Piston ring end gap question; UPDATE: machine shop screwed up my block!!

[arveetek]

I've finally had a chance to start assembling my engine, and I'm making good progress. I've got the camshaft and crankshaft installed, and all the bearing clearances have checked out just perfect. I'm using standard Clevite bearings.

I assembled the new pistons onto the original rods, and was getting ready to install the #1 piston. I checked the top ring end gap with the ring in the cylinder, and I measured a gap of about .032". My manual lists a max. gap of .0216". The second ring measured closer to specs, but still about .007" too large.

I was wondering if I have a problem here, or if my manual is incorrect. It is 14 years old. Sometimes specs like these change over the years.

Would some of you mind looking up in your manuals to see what the top piston ring gap max. is?

Thanks!

Casey

[ 04-09-2005, 09:16 AM: Message edited by: arveetek ]

[Brianlar]

According to my book, the top compression ring, 82/92, is 0.012 to 0.022. 93, and later, 0.010 to 0.020. Botton ring gap is, 0.030 to 0.040. Oil control ring gap is, 0.010 to 0.020. OK?

[arveetek]

Thank you!

That pretty much concurs with my book. Hmmmm...that means my top ring gap is .010" too large. My cylinders have just been bored .030" over and I have new .030" over Mahle moly rings. I wonder what the problem is?

Casey

[grape]

don't know much about these rings, but every other engine I've messed with was .0035" per inch of bore.

[arveetek]

Grape, according to my calculations, that means I should have a .013 to .014 gap (3.98 x .0035). That's within the specs in my book. I currently have .032. I called a couple of different shops to get their opinions, and it looks like I either have the wrong rings or a bore larger than .030" over standard.

I called Benny Avant today, and he said he would check into it, but he didn't call back. Looks like it will be Monday before I'll know anything else.

I don't have the proper measuring tools to check my bore....I wish I did. Something's not right here.

Looks like I won't be doing any more assembly this weekend. :( I'm getting rather tired of this project. I just wish it was done and over with.

Casey

[grape]

sounds almost like you got standard rings or at most .010" over. Those things shouldn't be that wide without filing.

[arveetek]

Well, it looks like the machine screwed up royally and over bored my block, well over past .030" over standard. :mad:

I was hoping that maybe I got the wrong sized rings, but I went ahead and checked the piston fit in the cylinder, and it's way too loose. I know I have the right pistons, because they're larger than my original pistons. I don't have the tools to measure it correctly, but I was able to insert one of the original pistons into the new bore, and it fits very, very loose. I inserted one of the new pistons, and it's only a little loose.

I assembled the piston onto the rod, installed the rings, and installed the piston into the cylinder. I can wiggle the piston inside the cylinder. It's way too loose. Like a worn out engine.

I can't decipher any of the markings on the new piston to tell me if they're .030" pistons or not, but they're definitely larger in diameter than the stock pistons. Plus, the box the rings came in has 030 printed on it. I must have the right pistons and rings.

I went ahead and measured the ring eng gaps on the second ring and oil ring as well, and they're all about .010" over the max. ring gap allowed.

The only conclusion is that the shop bored the cylinders past the .030" as stated. :eek:

So, I now have no engine to assemble. :mad: The shop is either going to have buy new pistons for me, or machine another block to fit my pistons. I'm not sure if my block is even useable now, because it could be bored even past .040" over! If the .030" rings are .010" bigger than the max. allowed, then that means it's possible that the cylinders are .020" too big already! That would be an over bore of .050". That would make this block useless, unless it was sleeved.

I guess I'll take my crank and camshaft back out, and take the block back to the shop on Monday and see what happens. I'll have them measure the bore, stock pistons, and new pistons. I'm confident we'll discover that the new bore is too large.

This is utterly disgusting. I'm quickly losing faith in this machine shop. I've never used them before. The shop I have dealt with since I was 16 closed up a few years ago. When I called this shop yesterday to discuss my ring end gap question, they told me that they never even measure the end gap...they just eyeball it! This shop has been around for years...you'd think that they wouldn't make a mistake like this. Oh well, stuff happens.

I just hope they don't give me trouble and they will make it right.

Looks like it'll be a few more weeks of down time yet! :(

Okay, venting off!

