As some know i am rebuilding my 390. I have got the motor stripped down just about completly to send it to the machine shop. When i took the pistons out today i saw that i had some broke rings. What does this mean? What could have caused them to break? What problems would this have caused?

 

Gouges in the cylinder wall would be the first thing I look for. If you have grooves in the walls you will need to look into getting sleeves if you plan to keep that block. Compression and oil loss would be signs of busted rings. How many of the rings where broke on each piston? Did the motor smoke bad before tear down? Did it run like crap or run on less then 8 cylinders?

 

Well...

There are a number of causes for broke rings. The ones I can think of at the moment, listed from most likely to least likely, are:
1. Ridge on top of cylinder, combined with
2. Over-revving, causing rings to hit ridge from #1
3. Ridge not removed upon rebuild
4. Ring ends hitting
5. Excessive ring side clearance
6. Bad piston rings
7. Excessive piston to bore clearance, causing excessive piston rocking. Hard on rings.
8. Water in the cylinder
9. Some combination of the above

You might have a slight score on the cylinder, but probably not. It will affect the way it runs more than anything. Excessive blowby, contaminated oil, low power. Just get it bored over and get new pistons and rings, it'll be fine.

I had a broken ring on an old 2.9L V6 I tore down. It had an ever so slight score on the cylinder bore, barely enough to catch with my fingernail. I'm sure a .030" overbore would have cleaned it up.

 

Ive also heared that detonation will cause broken rings.

 

the top ring in 4 of the 8 it smoked but not real bad. it smoked more when it was first started.

I have not check the walls yet but i will do that when i get off from work. If i need sleeves what would i need to get and from where. which would be the best?


It ran ok for the most part but it did have a miss every so often.

Well I have turned that motor up tight a few times, and i did leaves a little ridge when i took the pistons out so that might be the problem

I will still make sure that i check the walls good. I plan on getting it bored .030 so i think unless real bad that should clear up any marks.


oh yeah does anyone know the length of full skirted pistons, vs short skirted?

 

Yeah, detonation will cause it, I didn't think of that at the time.

If you need sleeves, the machine shop will get them for you. You wont have to mess with it.

The full skirt thing was only in 361FT motors. You dont have to worry about it. If you're using slipperier skirt pistons, made by any compotent aftermarket manufacturer, you're fine.

 

Yep FE66 is right about detination.....Its the #1 thing that breaks rings!!

And most people dont even know they have a cracked or broken ring as the motor usually runs...and may start using Oil..but they generally think its just normal wear and tear!!

Russ

 

that would have been me but now i know what was going on.

just got on more issue. When i took the old cam out i was looking over it and i saw about half way down the cam that it has deep grooves in it.

could the inside of that block be messed up because of this? If so what can be done to fix this?

 

You mean the camshaft now? Were these grooves on a lobe, or on one of the journals?

 

oh sorry about that yes on the old cam shaft in the journals

 

well one of the journals all the other looked fine as far as being smooth looking

 

Just sounds like to me..when rings cracked or let loose..Obviously some of the crap got into the pan and was sucked up into the Oil Passage's and went into the cam bearing in that one journal..and I would suspect it might have done damage also to the rod and crank journals as well..

But you wont know that until further inspection.... And if so..No biggy, New cam, Depending on how bad the cylinders are scratched.. Ya might get by with a light bore job ?? New pistons and rings..Crank turned 10/10 and new rod and main bearings..

Good Luck with your Project!!

Russ

 

good i plan on going .030 over with a crane 941 cam, and getting the heads done.

when i send the block and crank to the machine shop what would i need to tell them. so that i dont get something messed up.

how about the heads. What would i tell them i want done. I am going to get new springs, lifters and timing chain from crane when i order the cam. Should i let them put the springs in when they do the heads or should i wait and do it myself?


I have never done anything like this before. This is my first running motor rebuild. I have taken an old motor apart and put it back together, but didnt do anything else. (just wanted to see what was in at motor and how it worked, came apart and went back) (i am a visual/hands on kind of learner)

I am going to get a kit from summit racing for the rebuild.

could someone look for me at what they offer and let me know which one they think would be the best.

(this truck is not going to be race or anything like that just the bigger cam for sound around town.)

 

Yeah, the new cam, bearings, pistons, lifters, rings, etc, will make it like it never happened. It'll be fine.

On the block, tell them that you need it bored oversize to the next size to clean it up. Tell 'em you're shooting for .030" over if it will clean up there. On the crank, tell 'em you need it checked and ground undersize as needed.

On the heads, you want the valve seats ground with a 3 angle job, valves ground, retainers checked (I've seen damaged retainers before), valve guides checked and replaced as necessary, valve stems checked for wear. If you're getting Crane springs, be sure to tell them that. They might be able to get them cheaper than you can. In either case, let them put them in if you dont have a valve spring compressor. You might talk to them about porting too, as that adds a lot to performance. If you have the time, you can do it yourself with a bit of reading up on how to. Just get the heads back after cleaning, but before any machine work has been done, and port them yourself. Then take 'em back to the machine shop after you've ported 'em and have the rest of the work done. Or you could contanct someone who specializes in porting, which would save you time, and probably result in a better quality porting job.

On summit, the kit I'd buy is the FEM-MKP601N-311. This is a kit for a 360, but includes all the right parts including the correct higher compression pistons. I'd wait 'till all the machine work is done though, before you buy the kit. That way you know you'll need .030" over pistons, or .010" under bearings, etc.

 

cool thanks rusty

I got all the tools that i could think of when i started taking that old motor apart. 272 Y-block

I have the valve spring compressor.

I am going to send the block and crank this friday to the machine shop. to get that machined.

Now the freeze plugs I have got a slide hammer that i can take them out with. but when i get new ones are they pressed in at the machine shop or do i have to put them back in?