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Old 03-21-2009, 08:48 PM   #1
my75baby
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Default 350 flywheel vs. 400 flywheel

Hello all, I have a 1975 vette that someone put a Chevy SB 400 in. The car seems to have the chevy 350 flywheel which is bad b/c the 400 is externally balanced and the flywheel is different. My question is, how big is the chevy 350 flywheel and can I just swap out the old flywheel for a chevy 400 flywheel? are they the same size and will it fit into the old bell housing? I think the 400 flywheel is 168 tooth, I have no idea how many teeth the chevy 350 flywheel has. Any help will be appreciated.. thanks
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Old 03-21-2009, 08:58 PM   #2
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168 tooth flexplates are the most common for an auto equipped SBC. the difference between the 350 and 400 is, as you noted the internal or external balance. If you do have a 400 SB, then you need a flexpalte/flywheel that is compatible with the 400. If you had both to look at side by side, the 400 piece will have a large weight welded to it. The 350 plate may too, but it won't be near as large as the 400.

Same is true with the harmonic balancer on the front- there's a large weight cast into the balancer. Very evident if you look.
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Old 03-21-2009, 09:00 PM   #3
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What makes you think it has the wrong flywheel on it?
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Old 03-21-2009, 09:02 PM   #4
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The factory 400 was externally balanced and used a special flywheel, but a rebuilt 400
now can be internally balanced and use the same flywheel as the 350. The 350 engines
used both the 153 or 168 tooth flywheels depending on application. If your 400 engine is
not internally balanced and you are running the 350 flywheel, you would know it right away
due to the vibration.
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Old 03-21-2009, 11:08 PM   #5
my75baby
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Default Will a 400sbc flywheel fit in the 350 bell housing??

Thanks all, yes I have much vibration, no I do not have an auto car (= flexplate), I have a 4 speed (=flywheel). I know the balancer on front is correct (large groove out of it), I know this cannot be a 400 externally balanced flywheel (looking through the inspection area on tranny, there is no weight or counterbalance, looks just like a stock 350 flywheel), my question is, am I going to have a problem when I remove the stock 350 flywheel and swap it out for a 400 flywheel, in other words are they essentially the same except for the balancing weights?
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Old 03-21-2009, 11:54 PM   #6
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The weight on a flywheel for a 400 is cast into it. And the bolts will only line up in one position, so you can't miss.

Now, unless your current flywheel is toast, they make a plate that bolts on between the crank and the flywheel that has the correct amount of weight. It may be that I'm thinking of a plate for a one piece (late model) style crank, but it might be worth chasing a little, Be cheaper to throw that plate in there than replace the 'wheel.

And to answer your question, YES, the difference is the weight.
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Old 03-21-2009, 11:56 PM   #7
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Tow it a shop and have them find out what you have in your Vette by the sounds of it you don't know and slapping a 350 flywheel on it probably isn't going to do anything but waste time and money.

Vibrations come from many sources only an idiot would put a internally balanced ( 350) flywheel on a 400 externally balanced motor
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Last edited by MotorHead; 03-24-2009 at 07:51 PM.
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Old 03-22-2009, 12:05 AM   #8
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Default I agree

I agree that only an idiot would put a 350 flywheel on a 400 sbc, however only an idiot would put a 400 in a corvette in the first place, it truly does not belong there. I have looked through the inspection area of tranny and there are no weights, so I have a 350 flywheel. The 400 motor does have the correct harmonic balancer. It would seem that someone blew up the 350 motor and had a 400 and just threw it in. Its a 70-73 block, so 400's from that era did not usually come out of a manual vehicle (usually caprices, or wagons, and trucks) so the guy either didn't give it much thought or just pieced together what he had. the car absolutely has ALOT of vibration (so much so that the tranny mount bracket has to be re-welded to the frame. To top it off, the guy I bought it from had the bell housing replaced b/c it cracked (DUH), why doesn't anyone ever try to find the reason for something to happen. Additionally, it did not dawn on the mechanic who put the new bell housing on to figure this out either, and more annoying ,,, HE WAS THERE ALREADY and could have changed it!!. With all this idiocy going on, I was just checking to see if there were some other reason, like the 400 flywheel will not fit in the bell housing or something else? or maybe its just a bunch of people who shouldnt be working on cars or have tools?
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Old 03-22-2009, 12:18 AM   #9
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The 350 and 400 flywheels are the same except for the weights.

You can try the bolt on weight like TimAT said,

http://www.mcleodind.com/application...l_weights.html

and then get the right flywheel later if it works.

The original flywheel was 3986394 but looks discontinued.

You can find replacments by googling 3986394 flywheel.
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Old 03-22-2009, 12:23 AM   #10
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[QUOTE=TimAT;1569391499]The weight on a flywheel for a 400 is cast into it. And the bolts will only line up in one position, so you can't miss.

Now, unless your current flywheel is toast, they make a plate that bolts on between the crank and the flywheel that has the correct amount of weight. It may be that I'm thinking of a plate for a one piece (late model) style crank, but it might be worth chasing a little, Be cheaper to throw that plate in there than replace the 'wheel.
/QUOTE]

Yes there is a correct weight that can be added to the internally 350balanced wheel to make it compatible with a 400 engine. I believe I still have one left. Pioneer Engine Parts offers the weight.
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Old 03-22-2009, 08:59 AM   #11
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Default Weights?

