Broke Piston, replace just that one?

Quik95SC

SCCoA Member
Well after determining it wasn't the EGR valve or PVC valve, or vacuum leaks I did a compression test and found #5 cylinder only has 30 lbs of compression. Thinking I may have Blown HG's I took the heads off and noticed no bad HG, everything looked. Still has to be a reason for 30 lbs of compression. I went ahead and pulled the entire engine to look for a broken piston ring. Although not really thinking it was because the cylinder walls didn't show any signs of scratching of the walls. Took #5 piston out and rings look good. Then noticed a horrible thing. The entire piston was cracked on both sides.


I went ahead and took the rest of them out and didn't see any more indication of bad pistons or cracks. Engine only has about 10K miles on it.
I have ordered a new piston and a complete set of rod bearings. I have left the crank in it because the crank looks very good with no abnormal wear.

I have new rings already and plan on honing the cylinders to get the new rings to seat properly.

I know a question already.

What else should I be concerned/aware of in replacing only 1 piston if anything at all?

Coming to the Shootout, so do I need to give it a gentle break in period before I run in on the dyno and track? 300-500 miles or so on the new hardware?

Any suggestion are very welcome and solicited!

Smitty
 
Personally, i would replace them all. Would you replace just one shock if it went bad, and the others were fairly old? No because that one would out perform the others and it wouldnt feel right. This is just my opinion, but i would replace em all, and then break it in with a good 400-500 miles
 
xThunderbirdSCx said:
Personally, i would replace them all. Would you replace just one shock if it went bad, and the others were fairly old? No because that one would out perform the others and it wouldnt feel right. This is just my opinion, but i would replace em all, and then break it in with a good 400-500 miles


Even though the rest of the pistons only have about 8K mile on it and everything else looks great? Could it be just a bad piston and the rest will last the life of the car?

It sounds like we may be comparing apples to oranges. These aren't the original pistons anymore they are aftermarket .030 over. Like I said the other look in great shape, (normal wear for 8000 miles) 8000 miles is nothing for a piston.
 
xThunderbirdSCx said:
Personally, i would replace them all. Would you replace just one shock if it went bad, and the others were fairly old? No because that one would out perform the others and it wouldnt feel right. This is just my opinion, but i would replace em all, and then break it in with a good 400-500 miles


Using your scenario, if your shocks only had 8000 miles on them and one of them broke for some strange reason or a seal busted, would you replace all four of them? At $450 roughly!
 
I'm going against majority opinion here and agree with you quik. Under the circumstances and mileage I would just replace the one bad piston and go merrily on my way and not worry a bit. The only thing I would try to check out if something caused the piston to fail in the first place. Any sign of being lean or detonation? Good luck, John
 
Just validate the dimensions of the cylinder (eliminate a problem there leading to the cracked piston) and slap a new slug in there assuming it fits. You may want to make sure it's weight is the same as the others once it arrives.

I know the Sealed Powered stock replacements without a skirt coating are getting hard to come by so hopefully youre replacement piston is just like the others.

you might want to have that connecting rod checked as well, just incase the force that cracked the piston also caused some deformation of the rod. Then maybe take some time to polish the beams on those rods if it hasn't already been done. Here's an article on how to do that: http://chevyhiperformance.com/techarticles/74038/
 
Sounds like...

Sounds like you have a problem with the piston Skirts getting too hot. This is usually due to excessive cylinder temps and over-revving the motor.

I would get the pistons checked and re-install if all OK.

Find the reason before slapping a new piston in.

Don
 
Replace the ONE piston. I would want to find the cause though. You DID purchase the ones for the SC correct? I know the comp. ratio is lower then the NA but I dont know if the material is stronger.

Chris
 
Progress report

Well after cleaning all the pistons and rods up, I don't see any sign of fatigue or abnormal wear on either one. The Pistons are an after market, but not sure of the manufacturer, maybe someone can else can tell me.

