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#1
11-05-2018, 05:34 PM
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Mating a 455 to TH400 help
This is probably not the brightest of questions, but the worst one is the one you don't ask, so...
I am replacing a 400 engine in my 68 GTO with a 455. The transmission may be able to handle it, and I am looking forward to seeing the GTO on the road... However, I noticed the flywheel is a bit thinker than the one on the 400, and the difference is about a half an inch. Do I need to change the flywheel for the one that was on the 400 in order to mount the bell housing flush to the engine? Is there a spacer I can use? Any suggestions or things I am not thinking about, that I should be thinking about? Thank you in advance!
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Just a backyard mechanic of the dangerous kind... |
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#2
11-05-2018, 05:55 PM
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It sounds like you have a flywheel for a manual transmission on your 455. It's thicker than a flexplate for an automatic transmission, which you may have on your 400. So, it has to match your transmission, if that's your situation. You would use the thinner flexplate for automatic.
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Steve F. Last edited by Grand73Am; 11-05-2018 at 06:33 PM. |
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#3
11-05-2018, 07:26 PM
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Thank you for the reply. I was thinking that, but I didn't know if I could get a spacer to use the flywheel, or if there was anything else I was not thinking about.
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Just a backyard mechanic of the dangerous kind... |
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#4
11-05-2018, 07:28 PM
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What transmission are you using?
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Steve F. |
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#5
11-05-2018, 10:14 PM
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There is a difference between a flywheel and a flexolate. If you are replacing a stick shift with an automatic then you have to take the flywheel off and put on a flexplate and vice versa.
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468/TKO600 Ford thru bolt equipped 64 Tempest Custom. Custom Nocturne Blue with black interior. |
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#6
11-06-2018, 01:07 PM
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The 455 is being mated to a th400 that was hooked up to the 400 engine... So I do think it is a Flywheel to flexplate issue.
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Just a backyard mechanic of the dangerous kind... |
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#7
11-06-2018, 02:42 PM
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Is the replacement 455 stock, and is your 400 factory stock? If so the old 400's flexplate will work fine since both will require the original external balance flexplate. If the 455 was changed from external balance to fully internal balance when it was rebuilt, then the old flexplate will have to be zero balanced by a machine shop. Either way will work fine, you just don't want to mix external and internal balances up. Some shops always go for internal balance, and others only do it on request.
Post photos of the flexplate and the engine side of the flywheel and we can probably help confirm the balance if you have any doubts. Or you can take both to your machine shop for their determination.
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Mick Batson 1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon in progress. |
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#8
11-06-2018, 09:39 PM
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#1 Thin lightweight flexplates with big holes are for automatic transmissions with torque converters. #2 Big heavy (30 lb) solid flywheels are for manual transmissions with clutches. . They are not interchangeable.
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'70 GP Model J 462 Butler ported 87cc Edelbrock D-ports RPM intake with Holley Sniper 2 CompCams 236/242 hydraulic roller Tribal Tubes tri-y headers TH400 13" Continental "Jim Hand Special" 3.50 9" CurrieTrac, 245/45-18 Front, 275/40-18 Rear |
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#9
11-07-2018, 06:11 PM
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In GM terms, a "flexplate" is just an automatic transmission flywheel.
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#10
11-08-2018, 09:37 PM
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One issue that you may encounter is the that the center hole on the flywheel/flexplate may not be the same size as the crank.
IIRC some flywheels/flexplates(I think all of the early ones) had a 2.75" center hole and some, in the later years of production, had a 2.5" center hole |
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#11
11-08-2018, 11:01 PM
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The following are the flywheel and flexplate...
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Just a backyard mechanic of the dangerous kind... |
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#12
11-09-2018, 08:59 PM
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Swap the Flywheel for the Flexplate, use the Flex Plate bolts, Torque them with Blue Loctite and put it together.
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1968 Firebird 400 RAII M21, 3.31 12 bolt, Mayfair Maize. 1977 Trans Am W72 400, TH350, 3.23 T Top Everyone you will ever meet knows something you don't. Bill Nye. |
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#13
11-09-2018, 09:58 PM
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Quote:
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#14
11-10-2018, 12:17 AM
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Quote:
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1968 Firebird 400 RAII M21, 3.31 12 bolt, Mayfair Maize. 1977 Trans Am W72 400, TH350, 3.23 T Top Everyone you will ever meet knows something you don't. Bill Nye. |
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#15
11-10-2018, 11:38 AM
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The obvious solution is the leave the flywheel on the 455 and put a 4spd in the car....
 also don't mix up the bolts. they are shorter on the flexplate, longer on the flywheel.
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1979 Firebird Trans Am 301/4spd (Now 428) 1977 Firebird Formula 400/Auto 2007 Grand Prix GXP 5.3L |
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#16
11-10-2018, 11:14 PM
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Mine had a pilot bearing in the crank hole when I pulled the TH400, it did not interfere with the convertor hub.
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#17
11-12-2018, 12:42 PM
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I swapped them without any problem... Until I put the 455 in the car. The oil pan on the 455 is larger than the 400, and it sat on the frame without letting the engine mount line up by about 1/4 inch... So now we have to raise the engine and swap oil pans from the 400 to the 455...
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Just a backyard mechanic of the dangerous kind... |
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