main/rod bearing question

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moparfanatic56

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Need some info on main/rod bearings. i have seen some bearings that claim to give .001 extra clearance and I am curious as to whether this is beneficial or detremental. The journals are all standard and have been polished at the machine shop, I have not mic'd them yet to verify this. The motor is going to be strictly for street use. Any advice or info will be greatly appreciated.
 
The wording of your question is strange to me. "claim to have" you say. Bearings don't "claim" they have specified sizes and that is what they ARE to the limits of machining tolerance, unless someone made a dreadful mistake

"Street use" covers a lot of ground. If "Street use" is cruising, then I would run factory tolerance, refer to shop manual.

If "street use" is "wannabe" outlaw street racing then you want something else

Something in between would be say, the loose limits of factory out to + 1 or 2

With that in mind, a "standard" crank that has been polished WILL have a tiny amount of clearance added from that wear and polish.
 
.0025 is what I look for, and run a high vol, high pressure oil pump with a good oil pump, dist shaft.
 
67Dart273 said:
The wording of your question is strange to me. "claim to have" you say. Bearings don't "claim" they h ave specified sizes and that is what they ARE to the limits of machining tolerance, unless someone made a dreadful mistake

"Street use" covers a lot of ground. If "Street use" is cruising, then I would run factory tolerance, refer to shop
If "street use" is "wannabe" outlaw street racing then you want something else


Something in between would be say, the loose limits of factory out to + 1 or 2



With that in mind, a "standard" crank that has been polished WILL have a tiny amount of clearance added from that wear and polish.
Click to expand...


My wording was a little bit out there. What I am trying to find out is if this extra clearance will increase the journal lubrication or if it will be more harmful. This is not going to be a race car just a driver.
 
Like B6P said, .0025 is a good number for most. You need to measure your shaft, measure your bores and then put the bearing in the bores and measure and then calculate the clearance.

Let's say you do all that and a standard bearing gets you .001 clearance. You then would need to get a bearing with .001 extra clearance just to get to .002. If you measure everything, and it's all round and straight and to size you can run as tight as .0018 IF you use a 20-30 grade oil. I'm at .0020 on the rods and .0022 on the mains and use a 30.

.0025 is a good number but as oils get thinner you can make the clearance a BIT tighter. Some crap I've done at .0015 but that took a 0w20 oil and I was nervous. And the shaft diameter was well under 2 inches.

So you only need the extra clearance bearing if you need the clearance.
 
After a polish I would go with stock clearance bearings and check it from there.
Not that I am a pro or anything but I like about .0015 on rods and .002 on mains using a 10-30 or 5-30 oil (preferably a synthetic)

Remember, the thinner the oil the faster it returns to the pan and the quicker it removes heat.
 
TrailBeast said:
After a polish I would go with stock clearance bearings and check it from there.
Not that I am a pro or anything but I like about .0015 on rods and .002 on mains using a 10-30 or 5-30 oil (preferably a synthetic)

Remember, the thinner the oil the faster it returns to the pan and the quicker it removes heat.
Click to expand...
426 hemi`s came out w/ .003 clearance on everything, every one I had I set up that way, street or not. the race motor ran for 14 yrs, before being sold-still running, only problem I ever had was a thrust bearing falure, due to not installing it right.
 
They make some bearings oversize to compensate for any cutting on the crank journals... If you cut the crank .010", then you will need a .010" thicker bearing to get the proper/same clearance as the original...

Look up the clearance specs for the bearings in the factory service manual and go with those...

It is always stamped in the back of the bearing what size it is: STD, .010", .020"
 
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Thanks to everyone for all of the info it will steer me in the right direction considering it has been 35 years since I last did a rebuild.
 
famous bob said:
426 hemi`s came out w/ .003 clearance on everything, every one I had I set up that way, street or not. the race motor ran for 14 yrs, before being sold-still running, only problem I ever had was a thrust bearing falure, due to not installing it right.
Click to expand...


.003 is not excessive when you figure in how big the main and rod bearings are.
 
The factory clearance spec for main and rod bearings is .0005" - .0015"
Maximum allowable: .0025"

Per 68 Plymouth service manual...
 
I just checked clearance on my new scat crank with + .001 bearings. Clearance was .0019 to.0022 on the mains. Fone in my opinion, and I would have run them if they had been .003.
There is an axiom that Reher Morison repeats; If your bearings are too loose, only you will know. If they are too tight, everybody will know.
Fuel motors run .007 or so clearance. They run 70 WT tar for oil. So if you have race type clearances, a bit higher viscosity may be needed, depending on running oil temp, how loose they are, etc.
 
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