NAA connecting rod bearings.

[Dave Wave]

The saga of the cracked piston continues....

I pulled out the offending piston. It was not cracked all the way through. Hard to tell if it is a crack or a defect from when the piston was forged.

I took a look at the connecting rod bearings and they show wear. It has worn through to the copper in a couple spots.

Does the copper showing mean the bearing is at the end of its life or is it possible it will run for a long time that way (there was no knock and good oil pressure prior to the tear down)?

I meaured the crankpin at 2.278 . The spec on the pin is 2.2978-2.2986.

Does this mean I want .02 undersized bearings (2.2978-2.278 = .0198)?

Do the bearing really just slip into the connecting rod on a slot? No tools?


Thanks for the help,

-Dave

 

[souNdguy]

it's down this far.. and shows visible wear metals.. i hafta think I would replace them before I buttoned back up, unless i was plum broke, or had hay in the field and saw dark clouds... or was selling and didn't feel like the exercise was needed AND informed the buyer of their condition so HE could get the exercise.. ...

soundguy

 

[Ultradog MN]

After all the brewhaha the other day it looks like you want to at least replace your bearings after all.
Looks from your numbers the crank was ground 20 thou. undersize at some point but is still within spec for .020 under bearings. I assume you are taking two measurements on each journal 90° apart? Are you reading in tenths - ie .0001?
What does the end gap on the rings read? Did you measure them with a feeler gage. And if you have the ability to be measuring your crank, might you also be able to measure up your pistons and bores? Bores are likely out of round a few thou but give us some idea of sizes so we here can get a better "feel" for the rest of the engine?
You'd better drop your main caps one at a time too and have a looksee there. If you see scoring, wear lines or copper on your main bearings it's time to pull the engine.
I'm still hoping for the best for you on this matter but now I'm thinking you might be in this for a dollar and not for a dime.
But I don't hear the fat lady singing yet either.
Post back with some more numbers/info and we'll take it from there.

 

[Dave Wave]

Well the 2.278 is about as accurate as I can get it (several angles w/ a good caliper). My mic is too small to fit the crank.

Standby for the ring gap.

So my math is good for the rod bearings (.02 Undersize?)

-Dave

 

[TheOldHokie]

(quoted from post at 19:50:08 09/13/11)
Well the 2.278 is about as accurate as I can get it (several angles w/ a good caliper). My mic is too small to fit the crank.

Standby for the ring gap.

So my math is good for the rod bearings (.02 Undersize?)

-Dave

We had the caliper vs. mic debate a few weeks ago - see the archives. Factory crank shaft journal diameter on an NAA is 2.4974-2.4966. Crankpin diameter is 2.2978-2.2988. Forget the end gap - measure the piston wall clearance with a feeler gauge - that is the more important clearance. If that is OK then you can evaluate the rings.

TOH

 

[Dave Wave]

Ohh now you tell me :lol:

I just came back up from the barn.

The end gap is running about .02 It is the same with new and old rings.

I wonder if this thing had new rings in the recent past (it really did run well).

This was measured about 1/2 and inch down the cylinder (well as far as the second ring on the piston would let me push it in.


OBTW-Does my thinking look correct for sizing of the connecting rod bearings?


Thanks again for all your help.

-Dave

 

[Dave Wave]

The cylinder bores, as measured from the top, with calipers, are running about 3.445.

This might/might not be an accurate number as this is the area where the ridge reamer went to work.

These are .030 oversize pistons btw.

If I do not get called out to work, I'll see if I can come up with that piston gap.

-Dave

 

[Ultradog MN]

[i:654c4848f0]"Forget the end gap - measure the piston wall clearance with a feeler gauge - that is the more important clearance"[/i:654c4848f0]

You are correct - the piston to bore is the more important clearance. I was geting a little ahead of things - thinking he may still be able to reuse his old rings - thus the end gap.

 

[Dave Wave]

A quick question about checking the MAIN bearings:

To check them, just unbolt the cap one at a time? Use platiguage to check the clearance.

If I replace them, how do I get the old shell out from the block?

Just thinking ahead...

-Dave

 

[TheOldHokie]

(quoted from post at 11:04:32 09/14/11)
A quick question about checking the MAIN bearings:

To check them, just unbolt the cap one at a time? Use platiguage to check the clearance.

If I replace them, how do I get the old shell out from the block?

Just thinking ahead...

-Dave

Yes plastigage. But if more than one of the rod bearings were worn down to the brass/copper backing I suspect you will find that with the mains as well. If so no need for plastigage. The top side is simply rolled out around the crank and the new shell rolled in. The shells are keyed to the web in the block so the have to come out and go in on the the side with the tab.

