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Old 09-27-2021, 05:01 PM
Mark66 Mark66 is offline
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Default Big 66 flywheel interchange

I am planning to swap out the auto trans in my 1966 GP and install a factory Muncie 4 speed from a donor 1966 GP with a manual trans. It seems that the flywheel assemblies in the two cars are different as they have different part numbers - is this correct?
And assuming that I will have to use the flywheel assembly from the manual donor car, will it be necessary to rebalance the engine in the car it is going into?

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Old 09-27-2021, 06:16 PM
tom s tom s is offline
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You will need to check the balance on your flexplate to know if it has been zero balance.I would invest in a new flywheel over a used one.Over the years they develop heat cracks.Tom

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Old 09-27-2021, 07:49 PM
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stevep stevep is offline
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You need the manual transmission flywheel that came with the donor car

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Old 09-27-2021, 09:52 PM
Mark66 Mark66 is offline
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Thanks - looks as if I will have to have a professional take a look at my existing flywheel and see if it is worth saving.

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Old 09-28-2021, 12:01 AM
mgarblik mgarblik is offline
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If the existing manual transmission flywheel isn't hot spotted, heat checked, or badly grooved, it can probably be surface ground and re-used for regular street use and mild performance. A replacement cast iron flywheel is fairly economical as well. Depending on your time frame, I have a good stock flywheel that will be available late winter/early spring. Unless someone did some crazy balancing, the flywheel should be zero balanced and not require a re-balance to use in your auto trans application. One item that could be a bit of an issue is the pilot bearing counterbore in the crankshaft. You didn't say what engine the Grand Prix has in it and whether it was the original engine for the year and so forth. Some Pontiac auto trans applications do not have the pilot bearing hole fully machined and finished. To fix that without removing the crankshaft, any decent machine shop can make a bronze bushing to fit the existing hole in the crankshaft. A ball or roller bearing will be more of an issue depending on what you have for a size.

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Old 09-28-2021, 10:41 PM
Mark66 Mark66 is offline
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mgarblik - the engines in both cars are original to the cars - 389's. I have been told that virtually all 1966 (and probably other years as well) crankshafts were machined the same and could be used in either manual or automatic Pontiacs. This seems to be supported by the Pontiac Chassis Parts Catalog which only lists a single part number for all big 1966 Pontiacs with a 389 - 9782646. Worse case I would hope that I only have to install a pilot bearing.

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Old 09-29-2021, 10:07 AM
mgarblik mgarblik is offline
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OK, that sounds great then. The automatic car crankshaft will NOT have a pilot bearing installed as they don't use one. You will need to clean out the pocket for it in the crankshaft as it will be full of rust, dirt and crap. I would recommend a ball bearing for the pilot. Some people prefer bushings. I like the bearing because it makes it easier to engage reverse without grinding in most cases. BOP engineering sells the correct pilot bearing.

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Old 09-29-2021, 07:41 PM
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hobbygto65 hobbygto65 is offline
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The flywheel in the auto car is a flexplate. Pretty flimsy compared to the stick flywheel

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