Rod bolt torque after inspection
 

Guys,

During tear down of one of my 502s, I found a piston that looks like it kissed some parts at one point and time it's bad enough that I can't leave it even though compression and leakage were good. Compression was an average of 150 PSI across the board, leakage on this hole was 20 percent with air coming through the valves. No issues with the bore. Out of the two engines, this one was running perfect. I was absolutely floored to see this.

From my paperwork from the previous owner, it looks there was a valve issue that required repair on the cylinder head (it was not factory sealed like I thought). I guess some techs think it's okay to have left it this way?

Here's my dilemma: a friend has a stock 502 piston that I'll be able to use as a replacement. I'll have the piston swapped to the rod, rings reinstalled the way they came out, and back in it will go. My concern is with rod bolt torque. I hate disturbing the bottom end. What's the best way of ensuring the rod bolts have the right torque/stretch when it goes back together? The last thing I want to do it spin a bearing.

I'm half tempted to just rebuild the entire short block at this point, but the rest of it really doesn't need it. Had I not pulled the head, I would have never even known there was an issue. I'll post a pic from my phone in the next reply.

Thanks.

Pics


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Fresh marks on the pistons are from me poking at it.

Tom,
I have seen plenty of engines over the years that have run just fine for a loooong time with marks like that. I would almost say leave it alone.

That being said about the only way you can get the rod bolts exactly where they are is take a stretch gauge and measure them before you pull them apart and then tighten them back to the same exact stretch. If they are factory rods and bolts it should be fine just torquing them to the factory specs.

Thanks Bill.

The piston looks to have a gap and tear. I was worried that my much larger intake valves could be an issue.

mill get another pic of it straight on.

Looks like the ring land broke. Was there a chance it ran lean?

Doesn't appear to be broken. Damage is from something striking it. Like I said, this thing had good compression and leak down. Strange isn't it?

To my knowledge it has never run lean. I put in new BUP injectors last year as a precaution.

Closer pics...

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Looks like a broken ring land to me too. I had a very similar failure with a sbc I built. In my case, compression was a bit lower which caused me to pull the head. Insufficient top-ring end-gap and ring poping out of ring compressor as its going in the hole made the short list of possible causes. Replaced piston (had to grind off a little material to get down to the same weight as orig). 15 years later and its still running great.

I agree that using a bolt stretch gage is the best way to get rod bolts back to original torque.

I'd also remove the valves for at least that cyl and inspect. Many years ago, we used a blue ink like liquid (Persian Blue?) on the valve seats to see where valve was and wasn't contacting valve seat.

I'd hang the other piston on the rod and torque to factory specs. I'd also probably put the same bearings right back in if they look good. You will probably learn a lot when you remove that rod.

That piston has a broken ring landing. I had the exact same problem on one of my 502’s and it looked the exact same. I would replace the piston and fix the head. Mine scratched the cylinder wall also. I have to look I might have a spare piston with the metric ring pack. I ended up buying new pistons for the other engine.

If I can get to it, I'll have the piston out this evening.

Wouldn't a broken ring land affect running condition and compression/leakage? Compression was done last spring and the leak down was done last night. We're probably splitting hairs here because obviously the piston is fubar. It was late last night when I discovered this so I didn't look too closely at the bore on that side, though I ran my nail across the area and felt nothing out of the ordinary. I'll look more closely tonight.

At this point I'm leaning toward a refresh on both since they're out and I have the time.

You are right, cant believe I missed that :( That is where they like to burn through too.

I have seen several 502 pistons burned at that exact spot. JE makes a replacement that is a couple thousandths bigger. Pretty much a hone and go. I would go ahead and bite the bullet and freshen them up

That is exactly what I did on one engine. And the replacement piston uses a normal ring pack. I ran mine lean and that is what caused my problem! Good luck.

I think you should go with your gut here Tom. It’s always nice to have peace of mind on the water without wondering when the thing you didn’t do will bite you. You have the engines on the stands now, this is the ideal time to make them “Tom Z” worthy.

I have extra Mahle rings if you needed them. 6 sets 4.466

If you have to hone the cylinder I would take all 8 out and hone/ring them all. No way to be clean enough doing only one IMO

Thanks guys. I really appreciate all the feedback and advice.

This evening was spent undressing everything and getting it on the stand. Tomorrow afternoon I’ll get the pan off and the piston and rod removed.

Looking at the bore, there is some discoloration at the top where it looks like there has been an issue. The rest of the bore looks good and I think I can get by with honing everything. That will really be for the machine shop to give a final on. The game plan now is to get this one torn down to inspect the hard parts then send to the machine shop for analysis and cleanup, etc. While this one is at the shop, the other will go on the stand to be undressed and will be rotated out once the other one comes back.

If the bores are good I’ll stay at 502. If they need to be bored, then I’ll probably just bite the bullet and step up to bigger cranks and make them 540s. I’ll know a direction I’m guessing sometime next week.

