History; I bought a stock short block because I'm getting my overbored forged block refreshed at the machine shop. No big deal, huh? Just a stock 1.8 block, crank, rods and pistons - swap over the head, button it up and stay on the road while the "real" engine gets a once-over.

Well the swap-over went well, got the engine buttoned up and broken-in and decided to send it just for old times sake. Nothing crazy, after all it's a relatively new engine; 55% throttle, 170kPa,, 5700RPM - nothing dramatic (and it felt good). But then some expensive noises started and I thought, "Dammit, did I spin a bearing?" But the oil pressure was good and the motor still felt strong, just a clattering sound that followed the RPMs.

So I limped back to the garage and turned over the engine by hand and it spun freely until cyl's 2&3 were nearing TDC, then the crank seized up. It wasn't locked up solid, but there was significant resistance. I got a borescope and looked in all the cylinders, but nothing was contacting and the valvetrain was operating smoothly. Today I pulled the engine and found out what the problem is...the crank throws on cyl 4 are contacting the bottom of the piston! Not by much, but enough to cause the engine to seize up slightly. I have NEVER experienced this EVER before! I put some marker on the crank throw and rotated it through the contact (which you can see in the crappy cellphone picture below).


During the build, the engine turned freely and all of the bearings looked pristine after the teardown. All of the rod bolts were torqued to stock Miata spec. The only thing I can think is that this rod "stretched" (which to me seems weird 'cause I wasn't doing anything extreme). But, if the rod did stretch, then wouldn't the piston be FURTHER from the crank throw than before. Could it be that the wrist pin journal in the piston got pulled out of round, putting the bottom of the piston closer to the crank throw?

This seems awful weird. I appreciate any theories.

I don't (think I) have the necessary tools to check the rod lengths; just a HF digital caliper.

 

If the rods are too long they won't stay that way for long.

Sounds like exactly what happened to my car. I kept driving home and it eventually stopped "knocking" so I guess I auto-clearenced my skirts.

 

I'm tempted to do just that...let the piston/throw "auto-clearance" one another. What I'm concerned about is what caused this to happen in the first place. Am I facing imminent destruction of the piston? Will this happen again? Am I doomed to sub 5K RPM running on this engine?

Since if have the engine out of the car, I'm going to take a Dremel to the piston (it's not the skirt that's contacting, it's the lands for the oil feed to the wrist pin) and make some clearance. Another alternative is to swap in a stock piston that I have laying around and see what happens.

 

I don't see how it could be anything but a bent rod, I did a very rough check with a stick and am pretty sure 1/3 are bent. If I cared enough I'd do a compression test. Either way the noises I was hearing were bad and it needs to come apart anyway, I don't want to run it any more. If I had the space to tear it apart I would, should be easy to check once they are out. Hopefully next week.

I wouldn't start grinding on stuff, that's for sure.

 

A stretched () rod would surely move the piston AWAY from the crank? And into, or closer to, the squish area in the head where contact would be A Bad Thing, and probably Very Noticeable.

OTOH, a bent rod shortens the distance between wrist pin and big end journal, making a piston strike more likely.

 

Seems like a bent rod. What rods are in there?

 

Bent rods is my guess too.

 

Consensus is a bent rod. These were stock rods and frankly I'm a little impressed that my eBay churbo could produce enough "grunt" at lowish RPM's to bend a stock rod!

I'll finish the teardown tonight/tomorrow and verify the bent rod. In the meantime, I'll be extracting the Eagle rods from my "real" engine and using them. I was hoping to have the whole rotating assembly in my "real" engine balanced at the machine shop, but...

Next, let's see if I can destroy a stock piston in my temporary engine!!

 

"Well, there's your problem."

 

Lol, knew right away what your rods would look like.

Did the same thing myself, only much worse on 1&4.


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​​​​​Still running the same engine with rods only bottom and bought used pistons. GTX2867 gen 2 making 14psi.

If you rebuild! Check the oil squirters if you use them, mine got bent and cracked halfway, had to solder them.
Also, don't try to correct the counterweight on crank, the piston is the piece who took the real damage.
My rods got bent enough to touch the lower side cylinder bore, made a horrible ticking sound.

 

I read somewhere that rods get bent because of high(-ish) pressures at low(-ish) RPMs - as opposed to big PSI numbers at high RPMs. Like I said earlier, I was only 170kPa at 5700RPM. I guess that the stock rods are "delicate" than I thought.

Anyway, I'll take the forged rods out of engine #1 and put them into engine #2 - and I'll swap the squirters as well (no sense in taking chances there too) and see what happens. If nothing else, I'll get my engine pull/install skills down pat.

PS - you might ask "Why aren't you swapping in the forged pistons also?" That's because engine #1 - with the all-forged bottom end - has a 0.5mm overbore and the engine #2 is stock, otherwise I would.

 

Det will bend rods at lower boost. Look for signs on the pistons.

Otherwise, could just be making a lot of torque even at low boost if you are running a big turbo. If so, the good rods should fix it.

Note: above is not based upon direct experience.

 

Piston crowns and edges are clean, pretty and crater-free. And I purposely turned down the spark from my previous knife-edge settings.

My eBay 2871 clone has pretty lousy spool, so I'm skeptical about low-end torque - or maybe, I'm under-rating what I'm laying down. One of the mods I did while I was rebuilding after the spun bearing was to put in a 3-inch downpipe which is currently mating to the old 2.5-inch exhaust starting at the cat and out to the tailpipe. I wouldn't think that would add that much "grunt" - not enough to be bending rods.

 

wait what



red line was 11(?)psi wastegate pressure, blue line was 13psi on ebc on my old 2871 begi system w/ external wastegate, but this was my forged engine. Pretty sure you're definitely in rod goes noodle territory at 10psi.

 

I tried to get a Virtual Dyno plot of "the incident" for comparison, but it's not cooperating.

Anyway, the Eagle rods are on the stock pistons now, They'll be back in the block tomorrow, and the engin back n the car on Sunday.

 

Sweet, let us know how it turns out. You ever use an EGT for tuning? If you retard timing to get away from knock, depending on how "conservative" you go it is entirely possible to send EGTs through the roof. How much did you back off the spark in boost?

 

I've never used EGT for tuning. Once, a while ago when I first had the FM turbo kit installed, I tapped the manifold for a single EGT probe, but I never found an inexpensive thermocouple signal conditioner so that's as close as I've gotten to reading EGT.

I backed off the in-boost timing values approx 10 degrees, gradually from 100kPa on up.

Engine is back in the car, and had the first start yesterday. I didn't run it for long because there are still some bits-and-pieces that still need to be attached (PPF, driveshaft, exhaust, assorted dangling wires). I just want to get some oil circulating and make sure I had pressure. I'll do the "finishing touches" this evening and do a quick street run just to make sure nothing falls off.