8ba rod bearing problem
- Thread starter 3739ford
- Start date
cadillac512
Well-known member
The problem may be the rods rather than the crank. Make sure the caps are on the matching rods and that they are on the correct way 'round. Then install one at a time to see which is the problem. Finally, you may have to remove the rods and measure them for out-of-round and size. Also the crank throw can be measured in-place with a 2-3" micrometer to make sure the size is correct. Bearing thickness can be measured with a dial caliper or ball-end mic to make sure you don't have an incorrectly marked/packaged bearing.
Terry
3739ford
Senior Member
Caps were installed on correct rods. I went ahead and pulled all the caps off and picture 2 and 3 the bearing has a spot that hangs your fingernail. These were new bearings and wiped clean and moly lubed before installing. The motor has only made two complete revolutions with pry bar on the fly wheel.
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3739ford
Senior Member
It is a Mercury crank with 4" stroke that came with the 1CM block. The machine shop just polished it as it was in good condition. Had to bore the engine 3 5/16 to get a good clean bore. Reusing the original rods had the machine shop check for being round all were good. I had the machine shop install the new Egge pistons to the rods. When I picked the block and crank up I had the machine shop recheck the size bearings I needed. According to him I needed .20 under for the rods which I ordered. I cannot remember now where I ordered the rod bearings from.
Kohnke Rebabbitting
Well-known member
Kohnke Rebabbitting
Well-known member
Using a so called good enough crank, could mean it can very in a few tenths in size. Mic closely the pins that turn good, compared to the one that doesn't. As previously stated, check Radius, a .001 to tight is enough to lock a motor.
The black marks in your bearings, are the spots that are touching.
Herm.
3739ford
Senior Member
Well, sometimes my memory is not what it use to be. I went back to the barn and the old bearings were .20 under. I found the paper the machine shop guy wrote the bearing sizes down on I needed and it was .30 under. Here are a couple pictures of measurements from my cheap HF tool. The current bearings are .30 under.
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3739ford
Senior Member
He whipped out his little measuring tool and dialed it up on the journal and without hesitation said .30 under.
I found this site interesting: http://pbwdist.com/catalogs/Mahle_Clevite_Bearing.pdf
I found this site interesting: http://pbwdist.com/catalogs/Mahle_Clevite_Bearing.pdf
cadillac512
Well-known member
Those two measurements are of two different crank journals? The 2.106" is .032" undersize and the 2.115" is .023" undersize! It may be that you have a crank that's been previously ground to different sizes on different journals...was done sometimes back when. Obviously a dial caliper isn't accurate as a mic would be but close enough for what you're diagnosing. You may need to measure with a mic, and maybe either get different bearings for different journals (not the best but will work) or have the crank ground to .030" on all the journals (if the existing ones check out.)
Terry
cadillac512
Well-known member
Yes, stock rod journals are 2.139". My Clevite bearing book shows 2.1380"-2.1390" as limits for stock.
Terry
Terry
Lawrie
Well-known member
Dont use a vernier caliper ,USE a micrometer,
Measure the crank pin diameter,(using a Mic)
Measure the bore of the rod,
Then measure the thickness of the bearing shell, using a 1 inch mic and a 5/16 dowel ,then subtract the dowel diameter from the reading and that will give you the bearing thickness.
Start with the rod diameter subtract twice the bearing thickness and you should have the crankpin size plus the clearance.
Lawrie
Measure the crank pin diameter,(using a Mic)
Measure the bore of the rod,
Then measure the thickness of the bearing shell, using a 1 inch mic and a 5/16 dowel ,then subtract the dowel diameter from the reading and that will give you the bearing thickness.
Start with the rod diameter subtract twice the bearing thickness and you should have the crankpin size plus the clearance.
Lawrie
GOSFAST
Well-known member
Very accurate statement here, not only on an individual rod journal but sometimes an occasional cylinder bore also! (See photo below, it's just one brand of a "portable" boring bar used at the time for very this purpose)
Many of these type repairs were done with the engine still in the ride, it's how it was done back then!
Thanks, Gary in N.Y.
P.S. At one time I had the "portable" crank grinder along with the boring bar pictured here, I still own this bar, the crank grinder went a long time ago. In retrospect it wouldn't have hurt to hang on to both?? Nice conversation pieces today anyway! I have customers today have absolutely no idea what the bar was used for when I show it to them?
