Main bearing clearances
Thread Starter
Instructor
Joined: Jan 2011
Posts: 146
Likes: 2
From: Hillsborough NC
Main bearing clearances
Installed the crank tonight and checked the mains’ clearances with PlastiGage. The #1 read between 2-3 and #2-5 all read at 3 thousandths. Freshly lined bored/honed with ARP studs & lube. The forged crank is ground 10 & the bearings are from King Racing (MB 557XP 010).
A Car Craft article mentioned the PG read 0.0015 and the mic’d value was 0.0031. Several forum members say that PG measures only ½ of the clearance since you only put it on one side and you aren’t bending the crank just by tightening the caps. Seems to make sense but PG says nothing about this on their site.
Any thoughts? Regardless of the 1/2 reading, does 0.003 measured on PG jive with what you have done before? This will be in a 10:1 engine that should make 400-420 at the crank.
A Car Craft article mentioned the PG read 0.0015 and the mic’d value was 0.0031. Several forum members say that PG measures only ½ of the clearance since you only put it on one side and you aren’t bending the crank just by tightening the caps. Seems to make sense but PG says nothing about this on their site.
Any thoughts? Regardless of the 1/2 reading, does 0.003 measured on PG jive with what you have done before? This will be in a 10:1 engine that should make 400-420 at the crank.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
U have a good question dude. I have always thought the PG was the total clearance as most shop manuals say to use PG for ring & brg overhauls.
I only have specs for my '74 (which was an L48) from my GM shop manual as follows:
Mains: #1 = 2.4484"-2.4493", clearance = 0.0008"-0.0020"
#2,3,4 = 2.4481"-2.4490", clearance = 0.0011"-0.0023"
#5 = 2.4479"-2.4488", clearance = 0.0017"-0.0033"
Crankpin: 2.099"-2.100, taper new = 0.0003", service = 0.001"
Out of round new = 0.0002", service = 0.001"
Crank end play: 0.002"-0.006"
Rod bearing clearance: new = 0.0013"-0.0035", service = 0.008"-0.014"
So i read using your PG measurements - with the main caps fully torqued down and main brgs installed "dry" - your 0.0015" should be OK. U can turn the crank and measure with PG at different locations. This is still much easier (for me) than to measure in several different locations with a bore gauge then average them.
I only have specs for my '74 (which was an L48) from my GM shop manual as follows:
Mains: #1 = 2.4484"-2.4493", clearance = 0.0008"-0.0020"
#2,3,4 = 2.4481"-2.4490", clearance = 0.0011"-0.0023"
#5 = 2.4479"-2.4488", clearance = 0.0017"-0.0033"
Crankpin: 2.099"-2.100, taper new = 0.0003", service = 0.001"
Out of round new = 0.0002", service = 0.001"
Crank end play: 0.002"-0.006"
Rod bearing clearance: new = 0.0013"-0.0035", service = 0.008"-0.014"
So i read using your PG measurements - with the main caps fully torqued down and main brgs installed "dry" - your 0.0015" should be OK. U can turn the crank and measure with PG at different locations. This is still much easier (for me) than to measure in several different locations with a bore gauge then average them.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If the journals and cap bores are round (not egg-shaped), with new bearings, and you use the PG per the instructions, the results should be very close to the total diametral clearance. Essentially, you have a small circle inside a larger one...touching at only one point, with the PG positioned 180 degrees across from that contact point.
Excess oil needs to be removed before making a measurement; and the crank needs to be positioned per instructions.
Excess oil needs to be removed before making a measurement; and the crank needs to be positioned per instructions.
Thread Starter
Instructor
Joined: Jan 2011
Posts: 146
Likes: 2
From: Hillsborough NC
Thanks everyone for the replies.
My crank was ground 10 on a previous rebuild and then only polished prior to this current rebuild. Wasn't sure how much material comes off during a polish.
When taking the PG measurements, I wiped the assembly lube off the spot where the PG sat, but not off the entire bearing/journal. Going to wipe them all and retake the measurement today. Will post the results.
My crank was ground 10 on a previous rebuild and then only polished prior to this current rebuild. Wasn't sure how much material comes off during a polish.
When taking the PG measurements, I wiped the assembly lube off the spot where the PG sat, but not off the entire bearing/journal. Going to wipe them all and retake the measurement today. Will post the results.
Thread Starter
Instructor
Joined: Jan 2011
Posts: 146
Likes: 2
From: Hillsborough NC
new readings
Yanked the crank and wiped most of the assembly lube off. Checked the clearances again w/ PG.
The #1 bearing remained the same @ 0.003.
Numbers 2-5 all looked to have gone from 0.003 to 0.002 and maybe even 0.0018 or so.
Feeling better about the clearances except for the number 1 bearing.
Should I be concerned about the #1 bearing?
The #1 bearing remained the same @ 0.003.
Numbers 2-5 all looked to have gone from 0.003 to 0.002 and maybe even 0.0018 or so.
Feeling better about the clearances except for the number 1 bearing.
Should I be concerned about the #1 bearing?
Melting Slicks
Joined: Mar 2002
Posts: 2,164
Likes: 7
From: Beringen
Rule of thumb which is based on ages of experience of a dozen engine builders I've talked to : .001" per 1" crank pin diameter.
So if your mains are 2.45", you need .00245". If your rods are 2.100 you need .0021 to be perfect.
(Some combo's need something different, but most times that's not the case)
To get there you might need to mix and match bearings of+.0005.
I had perfect oil pressure at idle with that.
