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Old 10-05-2009, 11:59 AM   #1
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Question 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

Last edited by Faeflora; 10-05-2009 at 01:28 PM.
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Old 10-05-2009, 12:43 PM   #2
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WAT?

I build my motors to try and have a 0.0015" clearance on mains and rods. Not sure where you are getting those clearances, or you misheard what they told you. With a clearance of 0.0001" you would need to run like....machining oil or something...and then it would end up dieing anyway.
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Old 10-05-2009, 01:07 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.
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Old 10-05-2009, 01:37 PM   #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? :(
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Old 10-05-2009, 01:39 PM   #5
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Quote:
Originally Posted by Joe Perez View Post

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.
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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Old 10-05-2009, 02:02 PM   #6
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Probably. I know all of mine were beyond the range, but under the max spec.
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Old 10-05-2009, 02:25 PM   #7
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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?
So are you saying that your measured oil clearance is .004-.005?

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.
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Old 10-05-2009, 02:32 PM   #8
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A way stroked out drag motor that gets rebuilt after every race might run a clearance that large. Otherwise I can't see running more than 0.002" with any longevity (more than 2yrs).
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Old 10-05-2009, 02:52 PM   #9
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OK thank you for the information everyone! I'm ordering a stock crank from partsgroup.com
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Old 10-05-2009, 03:01 PM   #10
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Neo, what about the large clearances makes the motor have a shorter lifespan? Not that I'm worried, I doubt my motor will ever see the 10,000 mile mark, just curious.
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Old 10-05-2009, 04:17 PM   #11
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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.
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Old 10-05-2009, 11:00 PM   #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?
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Old 10-05-2009, 11:22 PM   #13
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The big players: ACL, KING, CLEVITE. All also make OEM bearings for various cars. They are all excellent quality.
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