Fresh rebuild bearing failure
 

Hi everyone,

I just finished a complete rebuild the other week. The engine ran great, but after 40 miles I had a light rod knock and metal in the oil. Would like some more opinions on root cause of the failure.

Motor is an NB2 with new cast pistons, eBay rods, ACL race bearings, Boundary pump. Ported the head myself then had a shop do the usual hone, deck, polish crank, and clean everything. Intended for N/A, not boost.

Assembly - measured main journals at small end of spec (1.9661-1.9663, spec 1.9661-1.9667), same for rod journals (1.7693-1.7695, spec 1.7393-1.7699), plastigauged mains at slightly loose, but well below service limit (.0020, spec ~.0008-.0014, limit .0039), rods at tight end of spec (.0010-.0015), spec .0012-?). Lucas assembly lube, Walmart 10W-30 conventional and primed the oil pump before starting.

Started right up, ran great. Did 40 miles for the FM breakin procedure, noticed a ticking, parked it. Changed oil, found larger metal bits in the filter than ideal. Pulled engine, rod bearings dead: https://flic.kr/s/aHsky3rfRs Also found some wear in cam journals and bores from debris: https://www.miataturbo.net/build-thr...2/#post1472397

Clearly the scratches to the shiny layer are from debris. The large gray areas seem too uniform to be debris wear. Bearings too tight? I plastigauged them again as-is and got .0010, .0008-.0010, .0017, .0018 front to back. Three rod journals have slight scratches in them now, worst in #1.

The mains mostly look ok, getting worse from front to back. Mostly looks like debris scratches. What concerns me is the front edge of 4 and 5, both have a significant but narrow wear band there. What is this?? Main journals are all fine, crank thrust faces are fine, thrust bearings have zero wear, thrust play is where I left it at .0065 (spec .0032-.0111).

So... why?

• leftover debris from machining that wasn't removed
• rod bearings too tight
• poor lubrication, not enough assembly lube
• ???

This was my first time building an engine, what might have gone wrong?

Right now I'm planning on getting the crank and cams repolished, taking the rods to work to measure new bearings with an actual bore mic, and I suppose at least separating the head and block so I can spray out as much of the oil system as possible.

Did you pull the oil gallery plugs from the block and heads when they were hot tanked?

Head, yes, block, no. Is that the two thick brass plugs on the front and back?

Yes on the back of the block. The other end of that hole should be where the oil pump delivers its load on the front of the block. I think of recall there being another one behind the starter somewhere on the side of the block.

Ummmm....
/me hyperventilates

What could possibly go wrong?

So, your machine shop didn't pull that plug out and run a bottle brush or rifle brush through the gallery to get any carbon and dirt and metal out of it before reassembling? Did they give you a discount? I think I would have paid the extra $20.

Check the pressure relief valve on the pump. Buddies fresh build with all clearances checked grenaded and we found the pressure relief valve was stuck open. Boundary claimed it was debris from the failure but the machine shop thought otherwise. Word of caution, disassembling the pump voids your warranty. Fun times.

I have a great story about Boundary pumps.

I had a customer a couple of years ago email me with exactly the same thing. New Boundary pump, new engine, relief valve stuck open. Luckily they caught it early. I referred him to Travis, who took the pump back and inspected, found FOD, and offered to sell the customer a new pump at a substantial discount. A few weeks later, the exact same thing happened. Fresh pump, stuck relief valve. At this point, the customer is pretty unhappy. He no longer trusts Boundary, and his machine shop, which swears up and down that they cleaned the block perfectly, refuses to use another Boundary pump.

Two weeks later, the customer emails me, sheepishly. His machine shop finally admitted to sandblasting the valve cover during the original assembly and not cleaning the sand out of the valve cover's PCV passages. The sand got into the engine and destroyed both pumps.

