Wrist pin walk

Hi everyone, can anyone help me understand how this could have happened? The engine was recently rebuilt and was not driven long before we noticed it had started to smoke out the breather and had excessive blow by so we ran a compression test and number 4 was low, pulled it back down and found this, guess you can imagine how the bore looks, it's a 1300 Toyota engine. 

Are those full floating or press in wristpins?

In reply to Pete. (l33t FS) :

Press in

  The small end of the rod is the wrong size, would be my guess.

Either the pins are worn or the pistons are worn, either way one of them is out by a thousandth or two.

Another thing that will push a pin out is a bent rod, wrist pin bushing not parallel to the crank.

Press fit pins suck.

I like the bent rod theory.

Vigo (Forum Supporter) said:

I like the bent rod theory.

One way or another, that rod is trouble.

Afternoon guys, thank you all for your suggestions, would it be ok to reuse the piston with a new rod?

In reply to shawone :

I'd be nervous about that, just because we don't know how much odd stress has been put on it.  Is the wrist pin ok?  It is fit to the piston, so they need to replaced as a pair.

Tbh I am not sure what is good, cause the pin did this to the bore

That's caused by the pin fit in the rod being too loose.  That can be an installation error, the pin being too small or the rod being too large.  The first and the last are the most common.  A bent rod can also cause that but it's less common.  Pistons and pins come as a set and that pin is shot so you'll need to replace both the piston and the pin. 

That block will probably need to be sleeved.  It's unlikely that an available oversize bore will clean it up and when pins walk out like that they often distort the bore in addition to scoring it.  You'll need to see if there's additional damage caused by pumping wrist pin and cylinder filings through the engine but at a minimum you're looking at a rod, wrist pin, piston and cylinder sleeve.

It's worth noting that there's absolutely nothing wrong with a properly done cylinder sleeve and I've built many engines with one or more sleeves in them and never had an issue related to them.

TurnerX19 said:

Press fit pins suck.

Not necessarily.  Floating pins can have a circlip go walkabout with similar results (Teflon buttons can be the answer there).

I have built and raced on engines with press fit pins, and have had no problems as long as you have good pins and the small end of the rods is correctly sized for a press fit.  But the first rule of press fit rods is to never try to mate them with pistons at home - get a pro shop to do it for you.

In reply to APEowner :

They could be additional damage. cause that score in the bore is fairly deep, so over bore is out of the question for sure.

Would got to either sleeve it or get a fresh block. Thanks 

You're not using that piston because that bore will have to be overbored.

If there's enough wall thickness, a machine shop might be able to offset the boring bar to mostly take metal out from that side. It all depends on how deep the gouges are, what the wall thickness is, and what oversize pistons are available.

Pete. (l33t FS) said:

You're not using that piston because that bore will have to be overbored.

 

If there's enough wall thickness, a machine shop might be able to offset the boring bar to mostly take metal out from that side. It all depends on how deep the gouges are, what the wall thickness is, and what oversize pistons are available.

The gouges are deep, too deep for overbore I believe.

One oversize piston would cause a balance problem.

Sleeve or block.

shawone said:

Tbh I am not sure what is good, cause the pin did this to the bore

Ouch.  Is there a used shortblock out there anywhere?

iceracer said:

One oversize piston would cause a balance problem.

 

Probably not but there's likely enough extra material on a piston that you you could even out the weight

Yeah, the big three would have single-hole oversizes all the time.  The difference in piston weight is usually fairly low, and as you point out, that can be remedied.

I too think that probably can't be fixed without sleeving, which is fairly "traumatic" and may require reboring the cylinder next to it due to warpage.  But that is an eyeball guess, not very well calibrated of a guess.