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Discussion Starter · #1 ·
I'm replacing my old 2 piece rear main block with a newer 1 piece rear main block. I built the engine from all aftermarket parts. I want to use my old transmission.

Does the flywheel/flexplate sit the same depth in the bellhousing for 1 piece or 2 piece blocks?
It looks like the crankshaft is longer on the 1 piece making the flywheel sit deeper in the bellhousing? Or is the 1 piece block made to account for this? OR do I need to measure everything? And possibly shim, or machine parts and new bellhousing? I was also think I might be able to flip the flywheel around if need be? Any information is helpful.
 

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Discussion Starter · #3 ·
I'm replacing my old 2 piece rear main block with a newer 1 piece rear main block. I built the engine from all aftermarket parts. I want to use my old transmission.

Does the flywheel/flexplate sit the same depth in the bellhousing for 1 piece or 2 piece blocks?
It looks like the crankshaft is longer on the 1 piece making the flywheel sit deeper in the bellhousing? Or is the 1 piece block made to account for this? OR do I need to measure everything? And possibly shim, or machine parts and new bellhousing? I was also think I might be able to flip the flywheel around if need be? Any information is helpful.
I was also thinking maybe the flywheels on the different cranks has a different offset to make the flywheels for 1 or 2 piece sit at the same depth inside the bellhousing?
 

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Discussion Starter · #6 ·
Cool thanks Eric. That makes sense.
Is there anything special I need to know about the pilot bearing in the crank where the transmission main shaft rotates in? I was just going to get one that fits the diameter of the main shaft end and inside of crank. It seems like if the main shaft of the tranny pushes on the crank so there is no clearance then the crank wouldnt get any oil on one side of the trust washer for the crank. So the end of the main shaft should not contact the crank? Is there a clearance? or should I not worry about it. I was thinking I could put it all together and check the crank thrust bearing clearance again from the front? Then I would know there is clearance between the end of the main shaft and the crank shaft.
 

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One piece seal and two piece seal flywheels and flexplates are identical except for the crankshaft mounting flange. The older two piece seals use a 3.5 inch bolt circle the newer, since 1986, use a 3 inch crank flange bolt circle. So these parts crankshaft and fly/flex do not interchange across 1985 and older to 1986 and newer.

You cannot use the one to two piece seal adapter without also using the older style 2 piece seal crank.

Keep in mind Chevy went to the one piece rear seal crank in 1986 as fix to 31 years of leaking rear main seals. There’s always these guys that say they never had an SBC with a leaky rear main seal so apparently Chevy spent money to fix this needlessly which is something big corporations are always willing to spend money on.

Note that 350 the carries a tiny amount of balance weight externally at the rear flange. The engines with two piece rear main seals have this as part of the crankshaft so their fly/flex is neutral balanced. For the one piece seal cranks this is bit of offset external balance is removed to the fly/flex so these are not neutral balanced.

Bogie
 

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Discussion Starter · #8 ·
One piece seal and two piece seal flywheels and flexplates are identical except for the crankshaft mounting flange. The older two piece seals use a 3.5 inch bolt circle the newer, since 1986, use a 3 inch crank flange bolt circle. So these parts crankshaft and fly/flex do not interchange across 1985 and older to 1986 and newer.

You cannot use the one to two piece seal adapter without also using the older style 2 piece seal crank.

Keep in mind Chevy went to the one piece rear seal crank in 1986 as fix to 31 years of leaking rear main seals. There’s always these guys that say they never had an SBC with a leaky rear main seal so apparently Chevy spent money to fix this needlessly which is something big corporations are always willing to spend money on.

Note that 350 the carries a tiny amount of balance weight externally at the rear flange. The engines with two piece rear main seals have this as part of the crankshaft so their fly/flex is neutral balanced. For the one piece seal cranks this is bit of offset external balance is removed to the fly/flex so these are not neutral balanced.

Bogie
[/QUOT
One piece seal and two piece seal flywheels and flexplates are identical except for the crankshaft mounting flange. The older two piece seals use a 3.5 inch bolt circle the newer, since 1986, use a 3 inch crank flange bolt circle. So these parts crankshaft and fly/flex do not interchange across 1985 and older to 1986 and newer.

You cannot use the one to two piece seal adapter without also using the older style 2 piece seal crank.

Keep in mind Chevy went to the one piece rear seal crank in 1986 as fix to 31 years of leaking rear main seals. There’s always these guys that say they never had an SBC with a leaky rear main seal so apparently Chevy spent money to fix this needlessly which is something big corporations are always willing to spend money on.

Note that 350 the carries a tiny amount of balance weight externally at the rear flange. The engines with two piece rear main seals have this as part of the crankshaft so their fly/flex is neutral balanced. For the one piece seal cranks this is bit of offset external balance is removed to the fly/flex so these are not neutral balanced.

Bogie
So is it faster and cheaper to balance a rotating assembly that has 1 piece crank because you only need to remove or add weight from the flywheel and the harmonic balancer? But on the 2 piece crank you have to balance by removing and adding weight from the crank counterbalances? So what it's easier to machine the harmonic balancer and flywheel/flexplate instead of machining the crank? Is this why gm did this? Then they can balance faster instead of needing to add heavy malory in the crank compared to bolting on weight to the flywheel/flexplate or harmonic balancer.
 

