TH400 in firebird v/s TH400 in bonniville help - Hot Rod Forum : Hotrodders Bulletin Board
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Old 10-30-2002, 04:20 PM
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Post TH400 in firebird v/s TH400 in bonniville help

what is the difference in a trans for a 68 firebird th400 than a th400 in a bonneville.
I have a 400/th400 from a bonneville and want to put in my firebird. please help. I know TH400 came factory for the bird, but is the bonn. different?
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Old 10-30-2002, 04:30 PM
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Turbo 400's came in different versions......short tail shaft and long tail shaft. The Firebird is a short and the Bonneville is mostly likely a long tail shaft. There are different seal sizes (output shaft) also. Different weight cars will have different stall ratings for the converter also.

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Old 10-30-2002, 04:40 PM
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If the bonn. has the long shaft how difficult would it be to convert to a short?
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Old 10-30-2002, 05:52 PM
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you would have to completely disassemble the trans to convert it to short shaft, but just because it came from a big car doesn't mean its a long shaft, i've seen them both ways.
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Old 10-31-2002, 04:02 AM
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Another, easier and probably less expensive way is to take the drive shafts to a good machine shop and have one made the correct length with the required yoke. Last one I had done was only 80 bucks.
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Old 10-31-2002, 08:59 AM
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So how can I tell a long shaft from a long shaft
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Old 10-31-2002, 09:49 AM
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Aren't the bellhousing bolt patterns different or does the Pontiac housing have both patterns like some other trans do?
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Old 10-31-2002, 05:02 PM
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you can get a plate to match them up
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Old 10-31-2002, 06:05 PM
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A short 400 is approximately 4 and 1/2 inches shorter than a long one. The mounting point where the transmission mount goes in 1 inch further back on a long than a short.

You should not need a adapter to bolt the transmission to either engine if they are both Pontiac powered.

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Old 11-01-2002, 04:05 AM
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Oops! I thought the trans was going form a Pontiac to a Chevy. My bad.
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Old 11-01-2002, 07:21 AM
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I have a th350 I could use.
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Old 11-04-2002, 11:19 AM
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A TH350 will require a different trans crossmember. The Bonneville will most likely be the long shaft, but should have the same output shaft, so you could use your existing yoke. To shorten the drive shaft would only be about $30. Oh, if you change to the th350 you will DEFINITELY have to change the front yoke on the driveshaft, too.
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