No, you can't alter the angular position of TDC & BDC by bending the rod -- those are fixed by the throws on the crank. The piston's linear axis of movement is constrained by the cylinder, so a bent rod basically acts like a shorter and fatter rod. The rod can hit the cylinder wall or the piston skirt can hit the crank at the bottom of the stroke.

Here are a few more pics from teardown:

https://www.miataturbo.net/attachmen...ine=1366755561
https://www.miataturbo.net/attachmen...ine=1366755561
https://www.miataturbo.net/attachmen...ine=1366755561

Cylinder 4
https://www.miataturbo.net/attachmen...ine=1366755561

so, being that the bent rod acts like a shorter fatter rod, then when the piston is at its highest position in the cylinder, its still lower than if the rod werent bent, right?

so, would this JUST make a bigger space for combustion to occur, but still combusting when the CRANK is TDC?

could that create any kind of noise? because it doesnt look the the rod was bent radically enough for the piston skirt to knock on the rotating crank.

sorry, im just digging to find stuff out... its not easy to do, not having the engine in front of me, im just really curious where the knock could be coming from, so im running all kinds of possibilities through my head. lol

could the bent rod have torqued the metal in some way down to the base, making a loose fit on the bearing, creating a larger clearance, and therefore a knock against the crank???

You got a pm Kinavo. I want those rods.

Oh hai, any new updates on that lamp?

As far as the rod goes, i'm guesssing it could make that sound since the weight difference to the one side could theoretically slap the cylinder at the wall. I'm guessing it also twisted sideways so it was harder to turn. At the very minimum ensure you get the walls honed and checked for roundness.

Yeah, with a twisted and bent rod, plus missing chunks of piston, there's any number of reasons why that motor would make noise.

Personally I'm not sure I'd reuse any part of that bottom end.

--Ian

I dont think there are missing pieces of the piston. It looks intact but I will double check. Nevertheless, I will have the cylinder rehoned and checked for roundness. I will have the crank checked and will replace the piston. Forged rods are already en route. All Rings and bearings will be replaced as well.

I appreciate the advice guys.

Regarding the hypothesis of the sound coming from piston slap, would the noise be as consistent or would it be more random?

Your picture of cylinder #4 has what looks like a missing chunk at about 4 o'clock. Is that just oil or something?

--Ian

My experience has been mostly consistent.

Interesting.


What you thought was a missing piece was in fact a carbon flake pushing to the side making it look like a hole.

This part of the piston touches the crankshaft.
Attachment 74931


If you inspect your crankshaft you probably find marks on the "counter weights" dunno what you call it (I'm from non-English speaking country)

that is whats making the noise. My engi made the same noise when i bent my rods.

You are correct sir. :)

https://www.miataturbo.net/attachmen...ine=1367188626
https://www.miataturbo.net/attachmen...ine=1367188626


Here is a pic of the bearings on that rod. Is that normal wear? It doesnt look good to me.
https://www.miataturbo.net/attachmen...ine=1367188626

Wow, it's a scary thought that a rod can bend on stock or sub 200 bhp.

Might be best to replace the bearings, don't know if that's normal or not.