Is .0015 measured with platigage too tight for a rod bearing clearance?
05-04-2014, 12:09 AM
#1
Is .0015 measured with platigage too tight for a rod bearing clearance? I am building an LS2 408 using an Eagle forged crank & rods and Clevite bearings. The only measuring tool I have is plastigage. I measured the mains and all of the bearings came out dead on at .002 w/the the plastigage. I feel pretty good about the mains.
I did one rod bearing and it came out to .0015 with the plastigage - maybe a hair larger at .0016. I was hoping to see somewhere around .002. I repeated and got the same measurement. It was getting late so I stopped there.
The engine has a low/mid range cam and will probably never see 6500 RPMs. It's more of a cruiser vs a track car.
Any thoughts on this? Is .0015 acceptable? Am I worried over nothing?
The other question is whether I should order some of the thinner (and expensive) bearings and swap in half a shell based on a plastigage measurement? I understand that plastigage has its limitations but I really don't see myself dragging all this stuff to a machine shop to have it measured.
Tomorrow I'll probably assemble and measure more rod bearings to see if they're all coming out around .0015 or if there are some significant variations.
I did one rod bearing and it came out to .0015 with the plastigage - maybe a hair larger at .0016. I was hoping to see somewhere around .002. I repeated and got the same measurement. It was getting late so I stopped there.
The engine has a low/mid range cam and will probably never see 6500 RPMs. It's more of a cruiser vs a track car.
Any thoughts on this? Is .0015 acceptable? Am I worried over nothing?
The other question is whether I should order some of the thinner (and expensive) bearings and swap in half a shell based on a plastigage measurement? I understand that plastigage has its limitations but I really don't see myself dragging all this stuff to a machine shop to have it measured.
Tomorrow I'll probably assemble and measure more rod bearings to see if they're all coming out around .0015 or if there are some significant variations.
05-04-2014, 09:13 AM
#5
TECH Fanatic
The Gm spec is .0009 to .0025". Just remember that the bigger the clearance the greater the oil leak at that point. This is in reference to overall oil pressure. The point is that pressure is a reflection of the total leak in the fluid column.
For a street engine your .0015" is not out of line. If you have an inside mike and a good oitside mike take a few measurements at various points in the rod big end. At points from about 20 degrees each direction from the oil hole you should not see a big difference. At the parting line the bearing should exhibit an "egg shape", or an ellipse.
I would be looking for consistency in all of the rods and journal dimensions as much as the actual dimension.
Another thing to look at is whether the bearings are tight in the rod or not. You should see some "crush" when inserted.
In my shop we would often mix and match pairs to get the most consistent dimensions.
For a street engine your .0015" is not out of line. If you have an inside mike and a good oitside mike take a few measurements at various points in the rod big end. At points from about 20 degrees each direction from the oil hole you should not see a big difference. At the parting line the bearing should exhibit an "egg shape", or an ellipse.
I would be looking for consistency in all of the rods and journal dimensions as much as the actual dimension.
Another thing to look at is whether the bearings are tight in the rod or not. You should see some "crush" when inserted.
In my shop we would often mix and match pairs to get the most consistent dimensions.
05-04-2014, 11:54 AM
#6
Thanks for the info guys. Still on the fence!
WRT the oil pressure, it's worth mentioning that I ported my oil pump and shimmed the spring for another 3 or 4 lbs so I should have plenty of oil volume.
I don't think I'll be going out and getting mics. I'd rather do the best I can with the plastigage.
WRT the oil pressure, it's worth mentioning that I ported my oil pump and shimmed the spring for another 3 or 4 lbs so I should have plenty of oil volume.
I don't think I'll be going out and getting mics. I'd rather do the best I can with the plastigage.
05-04-2014, 07:51 PM
#7
Short of buying a set of extra clearance bearings....you can create a bit more clearance than you have. Take the shells and install in the rods and tighten but don't torque. If you have access to a solvent washer, pick up a 3M scotchbrite pad and lightly scuff the bearing shells with the solvent washing as a cleaner/lubricant. This can typically add a few tenths of clearance and should get you to .0018-.0019.
