Hi, guys. I could use a little help determining what the trouble is with my engine. (Please pardon the length of the post).

I had a rebuilt engine put into my 73 about 7 years ago (basic 350, nothing wild). In that time I have put a whooping 15000 miles on that engine. Recently it developed a knock. I yanked the engine and discovered that the rod bearing on #6 piston is fubar (crank journal at #6 is toasted as well). Two other rod bearings are showing some copper color. It is smooth but it is not even (only on one edge of the bearing and not even all the way across).

The engine has a high volume oil pump (M55HV) and from day one it has given me about 60+ psi at idle when cold and about 30 psi at idle when warmed up. At cruise it puts out about 60 psi when warmed up. It was giving these same numbers even when the knock developed and right up until I pulled the engine and started disassembling it.

My thought is that it was either a poor assembly or possibly that the rods are too much out of round (I have not measured them yet).

I could not find the spec in the back of the FSM for that end of the rod. If somebody knows the measurement and the out of round tolerance for the crank shaft end of the rod, I would appreciate that information.

Is there anything else I should check to determine why it is that my engine ate that rod bearing?

Should I be concerned about the other two rod bearings that show smooth but uneven wear? (Main bearings are smooth and even. No copper showing).

Any help or suggestions will be appreciated.

Sincerely,

Mike.

 

Possibly crank not turned properly, leaving too large of a fillet at the edge of the bearing surface.

 

Pictures ?

 

and also have the crank checked for cracks.

 

What do the mains look like? You're laboring under an oil starvation scenario. If the mains look good, then it's not likely related to oiling unless you popped a gallery cap.

If the mains are gonners too, then you have a number of possibilities such as; grit or other debris in the oil (camshaft failures will do this), oil starvation, assembly error.

If the mains look good, but all the rods are trashed then it could be: too tight on the clearance, out of round big ends, twisted rods, detonation, machining errors, assembly errors.

Just something to ponder.

 

the crank was toasted where the #6 rod bearing was riding. i already ditched it for a replacement.

what about the rods being out of round? (or is that not very likely?)

i'm at work. i'll try to get some pics later.....

 

The mains are fine. Most of the rod bearings look fine. It’s just #6 is trashed and two others are unevenly worn.

I wasn’t really thinking that it was an oil starvation problem since it has plenty of pressure. I’m just surprised that the oil pressure did not drop off as #6 bearing got worse and worse and finally started making noise.

I haven’t pulled the cam shaft yet, but from what I can see of it from below, I think its fine.

If I popped a gallery cap, wouldn’t my pressure drop off?

 

With good mains, it sounds like you have a machining or assembly problem. Anything related to the oil (grit or pressure) would have taken out the mains as well as the rest of the rod bearings.

 

What about the large end of the rods?

Any body know what those should measure and what is permissible for out of roundness?

I could find every other speck in the back of the FSM for parts in that area except for the end of the rod that attaches to the crank…..

 

My opinion - You will need to turn the crank, resize the big ends of any of the rods showing bad journals and re-assemble with a basic overhaul kit.

I had to do the same thing on mine a few years ago. I had one rod journal that was making a slight noise. The tear down revealed a very unusual set of main bearings - .009" undersize AND one egg shaped rod journal like yours. The story was this was a rare factory reworked crankshaft. I believe it based on other things I have seen. Paint marks on the main cap area were also visible that looked to be factory rework.

I had the crank turned, some of the rods resized that were questiionable and a basic overhaul kit. Not much money and she runs better than new now.

Why ask why?

-Mark.

 

Here are some pictures.

I could have sworn that the wear was only on one edge, but looking at them again it would appear that it is on both edges and more than just two.

Oh well, #6 is clearly fubar and the mains look fine. At least that much I remembered correctly.

 

Your going to have to have the whole assembly done, everything. If you have one cap backwards or on another rod it will seize the shaft. New shaft, rods re-sized, and if you mess up a main cap position, a line bore too. Always keep the essential parts together, saves time and grief ...

 

I noticed that your rods have been balanced as I can tell they did some serious grinding on the rod caps, Probably after the rods were resized and from what I have seen at our shop is when that much grinding is done on the rods they will distort. Look at number 8 rod the bearing cap looked like they did a lot of grinding on it and that bearing looks like its ready to fail.

It does look like the clearances were a little tight.

The rods after grinding on them the nuts should have been loosened and retorqued and sized from there.

We useally cut the caps snug up the nuts, balance assembley, retorque the rods and size from there.

We use a belt sander to remove material as it looks much nicer the using a grinder!!!!!!!!!

 

I was going to say those caps looks like they have been hammered on. The caps should look nice and smooth after balancing not looking like someone took a angle grinder too them. Look at the other side of the bearing shells see if they are undersized. Make sure the uppers weren't mixed up with the lower halfs.
Either way its back to the machine shop

 

Exactly. That is why everything in that tray is positioned exactly as it came off of the engine, right down to which nut came off of which stud. I don't want to make that mistake.....

I guess it's pretty obvious that my rods are screwed up, but the mains seem fine so I think I’ll just keep the right caps in the right place and skip the align bore.

I already picked up a new crank kit, came with matching bearings so I guess that end is taken care of. Now I just have to get my rods fixed up.

Anybody have a ball park figure for having rods resized? Just wondering about how much I should expect to pay for that service.

Thanks again for the help and opinions.

Mike.

 

Having the big end resized is an inexpensive operation. I forget what I paid last time but it was so small that I never bothered to concern myself with it. Maybe $15 each?

-Mark.

 

We clean and resize the big end, dust the sides if the big ends, check for bend and twist and we charge 120.00 for that service.

Most reground cranks are on the extreme high side (less time to machine) and the rods may have to the high side and the same with the main line as that should be at least checked for sizing.

Rebalance the new crank is in order.

It might be worth your while to get some NEW plastigauge and check you clearances when assembling the lower end to make sure you have around .002 clearance every where.

Good luck with your build.

 

Thanks for the help, guys. I think i'm heading in the right direction now. Going to use a little PTO tomorrow and take my rods down to be checked out and fixed as required.

Appreciate the help,

MIke.

 

Why screw around with those rods? The caps look really hacked up to the point that you don't know much more than they probably caused you to wipe out the bearings. New stock-type rods aren't that expensive and will assure you you're getting quality pieces. Then have the whole rotating/reciprocating assembly balanced and your good to go.

 

The best price I could find for new rods was $200 and they came sized and balanced. I’ll see what the guy at the machine shop thinks about those rods and if he can do better than $200 for new rods, sized and balanced.

Thanks,

Mike.