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M-12 to GM 700R4 Swap

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Gadgetman1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gadgetman1 Quote  Post ReplyReply Direct Link To This Post Topic: M-12 to GM 700R4 Swap
    Posted: Mar/14/2023 at 10:36am
I’ve been told that the Javelin 390 pairs up with a GM 700R4 transmission very nicely with the correct adapter plate.  My M-12 transmission was already out of the car when I bought it, so I have no idea if it’s any good.  I asked one of the very few reputable local transmission shops if they could rebuild my Borg Warner automatic & they had no idea & called someone they knew that had done a lot of them.  They gave me a price of between $1000-$2000 just to rebuild it. That seems like a lot for something that’s so old & simple.  My thoughts are I want better drivability on the highway & less RPMs.  Is this a risky move or a great move?  I looked at some old threads on this & most of the pics & links no longer work.  Here are my biggest questions:
1) Is there an expert on this procedure for a ‘70 Javelin?
2) By swapping in a GM transmission, am I completely killing the value of this Javelin?  I have no plan to sell it, just want to enjoy it.
3) Will the cost of the swap be more than just rebuilding the stock automatic?
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sweatlock View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sweatlock Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 11:10am
I’ve never done this so I can’t help with the conversion itself, but I know it can be done. I’m sure Novak Adapters has something regarding it. 

https://www.novak-adapt.com/

Just a few points:
The $1K to $2K for the rebuild of the M12 is right in the ballpark. Nothing is cheap nowadays, simple transmission or not. If it’s simple, then some on here will suggest giving it a go yourself. However, if your heart’s set on an overdrive, the 700R4 is a good trans. With regards to hurting the value of your vehicle, it’s a moot point if you’re going to keep it. My only suggestion there would be to keep the M12 in case you do sell it and the next owner wants to put it back to original. Also, don’t cut the body structure to get whatever it is that you end up installing to fit. I would think modifying a bolt-on, like the trans crossmember is alright, but enlarging the shifter hole or the trans tunnel is a much bigger job. 


Edited by sweatlock - Mar/14/2023 at 11:22am
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Heavy 488 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 11:13am
The bell housing adapter alone only works on 72 and up V-8. Before that year you also need to locate a crankshaft adapter.
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Gadgetman1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gadgetman1 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 12:47pm
Originally posted by sweatlock sweatlock wrote:

I’ve never done this so I can’t help with the conversion itself, but I know it can be done. I’m sure Novak Adapters has something regarding it. 

https://www.novak-adapt.com/

Just a few points:
The $1K to $2K for the rebuild of the M12 is right in the ballpark. Nothing is cheap nowadays, simple transmission or not. If it’s simple, then some on here will suggest giving it a go yourself. However, if your heart’s set on an overdrive, the 700R4 is a good trans. With regards to hurting the value of your vehicle, it’s a moot point if you’re going to keep it. My only suggestion there would be to keep the M12 in case you do sell it and the next owner wants to put it back to original. Also, don’t cut the body structure to get whatever it is that you end up installing to fit. I would think modifying a bolt-on, like the trans crossmember is alright, but enlarging the shifter hole or the trans tunnel is a much bigger job. 

I just contacted Novak & unfortunately, they have absolutely nothing for an AMC 390.  They have adapters for the smaller V-8s & the 401, but nothing for the 390.  I tried calling Wilcap & their phone number must be disconnected.  I heard they were purchased by another company recently.  Back to square 1.
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sweatlock View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sweatlock Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 1:38pm
Someone more familiar with these types of swaps will have to chime in here. 

However, there is this thread about adapting a more modern OD auto trans to an older AMC V8:


It’s not a GM 700R4, but…
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Gadgetman1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gadgetman1 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 2:37pm
From my limited knowledge in researching this & speaking to technical support at Novak, the only adaptations is for the 401 & 360 & smaller.  Nothing for the 390.  They said that there was too much crankshaft variations to offer an adapter.  This car is certainly challenging my research skills!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 2:48pm
Any 290,304,343,360,390 and 401 built before 1972 model year had a flat flange crank. Bell pattern never changed. too many variations to them must mean more than 1.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 3:13pm
Use a 72 & up 401 crank in your 390 block. Then the adapter will work.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sweatlock Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 4:06pm
Originally posted by Gadgetman1 Gadgetman1 wrote:

From my limited knowledge in researching this & speaking to technical support at Novak, the only adaptations is for the 401 & 360 & smaller.  Nothing for the 390.  They said that there was too much crankshaft variations to offer an adapter.  This car is certainly challenging my research skills!

Novak is geared more towards Jeeps and their knowledge seems a bit limited to the mid to late ‘70’s. When they say “all 401’s“ they’re not thinking about a ‘71, which would have a flat crank. And when they say “all 360’s”, they’re not thinking about a ‘70 or a ‘71 360 with a flat crank. So to Heavy’s point, these engines have the same issue as with a 390. 

Plus Novak seems to be very enthusiastic about swapping in entire late-model GM LS powertrains, engine/tranny combos. But they do make a lot of adapters. 


Edited by sweatlock - Mar/14/2023 at 4:10pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2023 at 5:31pm
The Novak adapter is designed to work with engines that originally came with either the TF727 or TH400 transmissions and not the earlier 67-71 AMC V8s that ran the Borg Warner autos.

I have not used used the Novak kit either, but there are a number of ways to adapt the TF727 and TH400 to these earlier engines and the same principles would apply if using the Novak kit, this would be to use an adapter kit (but running an adapter kit for an adapter kit is probably not the way to go), modify the crank flange to take the later transmissions (this is a relatively straightforward machining operation, but the crank needs to be out of the engine), use a later crank (ok for engines like the 304, 360 and 401 which came with both crank variations but there is no later 390 crank, so you would need to build the engine with a 401 crank), have a custom torque convertor made (basically 700R4, TF727 or TF400 convertor with the snout modified to fit the earlier crank).

Essentially the 700R4, TF727 and TH400 convertor snout is to large to fit into the crank spigot and this is what you will need to correct. 


Edited by WesternRed - Mar/14/2023 at 5:33pm
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