far please??

    Also I guess I may as well get a new trans mount since the original is on the old trans I took out (and want to keep). Is this the one I need please?
    http://cgi.ebay.com/ebaymotors/NEW-M...=p4506.c0.m245

    or this one?
    http://cgi.ebay.com/ebaymotors/_Moto...=p4506.c0.m245

    Which I think looks more like the one I took off.


    cheers! Nick
    <br />'65 Skylark 2 door post with plans for a 425 dual quad and switch pitch but in need of pretty much everything - what have u?!

  2. #2
    Nick,

    The part # 70-2058-25 is the transmission mount you need from this link: http://166.82.96.9/homeframes.html. I have seen them as cheap as $9 elsewhere, but I can't find a link now.

    The 455 flexplate has a different bolt pattern and balance then your 425 flexplate. Your best bet may be to have a local machine shop drill new holes in your 425 flexplate in the correct bolt circle for the Poston converter. You could do it yourself if you have some precision measurement equipment. Also, it is important to note that the 425 flexplate will bolt up in 6 different positions on the crank. It is only in balance in the position where the index hole in the crank is lined up with the index hole in the flexplate. All of the Nailhead engines are externally balanced.

    Internal balance means the rotating assembly is balanced by managing weight on the crankshaft only. Internally balanced engines have a neutral or 0 balance on the flexplate and balancer since they are balanced inside the crankcase.

    Externally balanced engines use weight in different positions on the balancer and flexplate/flywheel as part the rotating assembly balance. Therefore, it is very important to have the weight in the same relative location when switching balancers and flexplates on externally balanced engines.

    It is always a good idea to have the rotating assembly re-balanced when changing the crank rods or pistons in an engine unless they are exactly the same in weight. Only the crank rods and pistons are needed to rebalance a internally balanced engine, but an externally balanced engine will require the flexplate, balancer, crank, rods and pistons to be re-balanced.
    Steve B.



    67 GS 525 Buick Stage IV
    66 GS Convertible
    65 GS HT
    63 Riv
    02 Subaru WRX Turbo
    03 Ford Cobra Convertible (Factory Supercharged)

  3. #3
    I just had to do this on a '64 Riv. Installed a '65 ST400 switch-pitch with a 12" converter. It's flexplate only had the holes for the 13"/'64 Converter. Wasn't a hard job, but time consuming making sure the measurements are correct. They are offset from the originals. Just go to e-bay or other sites to see a picture of where the converter bolts goes on a flexplate that has the dual holes.

  4. #4

    Smile

    Steve, Telriv,

    Thanks for the replies. Yes I was thinking redrilling may have to be the way to go. I didn't want to drill it though for fear of getting the holes slightly out and the whole thing way out of balance!! Isn't there a flexplate for hte 425 that accepts a 12" converter? I saw a Buick 350 flexplate on ebay but that was 13.5" across and not 14". So I guess the starter woudl not fit. I've got one of Tom Telesco's mini starters to go on but I guess they are not adjustable to line up with the half in smaller flexplate?

    I mocked up the converter on the flexplate and the bolt holes are about half a hole difference and have their own depressions. I've seen the dual, and triple hole converters on ebay - I think one of the links above was one but these are flat flexplates with no pressings.

    So drilling is the only way to go?

    Yes Steve, I do remember the keyway on the inner mounting circle. Thanks for the comprehensive info on engine balancing. Much appreciated.

    Thanks for the info on the trans mount - the rectangular one. Looks like the one that was originally on the car.


    Thanks, Nick
    <br />'65 Skylark 2 door post with plans for a 425 dual quad and switch pitch but in need of pretty much everything - what have u?!

  5. #5
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    Quote Originally Posted by Nick Stratta View Post
    Steve, Telriv,

    I've got one of Tom Telesco's mini starters to go on but I guess they are not adjustable to line up with the half in smaller flexplate?


    Thanks, Nick

    Telriv = Tom Telesco

  6. #6
    Ah! sori
    <br />'65 Skylark 2 door post with plans for a 425 dual quad and switch pitch but in need of pretty much everything - what have u?!

  7. #7
    Nick,

    The flexplates for the 401 in a 65-66 GS have the smaller bolt circle. It used the ST-300 trans with the 12" converter. That flexlplate has a slightly different balance then the 425, and they are hard to find because of the small number made. It would need to be matched balanced to your 425 flexplate if you were able to find one. So yes, drilling your flexplate seems to be the most expedient and cost effective solution.

    Always glad to help!
    Last edited by Dr. Frankenbuick; 09-01-2008 at 06:33 AM.
    Steve B.



    67 GS 525 Buick Stage IV
    66 GS Convertible
    65 GS HT
    63 Riv
    02 Subaru WRX Turbo
    03 Ford Cobra Convertible (Factory Supercharged)

  8. #8
    Steve,

    So, drilling it is then! At least that solves the issue of shipping one over to Ireland from the States.

    Are there any templates available or is it a case of carefully lining it up? How do you get the 3 holes drilled perfectly centred?

    regards, Nick
    <br />'65 Skylark 2 door post with plans for a 425 dual quad and switch pitch but in need of pretty much everything - what have u?!

  9. #9
    The 3 holes aren't perfectly centered from the factory Nick. The bolts are a 3/8ths. inch & the holes in the flexplate are larger, about 7/16ths. inch. There's a flexplate on E-Bay now that has the double drilled holes from the factory. Take a look at it to see how the additional holes you need are placed & this will give you an idea. Use the converter you are going to use. Use a piece of Styrofoam to make up the I.D. of the flexplate & the O.D. of the converter. Once you do this you can get an idea of what to do. Even if you get it slightly off the holes can be slightly elongated. The converter fits into the rear of the crank, not tight, but snugly. It won't be out of balance so much that you will notice it, unless you install the flexplate to the crank in the wrong position. I think by now you know the correct position.

    Tom T.

  10. #10
    Tom, thanks

    Yes the rusty flexplate for 150 bucks! I saw it last night. This one appears to be flat whereas mine has the indentations around the converter bolt hole. This little fact makes me concerned about drilling the new hloes - where to put them??

    Thanks for the info on stryofoam etc and that the holes are slightly over - Guess they have to be to get it on without binding. Can the two be spun to be balanced once bolted together and aligned if neccessary?

    regards, Nick
    <br />'65 Skylark 2 door post with plans for a 425 dual quad and switch pitch but in need of pretty much everything - what have u?!

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