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05-05-2014, 03:31 PM
#8
TECH Fanatic
You can hone the bearings, but be careful. Many of the contemporary bearings have a surface coat the the manufacturer does not want to see removed. I would check with the manufacturer prior to modification.
As for plastigauge, it is a good representation of clearance, just not an absolute. Look for consistency from journal to journal, and at 2 or 3 points in the rod bore.
Your rod clearance is ok, even more so if you plan to use a 5-30 oil for best flow. In many of the Stock - SuperStock and Comp Eliminator engines we plan on using a 0 weight oil for best flow and least windage.
In the end it is all up to you, just don't out trick yourself.
As for plastigauge, it is a good representation of clearance, just not an absolute. Look for consistency from journal to journal, and at 2 or 3 points in the rod bore.
Your rod clearance is ok, even more so if you plan to use a 5-30 oil for best flow. In many of the Stock - SuperStock and Comp Eliminator engines we plan on using a 0 weight oil for best flow and least windage.
In the end it is all up to you, just don't out trick yourself.
05-08-2014, 10:16 AM
#12
I don't get it. GM production spec is .0009 to .0025. If .0015 is too tight and .0015 won't work, why didn't GM make the spec something like .002 to .0025? Not trying to argue, I just don't understand why measuring .0006 inside GM spec (which has a range of .0016) isn't good enough. That's .0002 from being dead in the middle of the range. Maybe someone can explain this to me?
At first I was going to start buying undersized bearings and put half-shells together to get the clearance to around .002. I talked to a couple of guys that I know build engines (one builds about three street/hotrod SBCs a month, the other is a mechanical engineer that builds Miata race engines) and they said that was not necessary. Their reasoning was that it's well within spec, plastigage reads a little tight anyway, and the engine isn't going to live most of its life at high RPMs. I've never known any of their engines to ruin a rod bearing.
Also, it looks like GM production spec is .0009 to .0025 and in-service spec is .0009 to .0030. Does that mean that all bearings clearance themselves a little after being run?
At first I was going to start buying undersized bearings and put half-shells together to get the clearance to around .002. I talked to a couple of guys that I know build engines (one builds about three street/hotrod SBCs a month, the other is a mechanical engineer that builds Miata race engines) and they said that was not necessary. Their reasoning was that it's well within spec, plastigage reads a little tight anyway, and the engine isn't going to live most of its life at high RPMs. I've never known any of their engines to ruin a rod bearing.
Also, it looks like GM production spec is .0009 to .0025 and in-service spec is .0009 to .0030. Does that mean that all bearings clearance themselves a little after being run?
05-08-2014, 11:29 AM
#13
Oil clearance is there to provide a nice film/cushion to keept two pieces of meatal from turningint into 1
Anytime you add heat (which more power will) you need to add clearance ,which adds oil to remove that heat.
You dont want too much oil clearance, because then you run into a problem where excess oil loss from the rods,crank gets tossed up into the bottom of the piston,and now the rings need to deal with keeping all the extra oil out of the chamber.
Also plastigauge is at best a rudimentary tool
it tells you about one spot on the crank surface,and the crank is round .Not flat
You may have a issue 180* from where you checked,as well as taper on that journal
You need accurate mic and dial bore gauge to know the true oil clearances
Good luck
Anytime you add heat (which more power will) you need to add clearance ,which adds oil to remove that heat.
You dont want too much oil clearance, because then you run into a problem where excess oil loss from the rods,crank gets tossed up into the bottom of the piston,and now the rings need to deal with keeping all the extra oil out of the chamber.
Also plastigauge is at best a rudimentary tool
it tells you about one spot on the crank surface,and the crank is round .Not flat
You may have a issue 180* from where you checked,as well as taper on that journal
You need accurate mic and dial bore gauge to know the true oil clearances
Good luck
