Connecting Rod Side Clearance Not In Spec. - Dakota Durango Forum
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post #1 of 7 Old 04-06-2010, 05:52 AM Thread Starter
Ricker97
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Connecting Rod Side Clearance Not In Spec.

I started putting my motor (408) together and while checking the side clearance of the connecting rods that share the same bearing journal found it to be out of spec. when I checked it with feeler gauge between the two rods. The specs. should be .006" to .014", I have;
1&2 journal = .004"
3&4 journal = .008"
5&6 journal = .019"
7&8 journal = .020"

Any idea how to fix this?
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post #2 of 7 Old 04-06-2010, 03:15 PM
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To start with, are you sure the big end chamfer is oriented correctly. The large chamfer(the bearing and the rod) should be facing the crank, not the paired rod.
If all(rod & bearing chamfers) are OK the tight ones need to be machined to get the proper clearance. The loose ones, it has been a long time but IIRC typically .025-.028 is considered to loose by some people and need replaced. I have read about people running .030-.035 and claim to not have any problems. Some claim wider side clearance causes low oil pressure, others say it has no effect because oil pressure is determined by the rod bearing to rod journal clearance.

I will say I don't have a lot of experience here and am going by what I found years ago on the couple of builds I had done.
Keep us updated.

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post #3 of 7 Old 04-06-2010, 04:13 PM Thread Starter
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Yes the chamfer is oriented the right way and all the bearings are chamfered for a radiused journal. I brought my mics home from work last night and mic'd the width of the rods large end. I would have thought that the tolerances of these Eagle H-Beam rods would have been alittle tighter. I got;

#1 .927" #2 .930"
#3 .927" #4 .928"
#5 .926" #6 .925"
#7 .925" #8 .925"

If I switched the highs and lows around, the rod side gap should be;
1&2 journal .009"
3&4 journal .011"
5&6 journal .016"
7&8 journal .015"

If I did this and this is what I came up with, would the .015" and .016" be ok. The spec. are .006" to .014"
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post #4 of 7 Old 04-06-2010, 04:58 PM
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Scroll down to page 41 on this "How to hot rod a small block Mopar". BTW, I did some searches on the subject, and many run more clearance than you without issues. Also, It may not be the rods fault it doesn't fall into spec. Maybe the crankshaft grinder "kissed" the journal side a tad too much. Anyway... good luck and keep us posted.

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post #5 of 7 Old 04-06-2010, 09:06 PM
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Quote:
Originally Posted by Ricker97 View Post
Yes the chamfer is oriented the right way and all the bearings are chamfered for a radiused journal. I brought my mics home from work last night and mic'd the width of the rods large end. I would have thought that the tolerances of these Eagle H-Beam rods would have been alittle tighter. I got;

#1 .927" #2 .930"
#3 .927" #4 .928"
#5 .926" #6 .925"
#7 .925" #8 .925"

If I switched the highs and lows around, the rod side gap should be;
1&2 journal .009"
3&4 journal .011"
5&6 journal .016"
7&8 journal .015"

If I did this and this is what I came up with, would the .015" and .016" be ok. The spec. are .006" to .014"
The only thing I see with switching them around now is your rod and piston assembly's are probably weight matched and/or rotating assembly balanced as is. If they aren't, mix and match to get closer to what you need.

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If every American cared enough to purchase one USA made garment per year the impact would be $9 billion. That is a lot of American jobs.
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post #6 of 7 Old 04-06-2010, 09:34 PM
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0.008-0.0015 is optimal but 0.030 will run fine under say 7000rpm. It seems like Eagle has a tolerance of 3-4 thousandths, which kinda sucks, but I bet the crankshaft isnt perfect either.
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post #7 of 7 Old 04-09-2010, 03:30 PM Thread Starter
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I picked up some new Clevite H Series rod bearings, swapped some piston/rod assemblies around and put everything together. The rod side gaps look alot better. Still at the high end of the specs. but from what everyones telling me alot of people run alot higher with no problems. These turned out to be;
1&2 journal .012"
3&4 journal .011"
5&6 journal .015"
7&8 journal .014"

Thanks everyone for all the great input, much appreciated...Rick
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