I’ll post up what I find regarding the piston and rod tomorrow. Pics included.

Yes those nice rectangular dents in piston say broken ring to me. Almost looks like piece is still imbedded in piston. Take a magnet across the dents and see where it sticks. I've seen this before haha. I'd also peer under the valves of that cylinder and see if the pieces beat up the valve seats at all on their way out.

I might look at the heads from a curiosity perspective, but since I'm not using them, I probably won't. I posted pics of the combustion chambers earlier in the thread. This has run this way for a long time (70 MPH boat last summer) and I'm pretty sure it was put back together this way. Still speculation at this point so it'll be interesting to see what the piston looks like.

It really does take away from the fun, doesn't it? I had a trip last year where the injection started to have some issues. I had run out across the Bay about twenty miles and the port engine started after-firing and then set an alarm. In the end it was a problem with wire routing and a bad knock sensor. The whole thing of "am I going to make it home" really takes the fun out of it. You nailed it for sure, they need to be right! 'Tom Z: worthy!

Thanks for the offer on the ring sets!
:ernaehrung004:

Yeah, my first thought when I saw your pics is something has been bouncing around in there - looks very much like a chunk of the ring was in there for a minute. Good luck with your builds!

I have not gotten the piston out yet. I've been splitting my time on work, home, kids/remote school, all that. At lunchtime I went and got the pan off and timing cover off, removed the timing set and cam. The inside was pretty clean looking. While I was at it, I took a magnet to the top of the piston. There's for sure some metal on the top of the piston. If I have time, I'll yank the piston and rod tonight and get pictures.

Ouch...
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Bearings don’t look too bad. The two forward mains looked like some trash went through it (single wear line opposite the grooves shell) but the rest look okay. Crank seems happy.

This block has a lot of corrosion on the outside. It seems solid but it definitely went through a lightening phase. Hoping machine shop will give it the okay. The insides look great and outside of the one piston, the rest look almost new.

Damn!

Damn is right!

I asked around... no one has really seen one that bad. It's amazing the thing was still running well. Had I never took the head off, I wouldn't have known!

thats pretty bad!

so are you gonna put one slug in and cross your fingers?! :cool-smiley-027:

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I don’t like doing things half assed so it’s down to the bare block. Everything else looks great so I’m going to carry it over to the machine shop Monday to give everything a once over. I’m going measure bores, rods, etc over the weekend. It looks like I can get away with a hone and deck, swap the piston and install new rod bolts/resizing if needed.

If I can get around to it later tonight, I’ll grab pics of the rest of the bearings and the piston/rod assemblies.

It almost looks like the ring stuck to the cylinder wall. Did it sit a long time without fogging oil?

Not that I know of, but I’ve only owned the boat three years.

Wow, that's ugly. Mine had a much cleaner break and the top ring was pretty much still there. Suspect yours ran that way for a while.

Not saying this is definitely the root cause but when you go back together, do some research on setting top-ring end-gap. I was astounded by all the correction factors that come into play. Piston type (forged,cast.hyper), how close top ring is to top of piston (think KB), hp/inch3 ratio, marine or non-marine, raw water cooling vs closed loop, NA vs forced induction, likelihood of detonation, likelihood of over-heat, fuel type, where it's measured in the bore. The list seems to go on forever and the factors change as technology evolves. You can start off at .016" and wind up over .090" after all the corrections are applied.

Thanks for guidance. Yeah, that thing is pretty f’ugly, isn’t it? I have no idea of how long it ran like this.

When it goes back together, I’m going to follow JE’s instructions for setting piston to wall clearance and ring gap. So long as the bore isn’t bad, I’m sure honing can get me where I need to be for final clearance. I wish I was able to use a different set of rings, but if I stay with these pistons, I’m stuck.

I’ll probably go out and start taking some measurements this evening. I need to mark rods and caps.

Looking at that piston makes me feel ill. Lol

I wouldn't waste my time installing rod bolts and resizing rods. You can buy a new set of floating wrist pin rods for $300 and be money and time ahead.

I’m tending to agree.

Block is at the machine shop to be cleaned and pressure tested. It will more likely than not need to be bored. The piston ring really took a chunk out of it. To me it looks like the ring hung and then broke. At first glance the piston looks like it suffered from detonation, but I think having lost half of the ring caused the piston to lose support which in turn scuffed it. I’m surprised the engine ran as good as it did.

The crank appears to be fine so it looks like either a 509 or 511 depending on how bad the bore ends up being. I really need to figure out the rod situation. I know the idea is to replace the factory rods with new ones. Any ideas? I was thinking I’d stick with I-beans with screws vs bolt/nut. Honestly, there is too much information posted here on OSO to be able to make a decision, lol.

A set of the Eagle or Scat rods are all you would ever need for a stock 502. I personally would go ahead and get the 6.385" with floating pins since pistons have to be purchased anyway. Rotating assy should also be balanced