Ring gap, piston clearance, and final sizing questions.

1fstb1rd

Registered User
A little backround...I have a 89 motor, that had 120,000 miles on it. It ran good when it was pulled, I decided to freshen it up with a stock rebuild, pretty much stock pistons with new rings, new bearings and a few other things. The machine shop honed the bores, installed the cam bearings, polished the crank, and checked the rods and mains. The reason I went with a "stock" rebuild was to save some money by reusing the pistons, and the rods, and pretty much whatever I could within reason.

I have gotten all the parts and got ready for assembly. I realized i had an issue when I was checking the ring gap on the Sealed Power E-471K rings ( stock size) and the ring gap was like .031-.032 when stock it should be like .011,.012. checked my bore size, and im looking at roughly 3.8140 in each cylinder bore. After a few days of realizing that either I or the machine shop screwed up (most likely me) im trying to find out what the next step is. This is the first time I have ever done this with a SC motor, and my father is a chevy guy, so he is kinda in the dark as well. I checked my piston diameter, and it looks like each pistion is like 96.10ish, give or take. My manual doesnt state acceptable wear limits on the pistons. I put the crank in and put the rings on one piston to see what it felt like,and if i took my thumbs on the relief of the piston and push it down or up, I can get the piston to move in the bore. I dont know if i have the wrong size rings, or I'm going to need rings that compenstate for the wear and the hone off the bore. I tried to find out exact specs on the rings, but it just gives widths.

So at this this point, i can think of a few things that are not right.
1. Wrong rings.
2. Too much wear on pistons.
3. Combinations of 1 and 2.

So I guess does anyone have any insight as to what I'm doing wrong, or what I have done wrong; and what I need to do to correct it?
 
Not real sure what is going on but the stock bore of the cylinders is 3.81. What is the diameter of your pistons?
Piston to bore clearance should be 0.0040-0.0045 in. The piston should have a little movement without the rings installed.
 
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my tbird when i pulled the heads at 220 thousand miles barely had a ridge at the top of the cylinder so i doubt at 120 thousand miles it would be that bad unless run without oil i never checked ring end gap but what you are describing sounds like to much

ralph
 
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The piston diameter was like 96.10mm stock size is 96.88mm. I just don't understand why if I have stock size rings and my bore is still stock size that the ring gap would be too big. So I guess my pistons are below spec. With wrong rings. Me trying to save money is now going to cost alot more now. I'm just afraid I'm measuring sonething wrong or missing sonething.
 
How confident are you in that bore diameter measurement ? Sounds like the bore is too big for the pistons and rings. Do you know how much material the machine shop took off? Was there a ridge they had to remove?
 
Barely a ridge, almost nothing. They just honed it, which I don't see taking off much material. I'm going to call and find out
 
check your old ring gaps and see what they are at, if it ran with those it should run with new. I did the same kind of thing on an engine last year. When going back together I found my bearing tolerances were more than spec but my engine ran fine before so I slapped them in and havent had a problem.

On my 89 sc I could still see cross-hatches on the cylinder walls but when I had the machine shop check the bores, they had to bore it .030 to clean it up perfectly. That tells me that these engines were not perfect from the factory probably.

When it comes down to it your not going to get a good rebuild unless you spend the money. I knew that going into the one I did your way also.

chris
 
Like others have said put an old ring into the cylinder and check the gap then do the same with the new rings. You may have the wrong ones. I will go measure some of my old pistons and drop a ring in just to see what I come up with.
 
check your old ring gaps and see what they are at, if it ran with those it should run with new. I did the same kind of thing on an engine last year. When going back together I found my bearing tolerances were more than spec but my engine ran fine before so I slapped them in and havent had a problem.

On my 89 sc I could still see cross-hatches on the cylinder walls but when I had the machine shop check the bores, they had to bore it .030 to clean it up perfectly. That tells me that these engines were not perfect from the factory probably.

When it comes down to it your not going to get a good rebuild unless you spend the money. I knew that going into the one I did your way also.

chris

The cross hatch was still visable when disassembled. The motor ran great, i wanted to freshen it up. it didnt even need a rebuild. I would have went to .20,.30 over, except at this point in my life i cant throw the little money i have around. The machine shop couldnt tell me anything, and my old rings got thrown out by my step mother. Im on the phone with sealed power now, to find out if the rings i have are right, or if they have a ring that will have a better gap.

On a seperate note, does anyone know where the piston should be measured from? meaning diameter?
 
Like others have said put an old ring into the cylinder and check the gap then do the same with the new rings. You may have the wrong ones. I will go measure some of my old pistons and drop a ring in just to see what I come up with.

Thanks man, My old rings got thrown out by my step mom. Im just hoping that this is a case of wrong rings.
 
According to sealed power, my ring gap is on the low side of spec. I got .32, they said between .29 and .55..I dont know what to believe now. Im starting to really hate this crap.:mad:
 
It sounds like the bores are in decent shape ... but the pistons are measuring small! Can you recheck the piston diameter? Also, since you have different units for the two measurements - Maybe you used different tools for each measurement? Can you check the two tools against each other to verify their accuracy?
 
Yes, please stick to either SAE or metric to avoid confusion and delay!

.011-.012" is a misprint. OE spec is .011-.021". For a high performance application the high of that spec should be the target. .032 is loose but you have no choice as there is no alternative ring. If you want "blueprint" specs you have to bore and hone to size.

A lot of people seem to think that if they can see a cross hatch in the bores they must be in great shape. Wrong. If you can't see a crosshatch then the thing is completely worn out junk. Being able to see crosshatch just means it's still working.

As for honing out a 100K mile block, well you can do that but it's highly non-recommended. I can pretty much guarantee that cylinder wall taper, particularly near the top of the bore, is going to be ridiculous which will result in ring flutter and a not-so-hot seal. But it will work, and any compression loss will be much more the result of that taper and resulting ring flutter than it will be the .032" ring end gap.

The other thing I get concerned about on a 'refresh' like this is that regardless of what you think, piston to wall clearance is going to be well beyond spec for a hyperutectic piston. Ideal clearance is about 2.5 thou and you are probably beyond 4.5 judging by the amount of ring gap you have. This will allow the pistons to rock around significantly in the bore which will aggravate the aforementioned ring flutter and can result in skirt failure if you really hammer on it. Forged pistons go in at 4.5 thou but that is only because they grow significantly more when they heat up compared to the stock pistons.

Anyway, put it together and don't worry about it. It will probably run fine.
 
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