Hey guys, I started tearing down the '77 350 two bolt today, and now I have a question. what would cause the top rings on 7 of the 8 pistons to shatter, at first I thought it was from removing the pistons over a TINNY ridge (almost can't feel it) so after removing the first bank I pulled the other side after running the hone in each hole (pretty much smoothed it out), still didn't help, those four top rings were broken also.

the funny part is, you would think that if the ridge broke the top rings it would break the second rings also. after all they are the same ring (thickness). I didn't have to knock the pistons out of the holes, I just pushed them out by hand.

any Ideas what would cause this ????

 

One thing I can think of right away is that if the engine was rebuilt before, they may have had the end gaps on the top rings too small and when they heated up and expanded they broke.

 

I fairly sure the motor had been gone thru before, it had clevite 77 bearings in it and a melling HV oil pump, from the looks of it they put good parts in it during the rebuild. I hadn't thought of end gap but it makes sense, there was only one top ring unbroken and I still have it. maybe I will put it in the hole and check the end gap, I know it will be a little off because of wear but I should be able to get some idea.

 

i'll second the too tight end gap on the rings, are the pistons hypers?

 

Good point. Hypers don't expand under heat as much as cast piston and allow you to run tighter piston-bore clearances, so if the block was honed tight in the first place that "tiny ridge" may have been a lot larger than it would have been for cast pistons.

You have to remember that the top ring is the one that's subjected to all the heat, all the time. Over miles of use, they tend to weaken from so many heat cycles. That makes them more 'sensative' to any little ridge.

Remember to that just because the ridge seemed "tiny" to you, it only has to be able to close the ring .016" of an inch to cause the ring to close completely. That's only a .008" ridge that they need to catch on.

Then there's the possibility that they were broke before you even touched them. If there was a rebuild, then the previous person may have had NO CLUE how to properly install rings and broke them installing the pistons, or they just flat out ran the gap too small.

 

I believe theyr'e factory cast pistons, they are dished with 4 "eyebrow" reliefs in them. theres no way to tell if they were broken before I removed them.

thanks for the input guys, I thought I'd ask before I reassemble the motor using the same pistons. I'll make sure the end gap is right when I reassemble it, I plan on using new clevite 77's and felpro gaskets along with moly rings and a new HV oil pump.

also what cam do you recommend for that motor, it should be around 9.?:1 compression and I'll be running home ported 416's on top. this motor is going in a track only car so I want as much as I can run on those heads.

it's going in a lightened TA with 3.23 gears and a built TH350 with a 3000 rpm stall converter.

 

I would inspect those piston's rng lands very carefully with a magnifying glass before I reused them. If the rings broke while in use they may have bent or cracked the lands.

 

And take a close look at the rings themselves. If the rings don't show a lot of carbon buildup, they may have broken when you removed them. If there is carbon in the gaps and cracks, they likely broke while the engine was running.

If the pistons aren't damaged, you can re-use them if they still fit. If they are damaged at all, new Keith Black cast hypereutectic alloys pistons can be had for about $200 per set.

 

i've taken a lot of engines apart and seldom if ever find broke rings. at any rate i'd be very careful about reusing the old pistons. considering the time and effort required to verify the pistons are good and the fact you're running cast i'd just buy another set.