Broken Piston [Archive] - Chevy Nova Forum

: Broken Piston


73Hatchback350
18th-April-2010, 01:27 PM
hi everyone. I pulled my 350 for a rebuild and found that one of my pistons was broken. cylinder 7. On the outside of the block side the skirt is broken and the chunk was in my crank case. When I pulled the oil pan it was actually sitting on top of the pickup. Anyways I was wondering what could cause the piston to break. I haven't pulled apart the rotating assembly yet but so far the rest of the components look fine. Dirty but fine. Since I have to have the block machined now so I can replace the pistons would it cause any problems to put higher compression pistons in? Do you have any ideas about how much power I would gain? It is a stock 1973 block, I would also plan on having the heads ported. Just wondering about the power numbers I'm really just looking to get the engine running reliably. Thanks everyone

Paul Wright
18th-April-2010, 03:06 PM
You can't tell if they are "fine" by looking, especially if you don't know what to look for. Broken pistons can be caused by a number of reasons. You'll want to do some forensic analysis of the broken parts to find the root cause. I'd have the parts and block magnetic particle checked (magnafluxed) and pressure checked. Your heads might not be worth porting because it's too labor intensive and you can buy better heads cheaper.
Your horsepower depends on what you do and how much you want to spend.

Realistically you only keep the block and recycle the cast crank and stock rods. Buy a new, balanced rotating assembly and then have the block machined. You may find the cheapest route is to buy a new crate engine because these are mass produced more economically than one at a time methods. You also get a better warranty and all new parts.

73Hatchback350
19th-April-2010, 01:52 PM
Thank you for the advice. I was trying to avoid buying an all new engine at this point in time due to a lack of money. I was just going to try and replace the bearings and maybe the rings if they needed it but I guess I'll have to look more in depth. Do you think that there may be a problem with the block since the piston broke? Or could it just be faulty part? Is there any way to know without spending the money to have it magnafluxed? what is magnafluxing?

D-Man
19th-April-2010, 02:37 PM
Thank you for the advice. I was trying to avoid buying an all new engine at this point in time due to a lack of money. I was just going to try and replace the bearings and maybe the rings if they needed it but I guess I'll have to look more in depth. Do you think that there may be a problem with the block since the piston broke? Or could it just be faulty part? Is there any way to know without spending the money to have it magnafluxed? what is magnafluxing?

Magnafluxing is cheap.

As for the broken piston skirt....It could have been caused by a cheap-cast piston, too much taper in the bores, bores being too loose (allowing the piston to "Rock" too much in the bore), excessive detonation, overheating (heat distortion or deformation of the bores or pistons)...

FYI....I HATE trying to do things the CHEAP way!!:rolleyes:
If you can't afford to do it RIGHT, can you afford to do it a second time?:no:
More times than not, trying to go the CHEAP way out generally leads to more trouble down the road.:yes:

Paul Wright
19th-April-2010, 06:00 PM
I agree. You may think you are saving money by not spending it, but you may actually cost yourself much more by cutting corners. These pages are littered with expensive engine problems caused by "saving money". Believe it or not, the new GM crate engine is often more economical than rebuilding your old one.
Ever time you buy parts you are paying retail, you also pay shipping and Jegs and Summit have big handling fees. You pay labor charges for the machine shop, cleaning and measuring. If you don't have precision measuring tools you have to buy them. then there's the cost of the learning curve since you don't know what you don't know. When you get done you will find you got less than what you thought and it cost more than you imagined.

I'm an experienced engine builder and I can't remanufacture one for what I can get a new one for. It's why I don't make my own pencils. It's the economy's of scale and mass production.

73Hatchback350
21st-April-2010, 11:53 PM
OK, thank you guys. I appreciate all of your input. Since I'm going to have to spend the money anyways what do you think of having the block machining to 383. Would it be better to save a few dollars and get the block machined or would it be smarter to buy a new block? I've heard that the machining required to go to 383 is tricky and if not done right the engine may fail prematurely.

sonny1972
22nd-April-2010, 01:26 AM
Nothing too tricky about a 383, it is a really common build, little bit of clearancing is about all because of the additional stroke. It's Nothing that will make a block fail prematurely, unless you take off so much to get too close or into the water jacket, which can be fixed, probably not cheap though. The machinist sounds like he will be doing the work, so you should be fine. If your mostly just thinking about reliability, just stick with the 350. If you plan on getting the speed bug the 350 will still be fine, the 383 will make a little more power if both were built similar. It's all up to you, both are good ways to go. Oh yea, about getting a new block or not, take it to a machine shop that has a good reputation, after he checks it out, he should be able to tell you where to go from there. While taking apart the engine, bearings and other things will help tell you how the engine is too, if you know what to look for. Good luck with the engine whichever way you go, building one, or just buying one.

tnblkc230wz
22nd-April-2010, 01:36 AM
383s are pretty common now. They machining is not that tricky, but you have to check lots of clearances as you are assembling the block. The unbalanced rotating assembly will cost as much or more than a new GM 350 short block with cast internals.

74 hatchback
22nd-April-2010, 02:38 AM
Sorry guys but im cheap. The thing i did years ago to my 350 is i got a set of std. bore cast .100 domed LT1 pistons with rings in a kit from (PAW) performance automotive warehouse for $100 and went from 8.5cr to 9.7cr and i beat them to death, even had 11 to 1 with some camel heads, years later i took the engine apart to rebuild it and they were still like new. The engine build (coppied right from a magazine) using my ported open chambers made 340HP and 380TQ.

D-Man
22nd-April-2010, 11:12 AM
383s are pretty common now. They machining is not that tricky, but you have to check lots of clearances as you are assembling the block. The unbalanced rotating assembly will cost as much or more than a new GM 350 short block with cast internals.

I'm not too sure about that. I can get a BALANCED rotating assembly for around $800 and an UNBALANCED kit for around $650.

The balanced kit also comes with a NEW balancer and flexplate.:yes: All that would be required is to have the rotating assembly checked and the block machined/clearanced and your good to go. :yes:

What's a NEW GM shortblock going for these days?

tnblkc230wz
22nd-April-2010, 11:39 PM
I'm not too sure about that. I can get a BALANCED rotating assembly for around $800 and an UNBALANCED kit for around $650.

The balanced kit also comes with a NEW balancer and flexplate.:yes: All that would be required is to have the rotating assembly checked and the block machined/clearanced and your good to go. :yes:

What's a NEW GM shortblock going for these days?

That is pretty cheap. The new 4 bolt main roller short blocks can be had for about 1200.

D-Man
23rd-April-2010, 10:35 AM
That is pretty cheap. The new 4 bolt main roller short blocks can be had for about 1200.

http://www.adperformance.com/index.php?main_page=index&cPath=71_233

They can be found cheaper but sometimes the quality (fit -n-finish) will suffer as well.
I've had pretty good luck with the Eagle assemblies and Scat assemblies. Just have the machine shop give everything a good once over before ANY machining is done and you're good to go. If there's any problem found, they will exchange it out....no prob.