Well it finally happened. Last week the head of one of the screws on the OEM TB shaft broke off and had an adventure in the cylinder #4 combustion chamber.

I was driving near my house and messing with AE adjustments and blipped the throttle like 5 times in a row. All the sudden a loud buzzing noise happened in the engine bay which after ~5 seconds turned into a nasty engine knocking sound. I was close to home and the car didn't run for more than a couple minutes like this. Up until 6 months ago I was even using the Skunk2 TB to avoid this very thing happening, but went back to OEM after lowering my redline and to try and fix throttle sticking and idle issues. Kicking myself for that decision now.

So to start the diagnosis, I used an endoscope to look down the spark plug holes and found this:



So then I pulled the head and found this:




I removed the screw and sanded down the rough spots:




I am planning on sending it unless any of you can tell me why that's not a good idea or has sage advice like filling in the spot on the head with JB Weld.

 

I have been told that the piston banging the screw into the head has the potential to damage the bearing on that rod, raising the spectre of spinning that rod bearing and trashing the rod and crank. If it's a stock bottom end that's probably not a big deal, if it's a built motor then that calculation might change.

--Ian

 

Rotten luck! I lost probably three screws that way, only had a single indent on the flat face between piston and head. I heard nothing, and just replaced the TB (x3), never found that mark till the head came off a year or three later.

Point being that my bearings and crank gave no sign of noticing the events. Given your noises and multiple impact sites (it looks like, and apparently sounded like), your engine has suffered more than mine, and you may not escape so lightly, but I'll let you make the call on what if anything to do in response..

After the third shaft failure, a S2TB went on, and every BP since has had the TB replaced with that POS - but the POS issues are fixable, and preferable to ingesting a screw and the possible consequences of that - especially on a turbo.

 

I say send it

 

+1 send it

 

That first photo of the combustion chamber made me think for a moment that the whole screw was embedded into the casting. This amused me.

If you wanna be paranoid, grab a sample of the oil and keep it in a jar. Then change the oil (same brand & weight), and pull another sample after however many miles are on the oil that's in the engine now. Have Blackstone compare the two, and inform them that you're specifically interested in any signs of increased rod bearing wear.

It's a small risk of a big problem, but I'm with the rest of the folks here. If there's nothing expensive or exotic attached to the crank, I'd probably just run with it.

 

Nice.
​​​put a stamp on that and send it

 

Appreciate all the replies guys.

The motor is mostly built (basically just no forged pistons), so it would definitely hurt to have a rod grenade itself and trash the engine. But the very fact that it has forged rods and fancy rod bearings (King XP) makes me want to believe that the tiny screw wouldn't have damaged them. I'd imagine the cast aluminum the head is made out of is softer than the rod bearings and therefore would have incurred the damage--just like I found and is shown in the pics.

The screw was actually embedded into the head when I took it out. But it didn't take much of anything to remove it, so not "truly" embedded. Good idea on the Blackstone analysis. I've never actually had any of my oil sampled before, so I suppose this is a good opportunity for it.

I will post an update once I get the car running again.

 

I'm a software guy, not a mechanical engineer, but my understanding is that the alloys generally used for bearings are softer than aluminum.

If it were my engine, at a minimum I would pull it, take the oil pan off, and replace the rod bearings. That's mostly labor with what, about $50 in parts?

--Ian

 

I'm a software guy too so would appreciate an engineer's perspective. This is what I found...

According to King, their XP bearings have "the highest hardness level in the industry" with the top layer rated at 18.1 HV (this appears to be softer than aluminum) and the intermediate layer at 115 HV (this appears to be harder than aluminum). So if any damage to the bearing was done, I'd imagine that top layer took it all. Which would probably mean reduced lifespan and durability...but shouldn't be in danger of imminent failure? It's anyone's guess until/unless I actually pull the motor and inspect them. You do make a very good point about it simply being a pain in the ***, but not expensive to do. I will have to ruminate on that some.

 

As half an engineer (I have an assosiates) it is going to depend entierly on the forces applie over what area. The bearing to crank interface is a large surface, a screw head is a very small surface. Think the difference between the point and head of a thumb tack. you applie a force with your finger and can push the tack into something far harder than your finger because of the difference in surface area.

I would send samples to black stone to check bearing wear.

 

Engine is back together and running well. Only took it for a 15 minute drive so far but it feels fine. I did order some sample kits from Blackstone so I'm going to get an analysis done to make sure the bearings are still in good shape.

 

Thank you for not saying "just send it".

 

So I sent it. Drove ~6 hours to SLC (with my trunk open to hold my 245 slicks) at 4-4.5k rpm the entire way. Then raced all day Saturday and Sunday, and drove the 6 hours back home Sunday night. Zero issues.

I'm still going to get an oil analysis done out of curiosity, but she good.

 

How did the cylnder walls look? It looks like pretty mild damage compared to what I imagined.

I didn't know your were suppose to use countersunk screws in the pistons. LOL