Crank rod mains clearance
#1
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Crank rod mains clearance
I bought a polished balanced crank from lazzer that has .005" under rod mains clearance. Everything is exactly as he says it is and he's a good seller.
Anyways, my engine builder gave me a call this morning while checking measurements and said that the minimum acceptable clearance is .001". Lazzer said that he had a little extra polished off for turbo usage. My mechanic knows miata engines well but not turbo engines.
Is this extra clearance a good thing for my turbo motor? It has belfab robs, FM 9:1 weiscos. Does other machine work need to be done to the rods or block to accomodate for the extra clearance? I will be running a lot of boost with the goal of following in JayL's footsteps. Thanks
Anyways, my engine builder gave me a call this morning while checking measurements and said that the minimum acceptable clearance is .001". Lazzer said that he had a little extra polished off for turbo usage. My mechanic knows miata engines well but not turbo engines.
Is this extra clearance a good thing for my turbo motor? It has belfab robs, FM 9:1 weiscos. Does other machine work need to be done to the rods or block to accomodate for the extra clearance? I will be running a lot of boost with the goal of following in JayL's footsteps. Thanks
Last edited by Faeflora; 10-05-2009 at 12:28 PM.
#3
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I'm guessing there's been some miscommunication here.
Sounds to me like the crank has been machined undersize, which typically then requires bearings which are oversize to compensate.
The standard undersize cuts are .010" (0.25mm) and .020" (0.5mm). By this, I mean that the diameter of the main (or rod) journals is cut down so that they are that much smaller than the stock diameter.
If you try to run a crank which has been cut .010" under with stock bearings, then the oil clearance will be whatever it is normally, plus .010".
Conversely, if you try to use bearings intended for an undercut crank with a stock crank (or one undercut to a larger size than the bearings are intended for) then you'll wind up with wacky-small oil clearance.
The normal clearance for a rod journal is spec'd at 0.0011" - 0.0027", with an absolute maximum of 0.004".
In order for you to be getting 0.0005" clearance, you'd have to be doing something really bizarre, like trying to use the wrong size bearings for the crank.
Sounds to me like the crank has been machined undersize, which typically then requires bearings which are oversize to compensate.
The standard undersize cuts are .010" (0.25mm) and .020" (0.5mm). By this, I mean that the diameter of the main (or rod) journals is cut down so that they are that much smaller than the stock diameter.
If you try to run a crank which has been cut .010" under with stock bearings, then the oil clearance will be whatever it is normally, plus .010".
Conversely, if you try to use bearings intended for an undercut crank with a stock crank (or one undercut to a larger size than the bearings are intended for) then you'll wind up with wacky-small oil clearance.
The normal clearance for a rod journal is spec'd at 0.0011" - 0.0027", with an absolute maximum of 0.004".
In order for you to be getting 0.0005" clearance, you'd have to be doing something really bizarre, like trying to use the wrong size bearings for the crank.
#4
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Ha. I'm off a zero. Tenths .1, hundredths .01, thousandths, .001.
Yah I mean .004 clearance. Edited the original post.
So is .004 too much? If so, know where I can get a good stock crank? :(
Yah I mean .004 clearance. Edited the original post.
So is .004 too much? If so, know where I can get a good stock crank? :(
#5
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Yes, the miscommunication was me. My builder said what you said- that the absolute max was .004 and that was way too much for what I intend to do with the motor. Do you agree with that?
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In my humble opinion, that's too much clearance. It's not uncommon to run exceedingly wide clearances on race engines, though of course those motors get rebuilt a lot more than every 150,000 miles.
Sounds like the crank may need to be machined down, and a set of oversize bearings installed.
#11
Excessive oil clearance will drop oil pressure, especially in the head. You want to run a clearance that will allow good oil pressure, but you also want it big enough to give a bit of a cushion. Too tight in a performance motor and the power generated can cause metal contact which will over time pound out the bearings. Too much and the film will not have sufficient ability to maintain itself and will still cause metal contact.
All out stroked motors run a lot because they need a huge cushion for about 4-6sec so that the bearing doesn't pound out. We are talking >1k WHP revving out at like 11k RPM. For a mild motor that will see race time between 0.0015" - 0.002" is just about perfect in most import motors. On the flip side, a destroked motor with less piston accel can run tighter clearances and maintain a high rpm and power level.
For import motors in the 1.55ish to 1.7ish RS ratio range though 0.0015"-0.002" is ideal for both reliability and longevity. It might not last 300k miles with regular oil changes. But it will run for years, even with regular beatings.
All out stroked motors run a lot because they need a huge cushion for about 4-6sec so that the bearing doesn't pound out. We are talking >1k WHP revving out at like 11k RPM. For a mild motor that will see race time between 0.0015" - 0.002" is just about perfect in most import motors. On the flip side, a destroked motor with less piston accel can run tighter clearances and maintain a high rpm and power level.
For import motors in the 1.55ish to 1.7ish RS ratio range though 0.0015"-0.002" is ideal for both reliability and longevity. It might not last 300k miles with regular oil changes. But it will run for years, even with regular beatings.
#12
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^^^ That's what my builder said as well. He said you can run very heavy weight oil but it is risky with high power levels on a street build.
Sidejacking my own thread--
I've read differing opinions on bearings. Has anyone read/seen any evidence that aftermarket bearings perform better?
Sidejacking my own thread--
I've read differing opinions on bearings. Has anyone read/seen any evidence that aftermarket bearings perform better?
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