crankshaft main clearances question
Thread Starter
Drifting
Joined: Apr 2002
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From: Manhattan Beach Ca
crankshaft main clearances question
I measured my clearances on my new 383 and here are the results:
FRONT------------------------------------------------------AFT
------1----------2--------------3------------4----------5
---.0030-------0.0018-------- .0025-------0.0025------.0035
'69 Manual Spec 1 - 4 ( .0008 - .0020 ) 5 ( .0035 MAX )
max service limit .0040
My question is why is the large #5 location have different specs than 1-4? Shouldn't all the clearances be the same?
Any comments appreciated.
FRONT------------------------------------------------------AFT
------1----------2--------------3------------4----------5
---.0030-------0.0018-------- .0025-------0.0025------.0035
'69 Manual Spec 1 - 4 ( .0008 - .0020 ) 5 ( .0035 MAX )
max service limit .0040
My question is why is the large #5 location have different specs than 1-4? Shouldn't all the clearances be the same?
Any comments appreciated.
Last edited by Surfer69; Dec 10, 2004 at 09:33 AM.
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Yes that is where oil originates and you want it opened up a little more
How did you measure the clearances ?
My first explantion was not very good, I do believe the explanation I got was that it handles more load especially on a 4 speed car so that is why you want it opened more to get more oil
How did you measure the clearances ?
My first explantion was not very good, I do believe the explanation I got was that it handles more load especially on a 4 speed car so that is why you want it opened more to get more oil
Last edited by MotorHead; Dec 10, 2004 at 01:03 PM.
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From: The Top of Utah
Originally Posted by Surfer69
My question is why is the large #5 location have different specs than 1-4? Shouldn't all the clearances be the same?
Any comments appreciated.
Any comments appreciated.
*I* can't explain the production clearance differences in the main bearings. My Factory Service Manual shows, #1 and #5 as each being unique, and 2-4 as the same as one another. The Chevrolet Power book, makes no distinction to the position.
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Thread Starter
Drifting
Joined: Apr 2002
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From: Manhattan Beach Ca
That's interesting CFI because 1 & 5 are noticebly different than 2,3 and 4. I notice from plastigauge on 5 that the bearing is so wide it smashes the most where the bolts are which is around .0025 but tapers out to .0035 at the fwd end. I think that's why the 69 manual gives a larger spec because they know there is no way to get an even distribution since the two bolts are to one side only.
I'm trying to decide wether to spend another $50 bucks to get another undersize(or oversize?) set and make everything perfect. My last engine has/had an issue with low oil pressure, so I want to make sure clearances arent an issue on this 383.
I'm trying to decide wether to spend another $50 bucks to get another undersize(or oversize?) set and make everything perfect. My last engine has/had an issue with low oil pressure, so I want to make sure clearances arent an issue on this 383.
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From: The Top of Utah
It sounds like you are laying the plastigage in the wrong direction. The strand of plastic should be parallel to the bearing bore.
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PS. If I have misinterpreted what you're saying, check the bearing bores in the block, for straight, round, and diameter, for their full length.
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PS. If I have misinterpreted what you're saying, check the bearing bores in the block, for straight, round, and diameter, for their full length.
Last edited by CFI-EFI; Dec 10, 2004 at 10:07 AM. Reason: Add a PS.
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
In would further like to calrify what I meant in my previous post as well as my opinion why the bearing in question is bigger that the others . 
What I meant was "Yes" I agree with the previous post about that being the thrust bearing, next I elaborated on
this by stating that is where the oil originates for the the surface he is talking about. I can expand that even further : The rear main bearing is different from the other bearing in that it has a an extra surface that must be lubricated this area could be called the bearing flange. On the crankshft, in addition to the actual #5 (rear) crank journal there is an area behind this called the crankshaft thrust face. This area requires good lubrication because this is the area that stops the crankshaft from moving forward. In a 4 speed car this is even more important because there is more stress here as you bang through the gears.
With this in mind it kind of makes sense to have a little more clearance on this bearing to allow more oil to get to this surface.
What I meant was "Yes" I agree with the previous post about that being the thrust bearing, next I elaborated on
this by stating that is where the oil originates for the the surface he is talking about. I can expand that even further : The rear main bearing is different from the other bearing in that it has a an extra surface that must be lubricated this area could be called the bearing flange. On the crankshft, in addition to the actual #5 (rear) crank journal there is an area behind this called the crankshaft thrust face. This area requires good lubrication because this is the area that stops the crankshaft from moving forward. In a 4 speed car this is even more important because there is more stress here as you bang through the gears.