TOH

 

[Dave Wave]

Well finally some good news...

I pulled the rest of the pistons out.

It turns out #4 was the only one with real issues.

#4 had the cracked piston. It has the worst wear in the bearing, and has some interference damage where it was hitting the block.

#1-3 all look pretty good on the lower end. No scoring, one has enough wear to show a little copper. Much better overall than #4.

The crankpins measure out at #1-3 all at 2.280 #4 is at 2.278.

I tried to drop the cap on the center main but it would not budge. I will have to resort to a breaker bar to loosen it. I am not sure that is worth the risk of breaking a bolt, as the other bearings looked pretty good.

So the question is: what would cause the rod to hit the block...worn connecting rod bearing? Some of the damage looked old, some looked new. I am thinking about just dressing down the high spot on the rod (from the damage) and letting it it go at that.

Here are some pictures:


Thanks for the help,


-Dave

 

[Anonymous-0]

It seems strange that the rod bearings are worn at the bolted interface of the cap to the rod. I would have expected to see wear 90 degrees from there or at the bottom of the cap and the top side of the rod.

 

[Dave Wave]

Well I did not like my decision not to check the mains.

I put my breaker bar on it....and it came off as pretty as you please. The center main looks good and finally more good news..checked out at .002.

I'll try to get the rear main checked tonight.

-Dave

 

[TheOldHokie]

(quoted from post at 16:27:48 09/14/11)
Well finally some good news...

I pulled the rest of the pistons out.

It turns out #4 was the only one with real issues.

#4 had the cracked piston. It has the worst wear in the bearing, and has some interference damage where it was hitting the block.

#1-3 all look pretty good on the lower end. No scoring, one has enough wear to show a little copper. Much better overall than #4.

The crankpins measure out at #1-3 all at 2.280 #4 is at 2.278.

I tried to drop the cap on the center main but it would not budge. I will have to resort to a breaker bar to loosen it. I am not sure that is worth the risk of breaking a bolt, as the other bearings looked pretty good.

So the question is: what would cause the rod to hit the block...worn connecting rod bearing? Some of the damage looked old, some looked new. I am thinking about just dressing down the high spot on the rod (from the damage) and letting it it go at that.

Here are some pictures:


Thanks for the help,


-Dave

Two out of four rod bearings worn through to the backing isn't encouraging. Something was going on and it wasn't just on one cylinder. How many pistons had hit the head??? And is that closeup picture the #4 piston?

TOH

 

[Dave Wave]

(quoted from post at 16:45:25 09/14/11)

(quoted from post at 16:27:48 09/14/11)
Well finally some good news...

I pulled the rest of the pistons out.

It turns out #4 was the only one with real issues.

#4 had the cracked piston. It has the worst wear in the bearing, and has some interference damage where it was hitting the block.

#1-3 all look pretty good on the lower end. No scoring, one has enough wear to show a little copper. Much better overall than #4.

The crankpins measure out at #1-3 all at 2.280 #4 is at 2.278.

I tried to drop the cap on the center main but it would not budge. I will have to resort to a breaker bar to loosen it. I am not sure that is worth the risk of breaking a bolt, as the other bearings looked pretty good.

So the question is: what would cause the rod to hit the block...worn connecting rod bearing? Some of the damage looked old, some looked new. I am thinking about just dressing down the high spot on the rod (from the damage) and letting it it go at that.

Here are some pictures:


Thanks for the help,


-Dave

Two out of four rod bearings worn through to the backing isn't encouraging. Something was going on and it wasn't just on one cylinder. How many pistons had hit the head??? And is that closeup picture the #4 piston?

TOH

No pistons hit the head. The connecting rod was just kissing the block/sleeve area (see picture...the shiny spot). It left a wear mark on the connecting rod (see the picture of the connecting rod) The mark is on the top part of the connecting rod bearing portion.

The close up picture of the piston IS the #4. The crack (if it is one) does not seem to penetrate very far...not visible from below and not on the side (see picture). For grins I might dye check it to see if really is a crack.

So now what, assuming the rear bearing checks out (mid one was fine)?

Thanks,

-Dave

 

[teddy52food]

At one time that rod bearing went bad & the rod was loose enough to hit the block and the piston hit the head. The crank was ground to .020 to clear it up. The rod wasn't resized so it shows the wear opposite of where it should. If that rod was hitting the block now, it would be knocking. Was it?

 

[Dave Wave]

No, no knock. I was thinking it might have been old damage.

Thanks,

-Dave