8ba Center Main Bearing

W

waynet

Senior Member
Howdy folks,
I am re-assembling our '51 8ba with new replacement main bearings but the center main is soo.... tight that the crank won't turn. The main center bearing cap measures approx .003" shorter in height compared to the front and rear bearing caps. Can I shim the bottom of the center main cap safely?
 
Where are you measuring?
Any external measurements are irrelevant. Fit the bearing caps tight, without the bearings and accurately measure the ID. The vertical measurement is the most important. I had to get mine "align bored" which is complicated because the caps need to have about 0.005" machined off the mating surface.
 
Do you have the old bearing ? If so check its back side and see its size . I would not shim the cap .
 
waynet said:
Howdy folks,
I am re-assembling our '51 8ba with new replacement main bearings but the center main is soo.... tight that the crank won't turn. The main center bearing cap measures approx .003" shorter in height compared to the front and rear bearing caps. Can I shim the bottom of the center main cap safely?
Click to expand...
try another cap, the front and center are interchangable. should be easy to find someone with spares.
 
The caps posision and orientation needs to be the same as it was from the factory . If you have replaced one or more caps then you need to have the block line bored. If either is not done you will have a tight crankshaft at the very least.
 
Unless you have a precision dial-bore gauge, it is very hard to accurately measure the bore size. Do you have the necessary mics and know how to use them? If this is the correct cap for this location (not the front cap) and it is truly that far off, then the block will need to be align bored. If this is needed, make sure the shop knows flatheads and how to do this procedure.

Some machine shops will mill the bottoms of the caps (takes a special jig/setup in a milling machine - with a rotary table or a special fixture in a lathe). This is a pain in the butt - due to the cap indexing method that Ford used. Back in the day, it was a common procedure, and shops were setup to do it. Today, not many are setup to do this or will take the time.

Other shops will machine the whole bottom of the block. Both procedures can work - as long as the shop knows what they are doing and are experienced with flatheads.
 
Note the front and center caps are NOT inter changeable. Measure the bore diameter without the bearing to determine the difference compared to the front and center bores.
Now at least you will have something to work from.
Ronnieroadster
 
Where did that center cap come from, if it's what you took apart it should go back, did you dot them when you took it apart ?
 
A person could compare shell thicknesses to see if there is dimensional differences between the front and center shells. If this block was functioning with the previous shells then it makes a person scratch their head. If the crank has been turned then make sure you know the size compared to the new shell sizes. Most parts are precision made in this respect but mistakes have happened. It depends on the origin of the parts and who did the crank. I would check everything before going further. I still use T gauges and inside micrometers and get good readings after taking several to check myself. A person can usually get an idea if there are inconsistencies and proceed from there.

If there is .003" difference on the cap then I would wonder why it was working before. The condition of the old shell would come into play to see if it had more wear. The ,003" difference is enough to do that. I would wonder if it was overhauled before and what was done then. Some old geezer may have put yellow time saver on the journal and turned it till it fit. If these old engines could only talk.
 
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If it locks up by just torquing it up then it may be too tight for plasti-gauge. They recommend to put a little oil on the piece of plasti-gauge before cap installation and torque up so that it won't mar the surface of the bearing shell. The oil helps it spread out more evenly as pressure is applied.

Another consideration is whether that cap was ever align bored before. Machinists generally cut down all three caps about the same before align boring the journals as a set but there is always a human operating machines and some may not follow any set procedure.
 
I have a book that shows bearing dimensions, includes the thickness of standard size bearings. This size measurement can be measured with a ball micrometer, or a regular mic with a bolt, to accommodate the curve of the inside of the bearing.
With old stock bearings, who would know what confusion may have occurred.
I find that using shim stock or even paper instead of Plastigauge is more accurate.

Karl
 
just a thought. don't know how much faith i'd put into it, but...

i've seen it written that if one gets ahold of a center bearing made for one of the earlier blocks it might be too WIDE to fit properly, and could, i suppose, prevent the crank from turning due to insufficient end play...
 
the front and center main bearings are the same between the 8BA and the 59a type the widths of the thrust faces are different on the rear main

look at the markings on the old bearings to see if anything regarding undersize/ oversized is marked.
if the bearings you have in the center main are switched to the front does the tight bearing move to the front?
 
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