Casey

[wthif]

Casey

Sorry to hear about your troubles hope it isn't as big as you think. I have a 82 block on a stand in my garage that I currently don't want to part with, but.. I can check it out and see if it is ok to rebuild and if you pay for shipping you can have it as long as you find me another rebuild-able 6.2 high nickel block down the road and send it my way. No hurry on the replacement... I hope. Just want to give you another option. ;)

~thomas

[arveetek]

Hey Thomas, I really appreciate that!

I have two other blocks, an '83 and an '86, but I specifically wanted to use the '82 for the high-nickle reasons. If the shop decides it wants to machine a new block, I'm going to insist that they find another '82 block.

I'll keep that in mind.

Casey

[john8662]

Ouch! Bummer Casey, I was afraid you'd come to that conclusion when I started following this post. I hope your shop has the ability to make this right for you! I'll see if my two local diesel shops have any 6.2 red blocks sitting around if you end up needing one.

I can't believe that the shop wouldn't mic the holes before a bore to determine the amount of wear already on the cylinders. Had this engine ever been bored before, do the old pistons have any marking stamped on them being oversize?

[arveetek]

Originally posted by john8662:

I can't believe that the shop wouldn't mic the holes before a bore to determine the amount of wear already on the cylinders. Had this engine ever been bored before, do the old pistons have any marking stamped on them being oversize? As far as I know, this engine was totally stock before I started the rebuild. I didn't ask the shop to bore it to .030" over, I simply asked them to bore it enough to get the cylinders cleaned up. I'm assuming they started with what the stock bore should be, and then took enough off the cylinders to clean them up and get them perfectly straight. The result of this work ended up with a bore that was .030" over standard. Or at least that's what they told me. The paperwork says .030 and they even wrote .030 on the end of the block.

I was thinking that possibly they looked at the wrong numbers when setting up the boring machine. They might have looked at the .040 numbers by mistake. I'm just guessing here, but that makes sense. Still, you'd think that somewhere in the process of setting up the boring machine, and then checking again after the work was done, they would have realized it was the wrong size. Perhaps they looked the specs up, looked at the wrong column, wrote that figure down, and then never looked at the book again. Who knows.

Casey

[wthif]

Originally posted by arveetek:
Hey Thomas, I really appreciate that!

I have two other blocks, an '83 and an '86, but I specifically wanted to use the '82 for the high-nickle reasons. If the shop decides it wants to machine a new block, I'm going to insist that they find another '82 block.

I'll keep that in mind.

Casey Also might be able to work something out that doesn't require another 6.2 being sent my way. Or your shop can buy mine. :D

[BobND]

Are you trying to fit oversized pistons made to metric oversize spec's into a block bored to a decimal of an inch oversize?

[grape]

did they have the pistons, you can't bore a block without measuring the exact pistons you are using. If they had the pistons they get an F in engine machine work 101.

[gmctd]

Seems as tho they bored it for 6.5 pistons......

[BobND]

The commonly available 6.2 piston oversizes are .5mm, 1mm, and 1.5mm.

This translates into (roughly) .0197", .0394", and .059", respectively.

So, if they bored the block, in absence of the pistons, expecting a conventional American .030" OS piston, it ain't gonna fit!

[CleviteKid]

The piston end gap is related to the circuference of the bore in the block. So if the gap is 0.015" too big (0.032 - 0.017 = 0.015) then the difference in diameter is that excess divided by pi.

So Casey, your block is about 0.005" in diameter too big for your pistons and rings. The block is safe, but getting pistons and rings to fit it could be a bear.

If the shop did not ASK for the pistons and rings when they set up to bore it, then they are at fault.

But you asking them to bore it until it "cleaned up" may have left you with a unique bore size.

Good luck recovering from this little glitch.

Dr. Lee :cool:

[arveetek]

The shop bored the block, then told me it was bored to .030" over standard, and then I ordered .030" over standard pistons to fit it. The shop never asked for the pistons, never asked for the bore size, never suggested I have pistons first, or anything of that nature. I've never had a block bored before...this was my first time. Of all the engine rebuilds I've done, I've always been able to use the stock pistons in the stock bores.

If I had known that you needed to bore a block to match the pistons, I would have done that. The shop should have suggested that I do that. They did not.