PS : Use a micrometer and not plastigages, and measure at different place around the journal and in a cross (not near the oil hole) and use the largest diameter you get to determine the clearance.
So if your mains are 2.45", you need .00245". If your rods are 2.100 you need .0021 to be perfect.
(Some combo's need something different, but most times that's not the case)
To get there you might need to mix and match bearings of+.0005.
I had perfect oil pressure at idle with that.
PS : Use a micrometer and not plastigages, and measure at different place around the journal and in a cross (not near the oil hole) and use the largest diameter you get to determine the clearance.
Last edited by Belgian1979vette; Aug 10, 2014 at 12:40 PM.
Thread Starter
Instructor
Joined: Jan 2011
Posts: 146
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From: Hillsborough NC
For what it is worth, I am not sure the Car Craft article comparing PG accuracy with dial bore measurements came to the right conclusion.
They say "If you set the dial-bore gauge to the crank-journal diameter, the difference between the inside diameter of the bearing and the crank journal will be the oil clearance."
To me, the use of a dial bore guage measures the inside diameter which includes 2x the clearance. Reworded, the journal diameter plus the clearance on both sides.
I think that they should've said that 1/2 of the difference between the inside diameter and the journal diamter is the clearance.
They say "If you set the dial-bore gauge to the crank-journal diameter, the difference between the inside diameter of the bearing and the crank journal will be the oil clearance."
To me, the use of a dial bore guage measures the inside diameter which includes 2x the clearance. Reworded, the journal diameter plus the clearance on both sides.
I think that they should've said that 1/2 of the difference between the inside diameter and the journal diamter is the clearance.
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Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
For what it is worth, I am not sure the Car Craft article comparing PG accuracy with dial bore measurements came to the right conclusion.
They say "If you set the dial-bore gauge to the crank-journal diameter, the difference between the inside diameter of the bearing and the crank journal will be the oil clearance."
To me, the use of a dial bore guage measures the inside diameter which includes 2x the clearance. Reworded, the journal diameter plus the clearance on both sides.
I think that they should've said that 1/2 of the difference between the inside diameter and the journal diamter is the clearance.
They say "If you set the dial-bore gauge to the crank-journal diameter, the difference between the inside diameter of the bearing and the crank journal will be the oil clearance."
To me, the use of a dial bore guage measures the inside diameter which includes 2x the clearance. Reworded, the journal diameter plus the clearance on both sides.
I think that they should've said that 1/2 of the difference between the inside diameter and the journal diamter is the clearance.
It pretty simple, you get an accurate measurement to the 10 thou. of an inch (.0001 ) if you have 0.0001" mics and the dial bore gauge and mics are pretty reasonable these days. You measure the inside diameter of the journal housing with the bearing in and torqued down, to simplify things lets say you get 2.003" then you mic the crankshaft journal and you get 2.001" giving you an oil clearance of 2 thou. ( .002" )
The rear bearing #5 by the back of the engine is usually a little bigger.
Last edited by MotorHead; Aug 10, 2014 at 06:16 PM.
Thread Starter
Instructor
Joined: Jan 2011
Posts: 146
Likes: 2
From: Hillsborough NC
Hands down no way around it a dial bore gauge and mic is the best way to set oil clearance, I would not and have never built a engine any other way.
It pretty simple, you get an accurate measurement to the 10 thou. of an inch (.0001 ) if you have 0.0001" mics and the dial bore gauge and mics are pretty reasonable these days. You measure the inside diameter of the journal housing with the bearing in and torqued down, to simplify things lets say you get 2.003" then you mic the crankshaft journal and you get 2.001" giving you an oil clearance of 2 thou. ( .002" )
The rear bearing #5 by the back of the engine is usually a little bigger.
It pretty simple, you get an accurate measurement to the 10 thou. of an inch (.0001 ) if you have 0.0001" mics and the dial bore gauge and mics are pretty reasonable these days. You measure the inside diameter of the journal housing with the bearing in and torqued down, to simplify things lets say you get 2.003" then you mic the crankshaft journal and you get 2.001" giving you an oil clearance of 2 thou. ( .002" )
The rear bearing #5 by the back of the engine is usually a little bigger.
Really just wondering why, in the case you described above, is the clearance 2 thousandths and not 1. Seems to me you have a 0.001 clearance all the way around the journal, and you report 0.002 because of the way it is measured (diameter vs radius). Is that just the standard way to report a clearance?
Thanks for all the help and replies.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Agreed - I am not saying PG is better at all and I would rather have a dial bore/mic set.
Really just wondering why, in the case you described above, is the clearance 2 thousandths and not 1. Seems to me you have a 0.001 clearance all the way around the journal, and you report 0.002 because of the way it is measured (diameter vs radius). Is that just the standard way to report a clearance?
Thanks for all the help and replies.
Really just wondering why, in the case you described above, is the clearance 2 thousandths and not 1. Seems to me you have a 0.001 clearance all the way around the journal, and you report 0.002 because of the way it is measured (diameter vs radius). Is that just the standard way to report a clearance?
Thanks for all the help and replies.
Thread Starter
Instructor
Joined: Jan 2011
Posts: 146
Likes: 2
From: Hillsborough NC
Thread Starter
Instructor
Joined: Jan 2011
Posts: 146
Likes: 2
From: Hillsborough NC
Le Mans Master
Joined: Oct 2010
Posts: 6,938
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From: melbourne florida
2023 C3 of the Year Finalist - Modified
if you don't know exactly how to measure it use plasti gauge. make sure the crank spins easily. you could also have your machine shop measure it. watch them when they do it then you will know.