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34 Ford, 502 BBC 6 speed Tremec
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Discussion Starter · #1 ·
So i have the motor out of my 69 Chevelle that Ive had for 2 months and decided to go through it and frashen up this 50 year old motor. I just dropped the crank , block and heads off in a machine shop last week. Now Im trying to find a book that has the crankshaft clearances. I know a lot of the guys say generally rods and mains clearances go by the journal size. But I would like to see the actual specs in a printed manual. So I found an old Glenns repair manual today that went from 58-1967 and it did not have the clearances but it had the rod journal size. There was a footnote for the main journal size and when I read the foot note it said there was 3 different sizes to the main bearing journals. " I said what?" Would a manufacturer make a crankshaft with 3 journal sizes.? And Im thinking the 69 396 would have the same crank of course.

So my question is this true with the 396?

Also the crank has been turned , rods 30 and mains 20. (Im hoping that it just needs polishing). but it brings up the second question how does that affect the clearances, or is that not enough to make a difference?
 

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Our CJ2A had one crank journal .020 under, with the correct bearing to suit. Im guessing that a bum journal back then wasnt worth throwing a crank away at the factory.
By the 60s? I have a hard time believing they'd do that much re-shop. But, I'm only 40 so its just an educated guess. Perhaps the manual was printed with information from tearing down one car, and the writer used that variance as a C-Y-A??
 

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34 Ford, 502 BBC 6 speed Tremec
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Discussion Starter · #3 ·
So i have the motor out of my 69 Chevelle that Ive had for 2 months and decided to go through it and frashen up this 50 year old motor. I just dropped the crank , block and heads off in a machine shop last week. Now Im trying to find a book that has the crankshaft clearances. I know a lot of the guys say generally rods and mains clearances go by the journal size. But I would like to see the actual specs in a printed manual. So I found an old Glenns repair manual today that went from 58-1967 and it did not have the clearances but it had the rod journal size. There was a footnote for the main journal size and when I read the foot note it said there was 3 different sizes to the main bearing journals. " I said what?" Would a manufacturer make a crankshaft with 3 journal sizes.? And Im thinking the 69 396 would have the same crank of course.

So my question is this true with the 396?

Also the crank has been turned , rods 30 and mains 20. (Im hoping that it just needs polishing). but it brings up the second question how does that affect the clearances, or is that not enough to make a difference?
Now that I have the book back I can see that the difference is very minimal. seems they were allowing for more or less clearance. Possibly due to the distance from the oil pump?
Font Material property Number Paper Parallel
 

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Race it, Don't rice it!
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Not uncommon.
Bearing clearances are what you are looking for and so the formula goes by diameter. .001xJournal diameter so for you, .0027 is a good place too shoot for. In reality, anywhere between .002-.003 is good.
 

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Lots of things to consider here. First is that there are lots of tech changes since the original factory set up was printed. Newer bearing types are a consideration. I would go to Amazon and order one of the books on building big block Chevy engines and see what they are recommending currently.
Next, you mentioned that the crank was .020 mains and .030 rods already. This means its probably been rebuilt multiple times. Much better off to buy a new aftermarket crank that is a stronger alloy and has bigger fillet radaii. Get one with a nitrided surface for wear resistance. Since the rods have been thru a bazillion cycles and many of them at high speeds, you are better off to buy new aftermarket rods with ARP bolts than to think about reconditioning your rods. I would also look to see if there are oiling system upgrades or mods that people do to the engines.
 

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34 Ford, 502 BBC 6 speed Tremec
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Discussion Starter · #7 ·
Lots of things to consider here. First is that there are lots of tech changes since the original factory set up was printed. Newer bearing types are a consideration. I would go to Amazon and order one of the books on building big block Chevy engines and see what they are recommending currently.
Next, you mentioned that the crank was .020 mains and .030 rods already. This means its probably been rebuilt multiple times. Much better off to buy a new aftermarket crank that is a stronger alloy and has bigger fillet radaii. Get one with a nitrided surface for wear resistance. Since the rods have been thru a bazillion cycles and many of them at high speeds, you are better off to buy new aftermarket rods with ARP bolts than to think about reconditioning your rods. I would also look to see if there are oiling system upgrades or mods that people do to the engines.
I've got the Tom Wilson how to BBC. He doesnt include any GM spec sheets but he gives you Federal Mogul recommendation in the assembly verbiage. Thanks !
 

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My opinion is that bearing clearances are based on what you are doing with the engine. On an older engine like this for the street, I would be perfectly happy with .002 or a touch more. If this is going to the track, then nothing less than .003 will do.
 

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Discussion Starter · #11 ·
He didnt say which is better? # 1 is holding the oil on the bearing longer and keeping the oil pressure up. #4 is going to have lower oil pressure but its running the oil through the bearing faster. Could be keeping cleaner and cooler? This goes back to application, race motor or street motor.
 

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Pressure is not as important as flow..........basically. You can have too much of either. Generally engines which are expected to see higher rpms are generally set up with a little more clearance than grocery getters or cruisers. The general rule of thumb is that you need 10 psi of oil pressure for each 1,000 rpms. With most engines you are going to exceed that when idling, which is good. The reason for the 10psi per 1,000 rpm is that as rpms increase, so does centrifugal force. You need an additional 10 psi to overcome those centrifugal forces. You could actually have pressure, but no flow , so your guage would show good pressure but you have no flow or reduced flow.......which is bad. Basically 5,000 rpms needs 50 psi.

