main bearing bores
Thread Starter
Intermediate
Joined: Oct 2006
Posts: 34
Likes: 1
From: Leesburg, Florida
main bearing bores
I have a 1970 350 4-bolt bare block I'm fooling with. I wanted to check main bearing clearances so I installed new std bearings and a new scat crank. I could not get any crush on plastigage. I borrowed a dial bore gauge (I don't have much experience using this) and measured the main bores perpendicular to parting line without bearings installed. #1 2.6470, #2 2.6551, #3 2.6549, #4 2.6555, #5 2.6487. Also did it diagonally (i.e. 10 o'clock and 4 o'clock, 2 o'clock and 8 o'clock) and got 2.6540 and 2.6462. I did this several times and came up with very similar numbers each time so I'm either consistently wrong or I'm in the ballpark. The crank measures #1 2.4491, #2 2.4493, #3 2.4493, #4 2.4492, #5 2.4492.
What can/should I do about this? I' don't want to go to a machine shop until I'm a little more knowledgeable.
What can/should I do about this? I' don't want to go to a machine shop until I'm a little more knowledgeable.
Burning Brakes
Joined: Jan 2007
Posts: 1,057
Likes: 4
From: Concord Calif
I have a 1970 350 4-bolt bare block I'm fooling with. I wanted to check main bearing clearances so I installed new std bearings and a new scat crank. I could not get any crush on plastigage. I borrowed a dial bore gauge (I don't have much experience using this) and measured the main bores perpendicular to parting line without bearings installed. #1 2.6470, #2 2.6551, #3 2.6549, #4 2.6555, #5 2.6487. Also did it diagonally (i.e. 10 o'clock and 4 o'clock, 2 o'clock and 8 o'clock) and got 2.6540 and 2.6462. I did this several times and came up with very similar numbers each time so I'm either consistently wrong or I'm in the ballpark. The crank measures #1 2.4491, #2 2.4493, #3 2.4493, #4 2.4492, #5 2.4492.
What can/should I do about this? I' don't want to go to a machine shop until I'm a little more knowledgeable.
What can/should I do about this? I' don't want to go to a machine shop until I'm a little more knowledgeable.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Something is really odd. doing the math, #1 clearance should be .1979, and you plastigage didn't crush? I bet it didn't. Long step from .1 (almost .2) to .002
are you sure the crank is a standard size??
are you sure the crank is a standard size??
Thread Starter
Intermediate
Joined: Oct 2006
Posts: 34
Likes: 1
From: Leesburg, Florida
Thanks for the replies.
The caps are oriented correctly. Now, I can only say these are the caps that came with the block when I bought it so I cannot say they are for sure the correct ones.
Tim, I think you missed that the measurement is the bore without bearings.
I did also measure with the bearings but didn't see the point including that measurement in the post since it was obviously too big since the plastigage didn't crush. My point in including the measurements without bearings is that the spec (as far as I know) is 2.6410 with tolerance of +.0006 or -.0004. Mine are considerably larger, as you can see, and also seem to be not round based on the diagonal measurement. I don't know why.
So, assuming my measurements are correct can anyone recommend a course of action.
The caps are oriented correctly. Now, I can only say these are the caps that came with the block when I bought it so I cannot say they are for sure the correct ones.
Tim, I think you missed that the measurement is the bore without bearings.
I did also measure with the bearings but didn't see the point including that measurement in the post since it was obviously too big since the plastigage didn't crush. My point in including the measurements without bearings is that the spec (as far as I know) is 2.6410 with tolerance of +.0006 or -.0004. Mine are considerably larger, as you can see, and also seem to be not round based on the diagonal measurement. I don't know why.
So, assuming my measurements are correct can anyone recommend a course of action.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If you haven't had experience using a dial-bore gauge, you can get numbers that are several thousandths off without knowing it. Proper positioning of the gauge in the bore and properly rocking the gauge to find the smallest diameter at that measuring point are not simple things to do without training and practice. Get a friend with that experience to repeat your measurements and compare them before you trust your readings.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
OK-- you have the correct measurements- and you're still too big. Either someone has swapped main caps with you or it's a brand new never been worked on block. With the main bearing diameters out of round, I'd suspect it needs to be line bored. they cut a little bit off the main caps and then hone them back to the min/max bore diameter. Offshoot is, you get a block that the main bore is 100% straight. If the crank is as straight as the block, life will be good.
I think a 400 SBC used a bigger bearing bore too- maybe someone bored this block for a 400 crank? I don't know what the numbers are-- just thinking out loud..
Last edited by TimAT; Apr 10, 2008 at 05:25 PM.
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