well its been a few months but i finally pulled my motor and found this. both pistons were broke in the same place. this is a new one for me so if youve seen this before please let me know what i did wrong.

 

Yup, that would cause some significant blowby...

Looks like the ring end-gaps butted together.

Check the other 6, and look at the ends of the rings; if there's ANY telltale at all of them touching, then that's another clue.

Whatever you set the gaps to when you built it, add about .008" to it this time.

 

If you ran a compression test on that motor the compression in those cylindars would have been lower right? Just a general question.

 

thanks sofa, you were helping me with this problem originally. i bought this block assembled it was recently rebuilt. i spent my time learning how to build a heads (with your and every body elses help) and like a dumb*** i never went through the bottom end. so now im going to learn it.

so walk me through this.
the two pistons in the pics were the ones with the lower compression.

ill pull the rest. with them removed do i compress the rings and measure the end gap with a feeler guage ? what #s are good ?

and it was rebuilt cuz all the rods and crank are stamped with there respective #s.

im hoping to get away with new pistons and rings and honing. dont have the funds for a bore right now, unless i i dont have a chioce.

thanks for your infinite wisdome and patients.

 

well the good numbers for ring endgap have a lot to do with what kind of engine you have(ie. N/a street, blown, nitrous, drag.)

and what kind of rings you have the ring company has a formula to calculate the endgap for a specific bore and application

 

yes there was a big comopression difference all cylinders were at 120psi and the ones in the pic were 65 and 90psi.

so im getting new pistons and rings, looking through my usual mags, (jegs,summit) looks like speedpro or the summit, jegs brands are in my budget. my motor is stock bore, 9;3;1 comp. carbed 92 hyd roller street motor. are these good pistons and rings and how can i tell if i need pressed or foat pin.

and the bearings look good but should i replace them any way ?

and last what am i looking at that i will need a machine shop for ?

 

well im up to 3 broke pistons did stock camaro motors come with MAHLE pistons ?

 

Ummm, no....

To measure end gap, you put the ring by itself into the bore, and "square" it to the bore by pushing it down with a piston; and use a feeler gauge to check the gap between the ends. File or grind to fit.

Rule of thumb on forged pistons is .0045" - .005" per inch of bore on the top ring, and .004" - .0045" per inch of bore on the 2nd ring. Nitrous, blowers, etc. require larger gaps, sometimes up to .007" per bore inch on some pistons. FOLLOW THE PISTON MFR'S INSTRUCTIONS though; rules of thumb aren't worth much sometimes.

A little too much end gap is better than ANY too little. Too much, and a couple of stray combustion molecules will leak. Too little, and the ends of the ring will meet when they "grow" from their high temperature, and if the temp continues to rise past their "meeting" point, the ring will become larger than the bore; and the results may vary anywhere from what you've got (relatively benign) to MASSIVE cylinder wall destruction.

Examine the cyl walls CLOSELY for any sign of scoring, scratching, etc. If that has happened, then a bore job is necessary and inevitable. If not, then you can hone & run it.

Those are good pistons. If the bore is OK and the others are OK, I'd recommend just replacing the busted ones with the same part #.

Pistons are not "pressed" or "floating" pin; except that if they lack a Spirolox groove, then they can only be used with pressed pin rods. Since your setup has no Spirolox, your rods are pressed pin.

 

I had the same thing happen to my "New" engine. I found what looked like the rings were not gaped right and butted together. It broak my #6 and #4 pistons. I could not get the shop that put it together to help me out so I cut my losses and just did it myself.........

 

well chris, at least im not the only one weird stuff like this happens to. your rebuild turn out ok ? once you did it yourself.

sofa thanks again.
didnt think chevy put mahle pistons in, someone must of replaced pistons allready. looks like i only have 3 bad ones. i measured like you said ant the top ring is .007 and the bottom is .023. if im doing your math rite .0045 x 4" =.018 ? approx for the top ring. yes i measured and bore is still 4".

there are no deep scratches in the bores just lite lines through the cross hatch from the honing. the chunks of piston and rings stayed in place in the cylinders, thank god. my last motor the rings came apart and destroyed everything.

can you tell me whitch # on the pistons i need to identify them there are some stamped on the face and some cast inside across from mahle?

if i need to bore do you have a guestimate on cost in the SE USA.

thanks to everyone,
mike

 

OOfah! Did you say .007" gap on the top rings?? That's VICIOUSLY tight. Almost like somebody installed "file to fit" rings without doing any..... um..... filing to fit!

.004" per inch of bore is an old mechanic's rule of thumb. DON'T just use that as your only guide. Consult with the PISTON MANUFACTURER on what gap to use (NOT the ring manufacturer). Often they will call for wider gaps than that on the top ring. Especially if the motor is going to see nitrous or forced induction (both require wider top ring gap).

Keith Black hypereutectics are the most extreme example- they call for .026" top gaps even on stock 4" bore applications. I run mine even wider at a whopping .032" for use with my mild supercharged motor. No blowby or excessive oil useage with the wider gap.

As was said above- a little too wide won't hurt a thing. A little too skinny- ka boom!

 

No; I saidwhich works out to about .028" on a 350.

 

Yea, Spike, most out of the box rings are about .016-.018, at least the ones I got. I opened them up to about .022" just to avoid that nasty situation you've got.
Here's hoping you can get away with a hone and 3 pistons and a set of rings!

 

well checked a couple more top ting gaps all were .007 to .010. guess nobody gaped them. tried to hone the worst cylinder with no luck. and i cant find where mahle sells pistons other than in a set. so i guess im going to get that .030 bore ive alawys wanted

thanks to all, ill be back in a couple weeks when the block comes back from the machine shop and i start reassembling my first lower end.

mike