Bob, I'm not sure about your metric vs. USA specs question. If that's the case, then my .030" USA spec bore should be too small....unless I got USA pistons to fit a metric hole? :confused: I had no idea this could get so confusing! :eek:

I'm not really sure what happened. I thought, too, that maybe they used the 6.5L specs by mistake, but then that would have made the holes, really, really large....I'm not even sure it's physically possible to bore a 6.2L block to 6.5L specs plus .030" over, is it?

I'm loading up the block, with crank and one piston still installed, and heading back to the shop tomorrow. I'm also going to take one stock piston and one new oversize piston and let them measure everything. What a nightmare. I hope some good solution can be found.

Thanks for all your inputs.

Casey

[grape]

they get a huge F for not telling you to bring the pistons in before they bore it, that's absurd that they rebuild engines. Hell, out of your 8 pistons there may be as much as 5-6 tenths of a thousandth difference, which is a ton.

[arveetek]

I talked to Benny Avant this morning, and he said that the pistons he sent me are not metric. He said that if the shop had bored the cylinders exactly .030" over the standard bore size, then these pistons should have fit just fine.

Benny also said that he would take my current pistons back in trade on a new set of .040" over pistons, for about a $100. If the shop decides to go that direction, I'll wait until the new pistons arrive and have the shop bore the block to fit the new pistons.

Perhaps this machine shop will happen to have an '82 red block sitting around that they could machine to fit my current pistons? tongue.gif I guess that's just wishful thinking. They could then keep my current block and sell it to someone else. If we end up ordering new pistons, it'll probably be at least 2 to 3 weeks before I get the pistons and then have the machine work completed. :( I tell you what, this 3 day project has turned into one long overhaul! (I pulled the engine on Feb. 28).

I'm going to load up the block and head back to the shop soon. Wish me luck!

Casey

[arveetek]

Okay, I'm back from the machine shop.

The first thing we did was look up the bore for an '82 6.2L in their manual. It listed one stock bore figure at 3.980". I saw it myself. My manual at home lists several different possible bore sizes, but more on that later.

So, the shop simply added .030 to 3.980 and came up with the desired bore of 4.010". We measured the bore, and it's spot on at 4.010". We then measured one of the new pistons, and it's exactly .007" smaller at the widest point. According to their book, that's well within specs. They agreed that the piston seemed a little bit sloppy in the bore, but we used a .005" feeler guage and slipped it in between the piston and bore...it was fairly tight. (We couldn't find a .007" feeler guage....they said they don't use them much anymore...they called this form of measuring the piston-to-bore clearances "old school").

They said the piston fit inside the bore was fine, but agreed the ring gap was too large. They said that quality control on piston rings has been declining over the years, especially on GM gasoline engines. They couldn't say if that was true for the diesel engines or not. They said that sometimes they've had to get .035" oversized rings and then file the ends down to get the desired gap. They weren't sure if .035" oversize rings were even available for the diesel or not. They did say not to file down .040" rings...it won't work. The rings could get deformed (oblong) and not seal right.

So, according to their manual, and I saw the figures myself, everything's in spec, except for the rings. Conclusion: faulty rings.

I called Benny, and he had a set of rings that he masured inside a block they had bored .030" over themselves, and they measured a .022" gap. He said he'd send me that set of rings.

When I got back to the shop, I checked the bore size listed in my manual. It lists 6 different acceptable stock bore sizes, depending on what grade the block is, from Grade A to Grade F. The bore sizes range from 3.97585 up to 3.97885. That's just a hair smaller than the 3.980 figure used by the shop. What do the different grades mean? If the shop had used the smallest figure, I would have ended up with a bore of 4.005", which is .005" smaller than what I have. If they had used the largest figure, I would have ended up with a bore of 4.00885", just .002" smaller than what I have.

Also, my manual lists a max. piston to bore clearance of .0059", which is smaller than my current .007".

My manual shows different than theirs.....

According to their manual, they bored correctly and the piston to bore spec is okay. According to mine, the bore could be .002 to .005" too big, and the piston to bore clearance is .002" too big.

So...I called Benny back, and asked what figure they use as a stock 6.2L bore size. Drumroll please....3.976". Bingo! A .030" overbore would result in a bore of 4.006". I currently have 4.010. My bore is .004" too big.