Engines blow up sometimes. It's happened to me, it's happened to my machine shop. It happens to OEMs. Something wonky happens, usually right on startup, you scuff a bearing, heat builds up, the clearance you think you had closes up in a hurry, and it all goes south. Even on a race engine which I know will be abused, I still warranty it during the initial start-up process. Blaming it on a stuck relief valve is short-sighted at best. Correlation is not causation.

Same...

Thanks guys, and thanks for the story, Savington. I'll poke at the pump and try to flush and inspect it as best I can without voiding the warranty.

It doesn't look like they did. I didn't request it specifically, apparently it's not included in a hot tank and freeze plug replacement. Wish I'd known it's critical.

I'm dropping the cams and crank off tomorrow (at a different shop) to be repolished. It sounds like there's no reason not to strip the block again and have it hot tanked again at this point, is there?

That's a great post, thankyou.

Dann

"The mains mostly look ok, getting worse from front to back. Mostly looks like debris scratches. What concerns me is the front edge of 4 and 5, both have a significant but narrow wear band there. What is this??"

That sounds like you may need your block line honed to straighten it out. And, your crank may have a tiny bend in it. If so, you really need a good shop to grind the crank down to the next size (.010 undersize or more). Then get the correct bearing sizes.

Also - on the cleaning of the crank. I found that if you bend the little plastic tube of a can of brake cleaner at a 45-60deg angle, you can get it to feed into the tunnels that lead towards those little balls. That allows a direct and forceful spray to jet all the gunk out. I went through 2 cans before it stopped coming out with debris (mainly burnt oil). Don't forget to put a little oil back on (and in the passages) so it doesn't start to rust.

My first built engine was also a victim of leftover sand blast media.

Yes, great post mrmonk. I picked up a HF slide hammer yesterday, I'll get to cleaning. So do you guys think the bearing wear in the pictures was caused solely by leftover debris? The large gray areas look more like actual contact to the crank - poor lubrication on first startup? Clearances? And does this level of damage/the clearances I posted seem like it would cause the light rod knock I was hearing? I want to make sure I've found all the issues and their causes so I don't repeat the same thing!

I'll ask this shop to check the runout in the crank. I did have it balanced before, so I would have hoped they'd caught that. I didn't have the line hone checked/done because all the bearings that came out of it had zero abnormal wear, and I put it back together with OEM bolts, not ARP studs.

Alright guys. So I stripped everything down again. Pulled the galley plugs out of the head and block (thanks mrmonk, couldn't have been easier) and really blasted everything clean. Went through and cleaned everything that touches oil. Didn't pull the plugs out of the crank, but ran a couple cans of brake cleaner through it and got lots of black sludge out of it. Broke down the pistons and cleaned them.

Had the crank and cams repolished. Reassembled everything with a new set of ACL race bearings. Double checked them all with bore gauges and plastigauge and the clearances were about the same as last time.

Now, it still runs absolutely amazingly, but it sounds almost exactly the same. :crx:

Clearly the bearings weren't the source of the noise (although the amount of debris in there warranted the teardown, imo). In fact, ACL's brochure says that it's normal for the bearings to wear through the discolored layers and become shiny. In 40 miles though? Idk.

So what else could it be? It's definitely a bottom end noise... piston slap? I will admit I didn't measure p2w... just asked the shop to check the hone size and then dropped in new OEM pistons.

Here's the noise the first time around:
And the second time:
This time it was audible immediately after first startup once reaching normal idle. I drove it about 50 miles and changed the oil. Fairly glittery still, though I'd attribute it at least partially to the cams wearing their bores back in. No chunks this time. About 70 miles on the second assembly now and the noise hasn't changed - it's quieter than the first time but still there. Kind of a marbley rattle or clatter.

Wtf to do??

Neither video sounds particularly alarming to me, honestly. Terminal bottom end failure is really visceral, yours sounds nothing at all like that. I would pull a filter every 50 miles to check for metal flake, but honestly, if it were mine, I would run it and see what happens.

Looks good from here.

https://cimg5.ibsrv.net/gimg/www.mia...92a641643c.png

Run it a bit and see what happens. You may be pulling it back apart anyway so give it a while first.