With this in mind it kind of makes sense to have a little more clearance on this bearing to allow more oil to get to this surface.
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From: The Top of Utah
Originally Posted by MotorHead
The rear main bearing is different from the other bearing in that it has a an extra surface that must be lubricated this area could be called the bearing flange. On the crankshft, in addition to the actual #5 (rear) crank journal there is an area behind this called the crankshaft thrust face. This area requires good lubrication because this is the area that stops the crankshaft from moving forward.
Originally Posted by MotorHead
In a 4 speed car this is even more important because there is more stress here as you bang through the gears.
Someone had intimated that the reason for the additional clearance was the proximity to the pump. I wasn't addressing anyone, specifically, in mentioning the oil routing.
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From: NC,USA
I've got a question...
Do crank journals wear just like a cam lobe? The rod journals... the rod applies force on the top, bottom... left and right sides of the journal through out the swing! I wonder if this would put the same kind of wear pattern on the journal, given the rotation doesnt switch directions...
Wouldnt that cause the rod to try and walk forward on the crank? Thrust front to back, back to front?
Do crank journals wear just like a cam lobe? The rod journals... the rod applies force on the top, bottom... left and right sides of the journal through out the swing! I wonder if this would put the same kind of wear pattern on the journal, given the rotation doesnt switch directions...
Wouldnt that cause the rod to try and walk forward on the crank? Thrust front to back, back to front?
Thread Starter
Drifting
Joined: Apr 2002
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From: Manhattan Beach Ca
CFI, I layed the plastigauge in the fore and aft direction.
Rethinking, it doesnt make sense for it to taper even if the bolts are to one side of the bearing. Something must be causing the taper. I'll check for burrs.
The idea about needing more oil flow to the flanges sounds like a possiblilty for bigger specs. The actual spec for #5 is .0018-.0034.
Rethinking, it doesnt make sense for it to taper even if the bolts are to one side of the bearing. Something must be causing the taper. I'll check for burrs.
The idea about needing more oil flow to the flanges sounds like a possiblilty for bigger specs. The actual spec for #5 is .0018-.0034.
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From: The Top of Utah
Originally Posted by ZD75blue
I've got a question...
Do crank journals wear just like a cam lobe? The rod journals... the rod applies force on the top, bottom... left and right sides of the journal through out the swing! I wonder if this would put the same kind of wear pattern on the journal, given the rotation doesnt switch directions...
Do crank journals wear just like a cam lobe? The rod journals... the rod applies force on the top, bottom... left and right sides of the journal through out the swing! I wonder if this would put the same kind of wear pattern on the journal, given the rotation doesnt switch directions...
Ideally there will little to no wear to the crank journals. You have a soft copper bearing surface acting against a hardened iron or steel crank journal. Lapses in lubrication and contaminants in the oil cause almost all crankshaft wear.
Originally Posted by ZD75blue
Wouldnt that cause the rod to try and walk forward on the crank? Thrust front to back, back to front?
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From: The Top of Utah
Originally Posted by Surfer69
The idea about needing more oil flow to the flanges sounds like a possiblilty for bigger specs. The actual spec for #5 is .0018-.0034.
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Le Mans Master
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From: Las Vegas - Just stop perpetuating myths please.
Do the plastigage measurements repeat?
Originally Posted by Surfer69
I measured my clearances on my new 383 and here are the results:
FRONT------------------------------------------------------AFT
------1----------2--------------3------------4----------5
---.0030-------0.0018-------- .0025-------0.0025------.0035
'69 Manual Spec 1 - 4 ( .0008 - .0020 ) 5 ( .0035 MAX )
max service limit .0040
My question is why is the large #5 location have different specs than 1-4? Shouldn't all the clearances be the same?
Any comments appreciated.
FRONT------------------------------------------------------AFT
------1----------2--------------3------------4----------5
---.0030-------0.0018-------- .0025-------0.0025------.0035
'69 Manual Spec 1 - 4 ( .0008 - .0020 ) 5 ( .0035 MAX )
max service limit .0040
My question is why is the large #5 location have different specs than 1-4? Shouldn't all the clearances be the same?
Any comments appreciated.
Plastigage does have a shelf life. Can u find this date? And to verify measurements can u repeat them fairly accurately? But if u keep getting the same numbers either run the 0.010" oversize bearings (heck even mix & match with 0.020" over brgs) or get it back to a machine shop.