I called the shop back and explained what I found. They were surprised at the smaller figure. They haven't run into that before. They said they wouldn't mind boring the block to .040" over, but at this point, they don't want to give me $100 for the piston exchange. If I had bought the pistons from them, then they would be happy to exchange them for me. They did say they'd think about it, research into the 3.976 deal, and I should call them back tomorrow.

I asked about having pistons first before boring, and they said it's usually not a problem to bore first. Only when going to custom high-performance forged pistons and that sort of thing do they recommend having the pistons first. They generally use the piston mfg.'s specs for boring the block, so the numbers all come from the same source.

I would go along with them boring the block and me paying the extra for the new pistons...it's a learning experience for both of us really. I should have gotten the pistons first, they should have a better manual showing the correct figures.

So, right now, it looks like I'll be ordering .040 over pistons, I'll pay the difference, and the shop will bore the block to fit them at not cost. I find that acceptable.

Whew...what an object lesson!

So Dr. Lee, your calculation of my bore being .005" too big was right on the money!

Thanks for all your help! :cool:

Casey

[grape]

.007" is waaaaaaay loose on that size bore of engine....damn.

[grape]

.007" is waaaaaaay loose on that size bore of engine....damn. and you measure the piston diameter 1/2" up from the bottom of the skirt 90 degrees to the wrist pin, ie across the skirt. It's possible at the top near the ring lands they are .007" but thats not the standard place to be measuring.

[dieseldummy]

That's a bad deal on all parties. Even the little machine shop I use always wants the pistons to finish boring the block. They measure the bore and figure how much it will take to clean it up and then I order pistons in that overbore and then when I get them I take them the pistons and they bore the block. Simple as that... I never check things closely like you have, so it's a good thing that you are meticulous so 20K miles down the road your "overhauled" engine isn't worn out.

[arveetek]

Originally posted by grape:
...you measure the piston diameter 1/2" up from the bottom of the skirt 90 degrees to the wrist pin, ie across the skirt. That's exactly where we measured the piston, and that's where we learned that the piston was .007" smaller than the bore.

Casey

[arveetek]

Originally posted by dieseldummy:
That's a bad deal on all parties. Even the little machine shop I use always wants the pistons to finish boring the block. They measure the bore and figure how much it will take to clean it up and then I order pistons in that overbore and then when I get them I take them the pistons and they bore the block. Simple as that... I never check things closely like you have, so it's a good thing that you are meticulous so 20K miles down the road your "overhauled" engine isn't worn out. Yes, this has definitely been a big learning experience for me. If I had known any of this, I would have ordered the pistons before the actual boring, but, really, how was I to know? I know now.

I could have very easily assembled the engine as it is, and I'm confident it would have run just fine...but it might have had a lot of blow-by, low compression, and it probably would have smoked too much...pretty much where I started at!

If I had just now taken the engine apart and measured the ring gap and piston to bore clearances, I would come to the conclusion that the engine was worn out and in need of a rebuild!

Casey

[odee]

every shop I have talked to about boring any engine has always asked for the pistons and the block that way you get a given to go for on bore sizes for on a given cylinder. While on the subject of boring how big of an overbore will these red engines take? Hope everything works out.

[arveetek]

I talked to the shop again today, and they're still holding to their belief that they bored the block exactly where they should have. But, they're still willing to bore again at no charge, but I absolutely must bring in the pistons first! If they would have just told me that to start with, we could have avoided this whole mess. Oh well, you live and learn.

I believe that all 6.2L blocks, except for the 599's, can be safely bored to a max. of .040" over standard. That's where I'm going to be at. This is going to be the last rebuild this block can take, unless I sleeve it. Still, if I got 9 years and 120,000 out of a basic re-ring overhaul, I should be able to get 20 years and several hundred thousand miles out of this total rebuild! :D

BTW...the shop still claims that I don't need to bore and put in .040" pistons because I already have .007" piston-to-bore clearance, and that's what I need. I tried telling them that my book shows a range of .0035" up to .0045" in cylinders 1 to 6, and .0040" to .0059" in cylinders 7 and 8, but they wouldn't listen. So, when I do get the pistons, what clearance should I have them bore the cylinders to? I don't want too tight so that the pistons could swell and grab when they get hot, but I don't want them too loose so that I lose compression and have piston slap (which I belive .007" would probably do).