The piston has an H675 I believe on the side next to the wrist pin hole. This is the number the machine shop use to order the replacement piston with no problem. They also have a .75mm stamped on the very top of the piston as well. I then had the machine shop mic the bore and the rest of the pistons and all are well within specs with no signs of internal damage.

The machine shop said the engine was very clean and in great shape except for the one bad piston obviously. It had so little wear that I got all the pistons out with out having to use a ridge reamer, because it hadn't even wore a ring into the cylinder walls yet.

One question though do SC Pistons have more than the 3 piston rings? The pistons I have use a top compression ring, a bottom oil ring set and in the middle compression ring it actually has 2 rings in the 1 groove one of normal size and another very thin one as well. Just looker very strange, seeing as I had never seen 2 rings in 1 grrove except for the oil control ring.

Will keep you posted on progress. Should be together by Saturday, then I need to break it in with some low speed trips to Nashville and back several times(ie:until 500 miles or so) before the Shootout.

Thanks for the advise and knowledge!!

Smitty
 
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Quik95SC said:
One question though do SC Pistons have more than the 3 piston rings? The pistons I have use a top compression ring, a bottom oil ring set and in the middle compression ring it actually has 2 rings in the 1 groove one of normal size and another very thin one as well. Just looker very strange, seeing as I had never seen 2 rings in 1 grrove except for the oil control ring.

That is because it is a Total Seal gapless ring.
 
XR7 Dave said:
That is because it is a Total Seal gapless ring.


Dave I didn't get another set of the Total Seal gapless rings, I got a different set, will that be a problem? I wouldn't think so as long as they are for the SC Engine right!
 
Quik95SC said:
Dave I didn't get another set of the Total Seal gapless rings, I got a different set, will that be a problem? I wouldn't think so as long as they are for the SC Engine right!

Of course you are re-honing all cylinders and replacing all rings, right? Just make sure your piston side clearance is good.
 
XR7 Dave said:
Of course you are re-honing all cylinders and replacing all rings, right? Just make sure your piston side clearance is good.


Yes I am Rehoning all cylinders and replacing with all new rings and rod bearings.

Smitty
 
that's a number for a Sealed Power Stock replacement SC motor piston. And as dave said that's a total seal ring pack. .75 is an oversize piston. So that motor has been bored over already.

Was that crack along the same sides as the rod pin holes? I'm guessing detonation may have played a role. You better find a way to control that or another one could crack or worse.
 
I just wanted to say that Mike's point regarding the piston weight is pretty important. I would pay to have the rotating assembly ballanced. From what I've learned, buying a single piston makes it hard to get a matching set.

just my 0.02
 
If the piston is heavier then the litest one, it should stll be able to be cut within .4g of that. Even with a crappy set, they should still be relatively close. Just check the spread sheet (of course you know it WILL be different if you try to do it with rings since you are 2 rings less now) and go from there.

Chris
 
Great balance

seawalkersee said:
If the piston is heavier then the litest one, it should stll be able to be cut within .4g of that. Even with a crappy set, they should still be relatively close. Just check the spread sheet (of course you know it WILL be different if you try to do it with rings since you are 2 rings less now) and go from there.

Chris
Apparently the previous owner must have had it balanced, because I weighed the entire rod, pistons and rings on all 6 sets and they are all within .32 grams of each other. (Used a paint mixing scale.....very accurate!)

Looks like I got very lucky.

Yes I will fix the detonation problem, I think the previous owner was a stupid sh** with lots of money, but no common sense, and he ran it pretty hard with the detonation problems and I just happened to be the one stuck with it when it finally broke :mad: :eek:

Oh yeah got every thing back together tonight and it looks and sound really good (just turning over by hand) will finish it up on Saturday and start the break-in if I can find injectors and have them shipped to me by then!!

Again thanks for all the advise, it is very helpful.

I will make it to the shooout!!! :D

Smitty
 
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