Jump to content

main bearing clearance


Recommended Posts

  • Replies 21
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

So the clearance is about .004 small I need to be between .002 which mm undersized do I need? Just to recap I need undersized bearings because the crank is too tight correct? Bcuz normally their would be a plastigage strip this fat (     ), instead what I have is more like (  ). My engine spins perfect until the main bearings are torque so I'm just gettingan idea. Btw this is my first complete build so pardon my lack of intellect, and thanks frickin' ratsun people are awesome!!

Link to comment

So I 're measure, and this is what I found. The #1 smaller than .003, #2, #3, and #4 same under .003, and then the rear bearing is within spec of.002 . What does this mean? Can I mix an match bearings or am I going to need the journals foreground? Thanks again

Link to comment

If the crank is worn smaller than stock, then over size bearings are needed to make up the difference. Often the crank needs to be ground down to a known amount like 0.2mm or about 0.008" and then bearing shells of a known oversize can be fitted.


Measurements in mm and inches is confusing so stick to one.


I assume that your readings are by plasti-gauge? Try mixing the other bearing shells around for a better fit..

Link to comment

The crank journals are machined smaller. Then undersized bearings are fitted. Undersized bearings are thicker.


You don't need new bearings. 3 thousands clearance is good, the service limit is 6.2.


With A12/A14/A15 factory bearings being tri-layer they do not often wear out. Unless engine ran out of oil or was contaminated, the bearings usually do not need to be replaced. Rings last about 200000 km, bearings can last 800000 km.

Link to comment

But when I torque the bearings the crank locks up and will not turn?  and the clearances are smaller than.003. I don't believe the crank should be that tight shouldn't you be able to spin it with socket and ratchet?.

 I replaced cam, rod, main and honed the cylinder bore too

Link to comment

So I feel stupid but after awhile of torque and plastigage and checking cranks I mixed up the #3 bearing, so I believe I have everything in the correct order now, so but once again at full torque it won't spin. So I'm going to clean the bearings and try my other crank to see if it clears, if not I think my cranks going to the machine shop.. 

Link to comment

The crank was in spec when engine assembled. It sure didn't get larger, and if anything may be worn smaller, maybe.



Were the caps kept in order???? Instal all the top shells and torque the front and rear caps and see if the crank will turn. If it does, instal the others one at a time to find the tight one. Try swapping the tight cap with another.

  • Like 1
Link to comment

I tried torquing the mains, the rear will spin, I put an old bearing on the front, torque it got it to spin. Too tight in #2,3,4 still no go just lock. Clearances no good. I even tried to separate cranks to see, and I'm getting the same results. I feel like I'm going to need mismatched bearings or a align Bore. The caps are numbered 1 - 5, so I'm pretty sure I've got that right. Should I get my crank and block checked at the machine shop?? It just a bummer being on a budget and I need this running asap. Oh and are their differences in bearings depending on the year? I m under the impression that 78 to 80 a14s are the same?

Link to comment

There are very few reasons a crank would lock up when assembling an engine.


0) The idiot reason is assembling the crank dry.  Gotta have the bearings lubed- assembly lube, light grease, or hell just engine oil.  I have seen this mistake done.


1) Ive seen bearing shells that came out of the box bent, usually from shipping trauma.  Generally I've seen that, by putting the 2 shells together and noticing one side won't line up.


2) The most common reason, assuming all the right parts, is to have the main caps from the wrong block, in the wrong order, or backwards.  They were line bored at the factory, so if even one is in the wrong place, it can lock up the crank.


3) The next most common issue is putting undersized bearings on a standard crank.  Cranks don't get bigger (unless you weld on them) they get smaller so the clearances get bigger.  So if you put, say, .010 undersize bearings on a crank that hasn't been machined to .010, it'll lock up.  Badly.


4) Then we get to uncommon issues.  Assuming the bearings match the crank and the caps are on right, a bent crank could cause it to bind up.  But how the hell could you bend a crank without throwing it off a skyscraper?  I've seen cranks bent from overheating, but they also had very obvious heat damage and journal damage since the only way to overheat a crank is to have seized a bearing by having it run without oil.


5) And lastly, there's the extremely improbable case of  a warped block.  Once again, only way to do that is to severely overheat it, and in that case the camshaft won't turn either.

  • Like 1
Link to comment

0: bearings are lubed like a slip n slide

1: possibilities on bad bearings

2: okay I do have 2 blocks but I'm pretty sure I kept the originals, the caps go 1 to 5 starting from the front of engine. Thrust bearing in center.

3: I bought standard bearings

4: the bent issue I just speculated bcuz of the weird readings I was getting on the plastigage. And I have 2 different cranks and they can't both be bent

5: yeah, I doubt it's warped because the I install new cam and bearings. Everything spins until torque. So I'd, back to trial n error I guess

Link to comment

Okay thank you to everybody for your input and help I appreciate it like None other! :D update: tore it all the way down made sure everything was were it was supposed to be. Took out the rods and piston and torque the mains to see and what do you know spins like butter, so cranks good. Recheck ring gap and pulled the rods to find mix match cap on my rods. Lined em' up torque and and now we are all good! Lesson learned, definitely need to pay more attention to detail. Now time to finish assembly, timing and back to driving

  • Like 1
Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.