Casey

[john8662]

As far as I know (I don't have a 6.2 rebuild spec sheet, mainly because someone always outbids me on ebay!) the 6.2's didn't have a spec for different sizes for the #7 #8 cylinders. I'm probably wrong though, but I think they're all supposed to be same throughout. I know that the 6.5 does though, thats what I'm looking at. The 6.5 TURBO calls for:

.0035-.0049 in. cylinder clearance on #'s 1-6
.0040-.0054 in. cylinder clearance on #'s 7-8

I was floored the first time I saw this, I didn't realize GM overbored (or honed out) the hottest cylinders for heat. I now see why they do it, they really do get hot enough to grab the cylinder walls. The local engine shop here in town (that has the best rep for quality work on gassers) said they just bore them the oversize on all holes, never knew of the #7/8 oversize spec, said they never had any problems.

Since your 6.2 is going to be turbo it's new life after the rebuild I would at least get close to the 6.5 spec for cylinder clearance. I'm still trying to figure out how tight I want my new 6.5 to be, I've been told to use the spec, but I don't like the loose cylinders, I dunno, I'll probably make a compromise in between.

[arveetek]

John,

That pretty much lines up with what my specs show for the 6.2L. My list also shows tighter clearances for 82-86 6.2L's, and a little looser for 87 and newer blocks. I'm not sure why. And I still don't know what the different "Grades" mean.

I talked to Benny Avant at length today (what a patient man...he's quite busy but always takes the time to answer my questions). I was getting worried about having thin cylinder walls at .040" over, but he said not to sweat it. He's driving an '82 Suburban with a 6.2L bored .080" over standard, fitted with stock 6.5L pistons. And it's not a 599 block.

He said everyone has told him that you can't bore a 6.2L to 6.5L specs, but he's been driving this rig for 4 or 5 years, with about 30,000 miles on it. I saw this rig at the DP Rendezvous in 2001, and I always just assumed they used a 599 block, but it's just a standard 6.2L block. It's also got a Banks turbo on it. I'm beginning to wonder if there is any actual difference between the 599 blocks and earlier versions. Are the cylinder walls really any thicker on the 599's?

Benny also told me that in his experience, the 6.2L is more reliable and less likely to crack than the 6.5L. That's music to my ears! :D

Anyway, I ordered the new .040" over pistons. I'm also going to find out what the Diesel Depot recommends for piston-to-bore clearances, and use that to set my bore. I'll probably have 7 and 8 bored a bit larger as the factory does.

Some day, I'll have one sweet-running 6.2L!

Casey

[john8662]

Originally posted by arveetek:
I was getting worried about having thin cylinder walls at .040" over, but he said not to sweat it. He's driving an '82 Suburban with a 6.2L bored .080" over standard, fitted with stock 6.5L pistons. And it's not a 599 block.
Whoa! Who would have known, I figured that the 599 was the only block that could do this also.

Yeah, Benny is great to talk to, I don't know how he manages to make the time to talk like he does, I sure appreciate him doing it though!

[waorth]

Hi Casey,

not an easy life, uuhh?


I have 3 spec sheets for the 6.2

- 1985 Shop manual (1)

- 1989 CUCV Army manual (2)

- 1999 Hummvee engine manual (3)


Data for piston clearance (NOT compression ring clearance!!) as follows:

(1) two piston types
Bohn pistons
cyl 1-6: 0.00350 - 0.00453 *** cyl 7-8: 0.00402 - 0.00504

Zollner pistons
cyl 1-6: 0.00441 - 0.00543 *** cyl 7-8: 0.00492 - 0.,00594


(2) one piston type
cyl 1-6: 0.00350 - 0.00500 *** cyl 7-8: 0.00400 - 0.00550


(3) one piston type
cyl 1-6: 0.00350 - 0.00450 *** cyl 7-8: 0.00400 - 0.00500


Sorry I can't give you any tips. My list is just for comparison.

Good luck.
Walter

[arveetek]

Originally posted by waorth:
Hi Casey,

not an easy life, uuhh?

Data for piston clearance (NOT compression ring clearance!!) as follows:

(1) two piston types
Bohn pistons
cyl 1-6: 0.00350 - 0.00453 *** cyl 7-8: 0.00402 - 0.00504

Zollner pistons
cyl 1-6: 0.00441 - 0.00543 *** cyl 7-8: 0.00492 - 0.,00594


Good luck.
Walter Definitely not an easy life!! tongue.gif

My information is the same as your first reference....listing two different specs for two different piston types. My info comes from a 1991 Haynes Diesel Engine Repair manual.

