8BA tight main bearing clearance

D

Dirtslinger

Member
Im having some trouble with getting proper bearing clearance. I managed to buy a NOS Mercury 4" crankshaft along with King Standard bearings. Now Im getting a .001-.0015 clearance on the rear main using plastiguage. Using a mic and bore gauge, Im getting a clearance of over .003. The block has not been line bored. Im going to have a friend come help with a second measurement to make sure its not my tools. Looking for some help and ideas to get this thing together.
 
Plastigage does have its place, but a dial bore gauge and micrometer trumps it for accuracy. You can also set up a dial indicator gauge above the rear crank flange then pry upward to see how much the crank moves with rear bearing in and torqued. That will tell the tale.



Terry
 
I did have lube on the top of the journals to hold the plastiguage. The thrust clearance does seem to be on the tighter side also. I originally have purchased a used merc crank that ended up having a very sloppy thrust clearance, the latest crank is Nos.
 
I use brass shim stock and a micrometer to measure clearances.
Start by putting the crank in the block, lube, torque the mains, see if it move freely.
Check end play with feeler gauges.
Pull one cap at a time, and put in shim stock, When tightening be careful to look for when crank is hard to turn. Put in more or less to find true clearance.
Look for .001- .003". I like right in the center, .002"
Can be done with paper.
And- no I don't like Plastigauge.

Karl
 
Karl wolf, I am going to order some of those shim stack. Question is: although the manual states that clearance can be from .001-.003, would anyone run the rear main with that tight of a clearance? The front main is coming in around .0025. The middle is also on the tight side at .0015. Im starting to believe that the block needs to be line bored. Unfortunately I already have the valve train in and lashes set.
 
I always setup my flathead cranks for about .0025 clearance on all three. If anything, I try to have a bit more on the rear main - .00275 - due to the rear of the crank potentially getting hotter (due to the clutch) - and expanding a bit. The challenge is that sometimes the bearing shells are not perfect - I've had to swap sets around in some cases to "tune" the clearance a bit.

New or Regrinds: I purposely have my cranks ground for the clearance I want - and always have my bearings/block in hand and measured before grinding.

Do you have access to good mics and a 3-point dial bore gauge - and somebody who knows how to use them? If not, the shim stock method will work as well - and you don't have to rely on mics that you don't know. ;)

As others noted - I wouldn't trust plastigauge to determine my final bearing clearances. It is fine for a "quick check" (which you've done), but don't rely on it for understanding whether you're a thou or two off.
 
Dirtslinger said:
If I’m .001 too tight on a main, could I use one half of a .002 bearing to get that extra .001? Just a thought.
Click to expand...

Disregard my prior "half-shell" babble . . . I was temporarily insane! LOL

This still stands: First of all, I'd have the align-bore and clearances checked by a competent machine shop (with the right mics) . . . you may be chasing the wrong beast. ;)

Are you sure this block came with the main caps you're using?

D
 
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Dirtslinger said:
If I’m .001 too tight on a main, could I use one half of a .002 bearing to get that extra .001? Just a thought.
Click to expand...

I believe you would be going in the wrong direction from what your trying to do.

The.002 bearing would not be a larger diameter its a tighter diameter used to tighten up the clearance on a slightly worn journal.
Ronnieroadster
 
I’m following this as I have the same situation. I had block and mains checked and honed. Front and center are good and rear measures.001. My machinist said it was within spec. but he likes to see .002 min. He said I could try another rear bearing or “ massage “ the bearing. After reading ole Ron’s post I’m thinking that might be worth trying.
 
Yep - I had my head on backwards on that one! I've half-shelled a .010 crank with a .009 bearing . . . which gives more clearance. Trying to half-shell a STD crank with a .002 bearing gives .002 LESS clearance.

Thanks Ronnie and crew . . . I should have drunk more coffee before I babbled! LOL
 
Haha, yes I was going in the wrong direction. I do have my original bearings and they do look ok, I’ll have to try them out. They’re both standard, I could use one older bearing and the other the new one if that makes for better clearances?
 
I try to use the complet bearing to get the proper clearance. However I see no reason why 1/2 beraing shel wouldn't work. (.001/.0015) is not really OUT OF ROUND, is it????
G
 
Ol' Ron said:
I try to use the complet bearing to get the proper clearance. However I see no reason why 1/2 beraing shel wouldn't work. (.001/.0015) is not really OUT OF ROUND, is it????
G
Click to expand...

Yeah, I was thinking the same thing, but forgot that I've not had any bearings designed for a crank that was .001/.002 OVERSIZE . . . as that is what you'd need. All the .002 bearings we see are for journals that are worn and where a .002 tighter diameter bearing might help. This means the bearing is taking up clearance, not giving some back (which is what he was talking about).

You can purchase racing bearings for some modern engines that are for .009 instead of .010 journal sizes - this is how we 1/2 shell some of our engines (Think BBC cranks).

Anyway, this engine block, mains and crank just need to be accurately checked out - to first of all identify what the problem is. My best guess is that the mains are not right. Could be an alignment problem, could be the caps, could be the bearings themselves, could be the crank . . . or some combination of all these things. :D:D:D
 
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