Hey i can see ur gonna find out whats wrong if u have made this much effort to do it right. Just need to follow through here as most other repairs can be fixed with the block in the car. Yes details here will pay off in longivity or economicaly or maybe even more beach bunnies in the passenger seat with less time on jack stands.
Good luck.
cardo0
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From: The Top of Utah
Originally Posted by cardo0
But if u keep getting the same numbers either run the 0.010" oversize bearings (heck even mix & match with 0.020" over brgs) or get it back to a machine shop.
cardo0
cardo0RACE ON!!!
Le Mans Master
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From: Las Vegas - Just stop perpetuating myths please.
Not what i way trying to recommend.
Originally Posted by CFI-EFI
If he uses even one half of a 0.010" bearing in any position, the crank it going to be locked tighter than Fort Knox.
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For the 0.035" clearances a 0.020" oversize brg could get u back to spec while using the 0.010" oversize in another journal that has only 0.018" or 0.025" clearance. No, i don't imply to install a 0.010" oversize brg 1/2 with any other size brg 1/2. 
Though i have never done this myself i recall Grumby Jenkins doubling brgs as spacers for the destroking of 400" mtrs to 377" (3.48" crank in 400" block) - by stacking 400" crank brgs on top of 3.48" brgs. I recall he also prepared them using something like Scotch Brite pads to smooth and final fit/size them.
But if changing brg sizes to correct clearance differences between journals would lock the crank up please explain.
cardo0
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From: The Top of Utah
Originally Posted by cardo0
But if changing brg sizes to correct clearance differences between journals would lock the crank up please explain. cardo0
cardo0At first, I thought it was just a typo, so I didn't make much of it. Since you reiterate, look at the post you quoted. Using the number five main clearance as measured at thirty five TEN thousandths (three and one half thousandths) as an example. Now subtract the .010" (ten thousandths) bearing you propose. .0035 - .010 = -.0065. *I* don't call an interference fit a "correct clearance". You think that won't lock up the crankshaft?
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Le Mans Master
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From: Las Vegas - Just stop perpetuating myths please.
Okay i'm only off by a decimal place (or 2).
Originally Posted by CFI-EFI
First, using two closely matched but different sized bearing shells results in a clearance, close to their average.
At first, I thought it was just a typo, so I didn't make much of it. Since you reiterate, look at the post you quoted. Using the number five main clearance as measured at thirty five TEN thousandths (three and one half thousandths) as an example. Now subtract the .010" (ten thousandths) bearing you propose. .0035 - .010 = -.0065. *I* don't call an interference fit a "correct clearance". You think that won't lock up the crankshaft?
RACE ON!!!
At first, I thought it was just a typo, so I didn't make much of it. Since you reiterate, look at the post you quoted. Using the number five main clearance as measured at thirty five TEN thousandths (three and one half thousandths) as an example. Now subtract the .010" (ten thousandths) bearing you propose. .0035 - .010 = -.0065. *I* don't call an interference fit a "correct clearance". You think that won't lock up the crankshaft?
RACE ON!!!
My bad. :o I got the Chilton's out and see my error. Chilton's also has the same specs as posted and had a few paragraphs on measuring mn brg cleareance. Maybe this can help here:
- wipe brg dry, no oil
- Plastigage parrallel crank
- after torquing, measure p-gage at widest point and if flattened p-gage widens at middle or ends brg or journal has taper > then measure crank with a micrometer (but Chilton's doesn't elaborate how)
- use standard 0.001" & 0.002" undersize brgs to produce proper clearance > if fit too sloppy, then crank must be reground.
So i'm thinking that the undersize brgs (which i think really means larger/oversize brg to fill the gap) are the fix here. As u can tell i'm no brg expert but a good parts man (NAPA or maybe a auto-machine shop) can maybe find these for u. Anyways u will need to verify with p-gage again. And a little Scotch Brite can help smooth things out but be careful/conservative.
Now it's important to always check a crank for smooth drag-free rotation before closing up the surgery.
Good luck. cardo0
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From: The Top of Utah
Originally Posted by cardo0
Okay i'm only off by a decimal place (or 2).
My bad. :o I got the Chilton's out and see my error.
My bad. :o I got the Chilton's out and see my error.
Originally Posted by cardo0
then measure crank with a micrometer (but Chilton's doesn't elaborate how)
Originally Posted by cardo0
- use standard 0.001" & 0.002" undersize brgs to produce proper clearance > if fit too sloppy, then crank must be reground.
Originally Posted by cardo0
So i'm thinking that the undersize brgs (which i think really means larger/oversize brg to fill the gap) are the fix here.
RACE ON!!!