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Technical C4 trans to Flathead 8AB Speedway kit

Discussion in 'Traditional Hot Rods' started by Jellowe, Feb 11, 2021.

  1. Jellowe
    Joined: Jan 27, 2014
    Posts: 34


    Who has experience with the Speedway Motors conversion kit hooking C4 transmission to Flathead? I heard the Speedway flex plates tend to break and they are proprietary to the kit. Is this true?
  2. krylon32
    Joined: Jan 29, 2006
    Posts: 7,910

    Alliance Vendor
    from Nebraska

    I've used several of them building customer chassis but get no feedback as to the end results.
  3. sloppy jalopies
    Joined: Jun 29, 2015
    Posts: 5,152

    sloppy jalopies

    I broke a couple flexo-matic flex plates...
    i think it is that the flathead crank center is not centered in the belhousing's bolt pattern...
    has to be figured into the machine work... mine both broke just bigger than the bolt holes...
  4. wheeltramp brian
    Joined: Jun 11, 2010
    Posts: 1,314

    wheeltramp brian

    I used one and it all worked fine except the starter didn't mesh well with the ring gear.

  5. Flathead Dave
    Joined: Mar 21, 2014
    Posts: 3,632

    Flathead Dave
    from So. Cal.

    I am using one and I have several buds using this kit with ZERO problems. Never heard of this happening.
  6. 31wrecker
    Joined: Apr 27, 2021
    Posts: 32


    I just bought one. On backorder but they THINK it will be shipped next week. Fingers crossed. Flathead should be ready by then. Did you install yours yet?
  7. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,974

    jimmy six

    Flat-O Products sells them… are they different.
    Blue One likes this.
  8. Blue One
    Joined: Feb 6, 2010
    Posts: 11,256

    Blue One
    from Alberta

    And better in the way they work and in quality too.
  9. v8flat44
    Joined: Nov 13, 2017
    Posts: 974


    A friend used Flat-o-matic. Worked good, but his 41 sedan delivery was gutless with the c4. Rear gears ?
  10. rusty rocket
    Joined: Oct 30, 2011
    Posts: 4,540

    rusty rocket

    I have one with no problems other than shimming the starter. My combo is a slug off the line but works fine once it’s rolling. I think the kit comes with a stock stall speed torque converter, I always thought if it had more stall it would be better off the line. I’ve always had carb issues with my motor so maybe if that got worked out it would be better off the line.
  11. 55blacktie
    Joined: Aug 21, 2020
    Posts: 670


    If the only change was the installation of a C4, it should not have made his 41 "gutless." Maybe he needs a looser torque converter.
  12. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,974

    jimmy six

    Most guys changing to an automatic forget the hp to turn them. A flathead was 85-110 stock and maybe 140-150 hoped up. Since the trans needs an open drive line a lot of 9” are used which are power grabbers too and they want freeway gears and now have a “slug” from a light.
    Stockers had 3.78 or 4.11’s because 50-55 were highway speed and there were very very few interstates. You’ve done every thing to kill the power you have that’s why 200 hp overheads were so good. It took at least that much to run 3.00 gears with any efficiency in the 60’s.
    If your going to do it, an AOD or 200r4 both have ..67 and 4.11’s will give you a 2.75 freeway gear and a great off the line. There are a lot of rear end that aren’t power grabbers too many on foreign pickups and the 8.8 Fords which are very ez to narrow and come with discs or drums up to 11”.
    Think a little outside the box.
    X38 likes this.
  13. sloppy jalopies
    Joined: Jun 29, 2015
    Posts: 5,152

    sloppy jalopies

    i had a flexplate blow out [6"circle] ... still have a bellhousing and starter offset cone...

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