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Discussion Starter · #1 ·
I have a 97 7500 series truck that im taking the WEAK 366 out of and replacing w a 427 tall deck (which are both truck motors). The 427 i bought was an earlier model 2pc rear main seal block. Casting no 3999294 (68-84 model). The balancers are different b/c of yr models, so im wandering what the correct flywheel is i should use?
Ive noticed These med duty trucks had 2 types of flywheels (depending on eng size & yr models). One flywheel has a couterwieght and one does not. Wanting to know which one is correct for each eng? And the ealier model blocks dont have the same plug location for knock sensor on side of block that the newer tbi blocks have. But there is a plug hole a little lower down near oul pan. Does that location matter? Can anyone help me?
Need this trk back on the road…
 

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Both the 366 and 427TD are considered internally balanced.....but the difference is 1-piece rear main seal or 2-piece.

The 2-piece rear main flywheel is nuetral, it has no balance weight on it(a earlier 2-piece seal 366T would have been the same way.)

Difference comes with the 1-piece rear main seal design.....it eliminated the end flange counterweight and forced them to put a weight on the flywheel

bFlywheel-to-crank olt pattern is different between 1-piece seal and 2-piece seal cranks, so they can't get mixed up by mistake.

Since you are swapping to and using the 2-piece seal engine, then you need a plain internal balance("nuetral") flywheel to match the crank flange, along with the correct diameter for the clutch package you are using.

I can't help you on the knock sensor location, I have no idea if it will work correctly in that location.
 

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Discussion Starter · #3 ·
Both the 366 and 427TD are considered internally balanced.....but the difference is 1-piece rear main seal or 2-piece.

The 2-piece rear main flywheel is nuetral, it has no balance weight on it(a earlier 2-piece seal 366T would have been the same way.)

Difference comes with the 1-piece rear main seal design.....it eliminated the end flange counterweight and forced them to put a weight on the flywheel

bFlywheel-to-crank olt pattern is different between 1-piece seal and 2-piece seal cranks, so they can't get mixed up by mistake.

Since you are swapping to and using the 2-piece seal engine, then you need a plain internal balance("nuetral") flywheel to match the crank flange, along with the correct diameter for the clutch package you are using.

I can't help you on the knock sensor location, I have no idea if it will work correctly in that location.
Thank you. Ive been told all kinds of opinions, and the person i believed to be correct turns out is apparently wrong… glad to know the bolt patterns are different. I never new that.

Anybody out there know about knock sensor locations? If bottom of block port vs center of block port matters?
 

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There is a 2-piece seal counterweighted flywheel, but it is just for the external balance 454.
But it can be bolted by mistake to a 366, 396/402, and 427.....so just makes sure you get a Internal(nuetral) balance 2-piece rear main seel flywheel and you'll be fine.

Balancers may look different between the 366 and the 427, but both are internal balance and interchangeable if you need to in order to get belt driven accessories to line up
 

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Discussion Starter · #5 ·
There is a 2-piece seal counterweighted flywheel, but it is just for the external balance 454.
But it can be bolted by mistake to a 366, 396/402, and 427.....so just makes sure you get a Internal(nuetral) balance 2-piece rear main seel flywheel and you'll be fine.

Balancers may look different between the 366 and the 427, but both are internal balance and interchangeable if you need to in order to get belt driven accessories to line up
Luckily, i had an older parts 366 w 2 pc rear seal i got the balancer off of. My newer pulleys bolted up & line up fine on it. Now noticed older block doesn’t have same knock sensor port as newer blocks have. Dont know what to do about it? Basically all i need to know about now to drop it in…
 

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There is a 2-piece seal counterweighted flywheel, but it is just for the external balance 454.
But it can be bolted by mistake to a 366, 396/402, and 427.....so just makes sure you get a Internal(nuetral) balance 2-piece rear main seel flywheel and you'll be fine.

Balancers may look different between the 366 and the 427, but both are internal balance and interchangeable if you need to in order to get belt driven accessories to line up
Are the 2'piece rear. seal externally weighted flywheels the same for a 454 BBC & the 400 SBC ?
 

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Knock sensors can be very sensitive. In a HD truck I don't know if they operate like in a car. When GM changed to a plastic intake on one of the V6 engines the knock sensors had to be changed to match the harmonic of the new assembly.
 
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