Wow excellent, and how do I put this weight on? Would I still have to remove the tranny and bell housing to attach it? If so I think it would be just as well to remove the flywheel and put the proper one on no? THANK YOU soo much for all of your suggestions and knowledge, sometimes I know just enough to get myself into trouble, so at least I can now attack this thing with some knowledge. Any other ideas on the aftermarket weights would be greatly appreciated... thanks
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Old 03-22-2009, 10:28 AM   #12
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my scat 383 (400 type internals) had to have a diff fluwheel.. if I were you I'de ber nervous about bolting in a weight.. at least use locking bolts...
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Old 03-22-2009, 10:44 AM   #13
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Quote:
Originally Posted by my75baby View Post
however only an idiot would put a 400 in a corvette in the first place, it truly does not belong there.
I beg to differ, when built they're excellent performance engines and will spank other small blocks and most big blocks.
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Old 03-22-2009, 10:57 AM   #14
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Quote:
Originally Posted by L88Plus View Post
I beg to differ, when built they're excellent performance engines and will spank other small blocks and most big blocks.
I have a 400 sitting under a cover just waiting for the time to come to rebuild it and put it back to good use, I also have 323 rear gear already rebuilt and ready to install when the time comes.
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Old 03-22-2009, 10:59 AM   #15
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Default Flywheel part #

Hello all, I finally crawled under the car and spun the flywheel to find the number on it. The number is 50-6054, which I looked up and is in fact for a chevy 350. So it looks like the tranny and bell housing both have to come out to change this out. I hope the motor is not too badly damaged by running with the wrong flywheel.
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Old 03-22-2009, 11:00 AM   #16
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Quote:
Originally Posted by MotorHead View Post
Tow it a shop and have them find out what you have in your Vette by the sounds of it you don't know and slapping a 350 flywheel on it probably isn't going to do anything but waste time and money.

Vibrations come from many sources only an idiot would put a internally balanced ( 350) flywheel on a 400 internally balanced motor
tow it to the show because all of the vibrations could be something else or it could have messed it up by now to.
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Old 03-22-2009, 11:04 AM   #17
my75baby
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Default 400 idiocy

Well the idiot that put the 400 in my car used a 350 flywheel and probably toasted the great 400. The motor has a lower knock, probably the rear main bearing, this from running the car and probably getting on it. Who knows what kind of problems i'm gonna chase and how much $$ i will spend to find them. Wonder if it's even worth it to keep the 400 and try to fix / find the knock (hopefully not a wrist pin), or just take it out and put the proper motor back in. Such a shame.. If stupidity were painful, this probably wouldn't have happened.. (the flywheel appears to be aftermarket as well.. )
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Old 03-22-2009, 11:24 AM   #18
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Quote:
Originally Posted by my75baby View Post
Well the idiot that put the 400 in my car used a 350 flywheel and probably toasted the great 400. The motor has a lower knock, probably the rear main bearing, this from running the car and probably getting on it. Who knows what kind of problems i'm gonna chase and how much $$ i will spend to find them. Wonder if it's even worth it to keep the 400 and try to fix / find the knock (hopefully not a wrist pin), or just take it out and put the proper motor back in. Such a shame.. If stupidity were painful, this probably wouldn't have happened.. (the flywheel appears to be aftermarket as well.. )
if i was you i would pull the engine and tear it down to see what happened. i would say the main berrings are toast and the Crank has some sort of damage. and maybe the piston/connecting rods could be messed up.

but the heads, and valve train should be good
i would have a look at then engine and see whats going on before you get to replacing the weight being the engine may be pretty bad inside

Ryan
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Old 03-23-2009, 12:05 PM   #19
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Is it just me ... or is it deja vu all over again?

I see a response to the op that reminds me of a certain BS'er & ex CF member who hailed from Columbia MO & recently "exited" from CF. I'm reasonably sure it's Not the same sad sack; so I sure hope we don't have another one "in the making".

What seems familiar to me is Not found by parsing his reply's content. What's familiar is how this thread's responder asks lotsa rather basic motor questions in other CF threads ... and then with little time to even catch his breath, same fellow is giving tech motor advice in this thread.
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Old 03-23-2009, 03:08 PM   #20
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Quote:
Originally Posted by jackson View Post
Is it just me ... or is it deja vu all over again?

I see a response to the op that reminds me of a certain BS'er & ex CF member who hailed from Columbia MO & recently "exited" from CF. I'm reasonably sure it's Not the same sad sack; so I sure hope we don't have another one "in the making".

What seems familiar to me is Not found by parsing his reply's content. What's familiar is how this thread's responder asks lotsa rather basic motor questions in other CF threads ... and then with little time to even catch his breath, same fellow is giving tech motor advice in this thread.
If you referring to me i was just putting down what i had read in an book and what my dad was telling me. i know how and engine gets wrecked (watched more then 1 engine blow up) but when it comes to rebuilding them i am not to sure what i am doing. some will agree rebuilding an engine is a little more complicated.

Ryan
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Old 03-23-2009, 03:08 PM
 
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