Too much flow can pump all the oil out of the pan and if a particular engine doesn't have good return flow, the valve covers fill up and the pan is empty. Also too much flow can create windage around the crank as the oil flows back. Many people like to put high volume or high pressure oil pumps on their engines. An engine can only flow a certain amount of oil. Usually the amount of bearing clearance is the limiting factor. The oil can only flow when there is an avenue for it to flow and bearing clearance USUALLY is the determining factor......hence the video. Now it is possible that in some engines there are other restrictions in the oil passages due to their size or shape. Sometimes its the way the system is designed......ergo the order in which the components receive oil. By that I mean whether the crank receives oil first or the cam receives oil first and then the other components feed off of them. Most people consider a "priority main" as the best set up. That means the main bearings receive oil first. Most factory engines aren't priority main systems. As I mentioned earlier, people believe that a high volume or high pressure oil pump will cure their problems. Lets face it, with millions of Chevy engines out there, sometimes changing the pump has helped someones problem. The basic point is this. If the engine cannot flow more oil, then adding a high volume pump will only allow the bypass in the pump to open farther and you use HP to pump the oil in a useless circle. If your engine has sufficient oil pressure at all rpms, then a higher pressure pump will use additional HP and basically accomplish nothing.

On the 500 Cadillac engine I'm building, its known that the last two cylinders in the block are usually problematic at higher (above 5,000 ) rpms because they are the last to receive oil. Builders are learning to use some aftermarket cam bearings that have a groove on the outside and a second hole that is clocked to a position where it provides the best "wedge" to the camshaft journal. This is why I suggested looking to see what things may be available for your Chevy Big Block that may be better than the original stuff. Also, on my block I started at the oil pump mount and radiused the steps in the machined passages and bought a gun drill that I could run thru the oil passages and open them slightly. Maybe it will help, and maybe it won't but I definitely cleared out any cast in obstructions and made the passages capable of flowing even if the bearings will be the ultimate decider of actual flow.

Different people will recommend different bearings and different clearances. Clevite and King are well known. I would comb the manufacturers website and see what they recommend before listening to anyone else. The question you asked about which flow rate was best. The higher the rpm you are turning the more flow you need, but you have to decide whether less flow is better when 99% of the driving is crusing. So actual clearance will be an individual choice.
 

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Discussion Starter · #13 ·
Pressure is not as important as flow..........basically. You can have too much of either. Generally engines which are expected to see higher rpms are generally set up with a little more clearance than grocery getters or cruisers. The general rule of thumb is that you need 10 psi of oil pressure for each 1,000 rpms. With most engines you are going to exceed that when idling, which is good. The reason for the 10psi per 1,000 rpm is that as rpms increase, so does centrifugal force. You need an additional 10 psi to overcome those centrifugal forces. You could actually have pressure, but no flow , so your guage would show good pressure but you have no flow or reduced flow.......which is bad. Basically 5,000 rpms needs 50 psi.

Too much flow can pump all the oil out of the pan and if a particular engine doesn't have good return flow, the valve covers fill up and the pan is empty. Also too much flow can create windage around the crank as the oil flows back. Many people like to put high volume or high pressure oil pumps on their engines. An engine can only flow a certain amount of oil. Usually the amount of bearing clearance is the limiting factor. The oil can only flow when there is an avenue for it to flow and bearing clearance USUALLY is the determining factor......hence the video. Now it is possible that in some engines there are other restrictions in the oil passages due to their size or shape. Sometimes its the way the system is designed......ergo the order in which the components receive oil. By that I mean whether the crank receives oil first or the cam receives oil first and then the other components feed off of them. Most people consider a "priority main" as the best set up. That means the main bearings receive oil first. Most factory engines aren't priority main systems. As I mentioned earlier, people believe that a high volume or high pressure oil pump will cure their problems. Lets face it, with millions of Chevy engines out there, sometimes changing the pump has helped someones problem. The basic point is this. If the engine cannot flow more oil, then adding a high volume pump will only allow the bypass in the pump to open farther and you use HP to pump the oil in a useless circle. If your engine has sufficient oil pressure at all rpms, then a higher pressure pump will use additional HP and basically accomplish nothing.

On the 500 Cadillac engine I'm building, its known that the last two cylinders in the block are usually problematic at higher (above 5,000 ) rpms because they are the last to receive oil. Builders are learning to use some aftermarket cam bearings that have a groove on the outside and a second hole that is clocked to a position where it provides the best "wedge" to the camshaft journal. This is why I suggested looking to see what things may be available for your Chevy Big Block that may be better than the original stuff. Also, on my block I started at the oil pump mount and radiused the steps in the machined passages and bought a gun drill that I could run thru the oil passages and open them slightly. Maybe it will help, and maybe it won't but I definitely cleared out any cast in obstructions and made the passages capable of flowing even if the bearings will be the ultimate decider of actual flow.

Different people will recommend different bearings and different clearances. Clevite and King are well known. I would comb the manufacturers website and see what they recommend before listening to anyone else. The question you asked about which flow rate was best. The higher the rpm you are turning the more flow you need, but you have to decide whether less flow is better when 99% of the driving is crusing. So actual clearance will be an individual choice.
This will be my every day driver. I was going to go with a stock GM oil pump and timing chain. for that matter. Clevite Bearings are what I usually use, and they should be available. The crank has been cut and thats what is in there now. You are putting a lot of effort into the Cad motor. hope it turns out well for you! Thanks for the info!
 
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