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Old 07-21-2010, 06:12 PM   #1
Farmallgray

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Piston clearance, ring gaps

I'm building a pulling garden tractor with a 68 cube 3cyl turbo'ed engine as I have posted some questions before about. I did a search and I see on Cummins builds it was recommended .007-.009" on daily drivers and
.015-.020" for piston clearance on dedicated pullers.

I have seen for gas engines they normally recommend so many thousandths per inch of bore size. My engine is only 78mm bore compared to ??? on your big cummins motors. So I'm looking for a similar rule of thumb for hot diesels.

What is the bore size of a cummins? I could calculate from that.

Is there a similar rule of thumb for ring end gaps?
 
Old 07-22-2010, 12:52 PM   #2
Farmallgray

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Those figures you gave are just for the ring gap? My bore is about 3-1/8" which would be about .021-.023" for ring gap.

Those figures I quoted were for piston to wall clearance rather than ring gaps.

I sent the second rings to Total Seal and had them convert them to gapless. I'm also having the pistons coated with ceramic on the tops and the moly on the skirts.

Other guys running these engines typically see EGTs in the 1700 range.

Still need a figure for piston to wall clearance. I'm running standard pistons but I'll have the cylinders honed for more clearance.
 
Old 07-22-2010, 01:55 PM   #3
GOT-Torque
 
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I seem to remember cummins specs were up to about 0.006 or 0.007 on a stock engine (piston to wall clearance). I took my current engine to 0.008" p/w with ring gaps in the 0.030" area. Haven't had it apart in two seasons, but it runs great. EGT's have been up to 2000 plenty of times... I wouldn't go any smaller than this (once scaled down for your bore size obviously).

Do you have any more info on your build? I've been wanting to do a garden tractor puller as well...
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Old 07-22-2010, 01:59 PM   #4
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Forged/cast might impact your decision also.
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Old 07-22-2010, 02:08 PM   #5
paulb
 
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Bore is 4.02. Standard honing diameter is 4.0165 +- .0008. These specs are from the 05 Dodge FSM. Top ring end gap, .010 - .014, second ring .033 - .045.

For high performance applications these are considered to tight. .007 - .009 is more generally excepted, but with coated pistons (sides AND tops), I go with .005 - .006. The coated tops helps keep the piston expansion down, and the coated skirts help reduce scuffing.

I use .023 for the top ring end gap. The second I leave at the factory end gap. The cummins top rings out of the box seem to be around .012.

These are spec and values based on a 05 CR.

Hope this helps.
Paul
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Old 07-22-2010, 06:24 PM   #6
Farmallgray

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Quote:
Originally Posted by GOT-Torque View Post
I seem to remember cummins specs were up to about 0.006 or 0.007 on a stock engine (piston to wall clearance). I took my current engine to 0.008" p/w with ring gaps in the 0.030" area. Haven't had it apart in two seasons, but it runs great. EGT's have been up to 2000 plenty of times... I wouldn't go any smaller than this (once scaled down for your bore size obviously).

Do you have any more info on your build? I've been wanting to do a garden tractor puller as well...
There is a forum for diesel garden tractor pulling;
dieselgardentractorpulling : Diesel Garden Tractor Pulling
Lots of good info there.

I have been posting pics of mine on the IH registry forum since I started it;
IHREGISTRY FORUM: Pullers Cafe
Look at the archive pages listed at the top of the page too. If you want any more info or help on a build just email me. farmallgray at aol.com (change the "at" to @).
 
Old 07-24-2010, 12:50 PM   #7
Mike Holmen

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You guys know what width of the typical stock cummins first oversized piston? In the cummins manual they have different specs for the first and second rings? What ring gaps are typically used? Good information guys.
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Old 07-24-2010, 09:30 PM   #8
Farmallgray

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Quote:
Originally Posted by paulb View Post
Bore is 4.02. Standard honing diameter is 4.0165 +- .0008. These specs are from the 05 Dodge FSM. Top ring end gap, .010 - .014, second ring .033 - .045.

For high performance applications these are considered to tight. .007 - .009 is more generally excepted, but with coated pistons (sides AND tops), I go with .005 - .006. The coated tops helps keep the piston expansion down, and the coated skirts help reduce scuffing.

I use .023 for the top ring end gap. The second I leave at the factory end gap. The cummins top rings out of the box seem to be around .012.

These are spec and values based on a 05 CR.

Hope this helps.
Paul
So these figures are what you use for a 4.02" bore, correct? So I should calculate a percentage of those for mine, right? My bore is right around 3" so if I multiply those figures by .75 then it should be eqivilant for my application.
 
Old 07-25-2010, 01:05 AM   #9
paulb
 
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Quote:
Originally Posted by Farmallgray View Post
So these figures are what you use for a 4.02" bore, correct? So I should calculate a percentage of those for mine, right? My bore is right around 3" so if I multiply those figures by .75 then it should be eqivilant for my application.
You would probably be pretty close on the end gaps doing that, but not on the piston clearance. If your pistons are of a similar construction and material to the Cummins pistons, then you should use the above mentioned spec. The bore size won't have that much effect on piston clearance. At least the difference that we are talking about. If your pistons are forged, you would want to go looser. If they have expansion control built into the pistons (typically metal rings cast into the piston), you could go tighter. Sorry if I'm sounding vague.

Paul
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Old 07-25-2010, 09:01 AM   #10
Farmallgray

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I don't know how to tell if they are forged or cast. I know the rods and crank are forged but I can't tell with pistons. They are OE turbo pistons and the have steel inserts for the top ring groove and they also have a steel band
inside in the skirt area. I'm guessing that may be what those are for.
 
Old 07-25-2010, 01:51 PM   #11
Farmallgray

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Just for curiosity I checked the specs in the factory service manual.
Factory spec for piston to wall clearance is .0009-.0016" with a max wear limit of .0075". Ring gaps are .012-.018" with a max wear limit of a whopping
.049" which seems like an awful lot!
 
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