I think I might shoot for .004" clearance on cylinders 1 through 6, and .005" on 7 and 8. That would put me right in the middle. Or should I start at the bottom of the specs, and aim for .0035" in 1 through 6 and .0045" on 7 and 8? :confused:


Casey

[arveetek]

Okay, let's see if I can figure this out....

The shop bored my block .030" over 3.980" to reach a bore of 4.010".

The pistons I receieved from the Diesel Depot measured 4.003".

If the shop had bored the block .030" over the Diesel Depot's figure of 3.976", I would have ended up with a bore of 4.006" instead, which would have left me with .003" clearance on my current pistons. That tells me that the Diesel Depot probably uses a figure of .0035" for the piston-to-bore clearances.

Again, if I had the shop bore the block to .040" over the DD's 3.976" figure, I would end up with a bore of 4.016". The new pistons ought to measure out at around 4.013", again leaving me with about .003" clearance. I believe this tells me I ought to use the .0035" figure, at least in cylinders 1 through 6, and .004" in 7 and 8.

What a learning experience!!

I'm growing tired of learning...I'm ready to start driving again! ;)

Casey

[BARRAZA]

Sorry to hear about your troubles. I wouldn't take a lawnmower to a shop so incompetent. Does anyone else find it laughable that a shop that bores blocks without pistons in hand, and that measures cylinder wall clearance with FEELER GAGES :eek: would complain about piston ring quality conrol. I nearly fell out of my chair when I read that. They wouldnt know QC if it hit them in the face.
If you had put that motor together with that large of clearances it would have been a quart every hundred mile pig. A long time ago I kept my college era junk running with some pretty scary junkyarding and scrounging, but I wouldn't have dreamed of putting together an engine with that much piston to wall clearance.

[mhagie]

OK To add to the confusion I am looking at my 1985 GMC Light Duty shop manual X-8532 copyright in 1984 so it is a early edition.
And it says
CYLINDER BORE DIAMETER 100.987-101.065
OUT OF ROUND .02 MAX
TAPER THRUST SIDE .02 MAX
PISTON BORES 1-6 BOHN PISTON .089-.115
1-6 ZOLLNER PISTON .112-.138
BORES 7&8 TO BE FIT .013 LOOSER
PISTON RING GAP TOP .3-.55
2ND .75-1.0
NOTICE ALL DIMENSIONS ARE IN MILLIMETERS UNLESS OTHERWISE STATED
I DON'T HAVE A CONVERSION CHART HANDY SO I'LL LET YOU DO IT.
GOOD LUCK TO YOU CASEY, IT WILL WORK OUT IN THE END.................MERLE

[odee]

How do you tell the difference in the piston brands? What would be a good gap to go with on a rering job?

[arveetek]

Originally posted by odee:
How do you tell the difference in the piston brands? What would be a good gap to go with on a rering job? The Zollner pistons look a lot different than standard pistons. They have a steel band that surrounds an inner aluminum core. The upper compression ring land is made of steel.

I'm assuming that the Bohn pistons are the same as all other pistons such as Sealed Power, Mahle, etc.

You want the ring gap to be as small as possible, with a min. of .010" and a max. of .022". The smaller the better, as it will hold compression better.

The second ring has a larger gap, though, with a .030" to .040" range.

Casey

[arveetek]

I received the new .040" over pistons this week, and had the shop machine the block again. This time, they measured each piston, and bored/honed each cylinder to fit each piston. I had them make the piston-to-bore clearances be .0035" for cylinders 1 through 6, and .0040" for 7 and 8.

Now my top ring end gap is .020", right within specs. The instructions that came with the new .040" over rings said I needed a minimum gap of .016", so .020" is pretty good, and below the max. gap of .022" for a stock 3.976" bore. I now have a bore of 4.016", so I don't know if that raises the max. gap allowed or not.

The second ring and oil ring are within specs as well.

Yes! Finally I can start reassembly. And the new pistons fit like a glove and aren't sloppy in the holes.

What a learning experience. Or, as my dad would call it, an "object lesson." :rolleyes:

Casey

[DmaxMaverick]

TTT for Arveetek