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Discussion Starter · #9 ·
One piece seal and two piece seal flywheels and flexplates are identical except for the crankshaft mounting flange. The older two piece seals use a 3.5 inch bolt circle the newer, since 1986, use a 3 inch crank flange bolt circle. So these parts crankshaft and fly/flex do not interchange across 1985 and older to 1986 and newer.

You cannot use the one to two piece seal adapter without also using the older style 2 piece seal crank.

Keep in mind Chevy went to the one piece rear seal crank in 1986 as fix to 31 years of leaking rear main seals. There’s always these guys that say they never had an SBC with a leaky rear main seal so apparently Chevy spent money to fix this needlessly which is something big corporations are always willing to spend money on.

Note that 350 the carries a tiny amount of balance weight externally at the rear flange. The engines with two piece rear main seals have this as part of the crankshaft so their fly/flex is neutral balanced. For the one piece seal cranks this is bit of offset external balance is removed to the fly/flex so these are not neutral balanced.

Bogie
1 piece seal crank might be better because you can run a neutral balance flywheel/flexplate with 1 piece seal bolt pattern and neutral balance harmonic balancer, then get the rotating assembly balanced by the crank counterweights. I think the extra counter weight that was on the flywheel/flexplate on the 1 piece set up will now be weight added to each counterbalance on the crank. This gives a balance job where there is no counterweight outside the block and its all on the crank. The rotating assembly only uses counterbalances opposite of the piston rod assembly. You could never do this with the 2 piece crank because that offset weight is always on the crank outside the block, away from the piston rods assemblies. Has anyone ever tried removing that extra weight on the 2 piece crank and then balancing the counterweights on crank instead, before they invented the 1 piece seal crank? Seems like this would have less twisting on bearings because the counterweights on the crank more perfectly match the bob weight of the piston rod assembly.
 

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The factory uses techniques that minimize cost but provide suitable performance. Yes you can remove the external weight and replace it with a bit of Mallory Metal (chunk of tungsten by any other name) but that requires drill a hole into a counter weight, pressing the Mallory Metal in then either peening or welding it in place. Expensive material and process, it is done on serious race engines where cost isn’t as much a consideration as it is on street through even moderate race engines. You will note that for high speed endurance racing speciality crankshafts are used with center main counter weights that are not on production and “moderate” competition crankshafts. The added counter weights are to remove the angular twist on the inboard crank pins that is taken on the center 3 mains which are also the mains that usually receive 4 bolt main caps. These center crank pins depend upon the 4 piston and rod assemblies to establish balance on the crank. However that balance is offset rather than directly in opposition so inside the crank there is the existence of a lever arm by the distance of the crank pins from each other. This puts a twist on the shaft the is reacted by the center 3 mains, again hence the 4 bolt caps on these mains.

I don’t like to balance the crank without the damper or Fly/flex being used, sometimes these end devices offer up unexpected balance issues. But this is more critical of high RPM engines that go high and stay there a while. To a large extent a drag racer doesn’t stay at high revs long enough to sweat blood over the balance, not that you want to be sloppy but to the bigger need of accelerating the crankshaft often it is more important getting the weight minimized for the needed strength than achieving balls on counterbalance of the assembly.

In theory a neutral balanced crank assembly should accept a neutral balanced damper and flywheel or flexplate it’s just sometimes those parts aren’t as neutral balanced as you might wish. With a one piece rear seal fly/flex for a neutral balanced shaft you must shop for a fly/flex that uses the bolt on offset weight rather than the welded. Not that weld can’t be ground off but here you are back at a don’t know if the device is in balance. I suppose you could put it on your motorcycle wheel bubble balancer to see or check as you go how good is good. I admit in the middle of the night of the last hours before having to present machine and man for tech inspection I’ve done such things and the machine and driver lived to tell so it becomes mechanic’s tales in the pits. Everybody gets a “holy sheet” laugh out of it.

Bogie
 

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Look at this attachment. You may also need to chage your starter. Your input shaft should slide into your pilot bearing and NOT put any pressure against the crankshaft. If it does, you have the wrong pilot bearing. It will quickly become a problem.

 

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Is there anything special I need to know about the pilot bearing in the crank where the transmission main shaft rotates in? I was just going to get one that fits the diameter of the main shaft end and inside of crank. It seems like if the main shaft of the tranny pushes on the crank so there is no clearance then the crank wouldnt get any oil on one side of the trust washer for the crank. So the end of the main shaft should not contact the crank? Is there a clearance? or should I not worry about it. I was thinking I could put it all together and check the crank thrust bearing clearance again from the front? Then I would know there is clearance between the end of the main shaft and the crank shaft.
All I do professionally is gearbox related. Be careful if you order a "Main shaft pilot bushing" . The INPUT of your trans is what youre actually discussing, and its professional name is Maindrive gear, along with a handful of others. The bushing or bearing youre thinking of is a Crankshaft pilot bushing/bearing, OR an INPUT pilot bushing/bearing. If you order a mainSHAFT pilot bearing, you're going to get the bearing that goes between your input and output shaft.

be careful installing the pilot bushing/bearing into your crank. If it fits TIGHT, stop. Its a snug fit.

Bellhousing concentricity is also extremely important.
 
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