Im working on a sbc for circle track (355) I have lots of manuals and they all state .001" per 1" of bearing Diameter, obviously this is a general rule of thumb and a safe number... I'm in the middle of measuring bearing clearance for my oval track engine, right now I'm getting between .0014 to .0023 of oil clearance on the 2.4500 mains this motor will be revving around 6000rpm should I be looking oversized bearings to get another .001 of clearance or run an oil that best suits the tighter clearances honestly if it was in a daily I would probably run it but I'm having a hard time deciding on tight or loose I know the Hondas run very tight bearing tolerances and oils have gotten better since the sbc came out... this crank came from scat with the journals a little big maybe they know somthing I don't.... run it or not? Thanks
Bearing clearances sbc
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Sbc355
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Im working on a sbc for circle track (355) I have lots of manuals and they all state .001" per 1" of bearing Diameter, obviously this is a general rule of thumb and a safe number... I'm in the middle of measuring bearing clearance for my oval track engine, right now I'm getting between .0014 to .0023 of oil clearance on the 2.4500 mains this motor will be revving around 6000rpm should I be looking oversized bearings to get another .001 of clearance or run an oil that best suits the tighter clearances honestly if it was in a daily I would probably run it but I'm having a hard time deciding on tight or loose I know the Hondas run very tight bearing tolerances and oils have gotten better since the sbc came out... this crank came from scat with the journals a little big maybe they know somthing I don't.... run it or not? Thanks
No but wi will check that out, thanks
You say you have between .0014" and .0023" clearance. What kind of measuring instruments? What is causing the variation? Usually the spread would be tighter on factory parts.
If it was mine I'd prefer .0015" to .002 clearance, with preference to the .002". I also wouldn't run a real heavy oil viscosity.
If it was mine I'd prefer .0015" to .002 clearance, with preference to the .002". I also wouldn't run a real heavy oil viscosity.
Measuring crank with outside micrometer then zeroing a bore guage off the outside mic I had the block aline honed they're all within. 0005 or less but the scat crank does vary more then I would like for the money 2.4488 is my largest on the crank and 2.4478 is the smallest I've measured it multiple times I've also used plastic guage and it tells the same story I have snap guages but it's hard to be consistent I don't have inside micrometers I wish I did
My thoughts exactly it took a while to get this one
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Is the 2.4478 on the rear main journal,if so,use a standard bearing on that journal and the others uae a bearing for .001 added clearance
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Agreed on having the crank mic’d by a shop and see if it needs a little cleanup. Don’t be surprised if its fine though. Are you able to measure the same housing bore or journal, say 4 to 5 times, and get the same number. Dont be surprised if not. It’s tough to pick up a set of mics, bore gage, etc and get shop repeatable numbers. God knows I’ve struggled with it myself.
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I was just reading a machinist post today where using the ratchet to set the mic to the standard can apply a few “ten thou” to the mic due to frame flex. Also was mentioned by several that just holding the mic with your bare hands will change it another few “ten thou” due to heat. It’s crazy but stack those two up and you’re already at a half thou variance due to two small things.
For me, using an external micrometer to measure an outside diameter is very repeatable and straightforward. Those are the only measurements the OP posted. The hard part is measuring a hole of that size. But he's using a bore gauge, and to me that's the best way to do repeatably.
Most crank guys will tell you don't try to polish .001" or less from a crank journal. It's really hard to keep it round over its entire length. I do believe you can get bearings that will fit .001" looser. Don't know how hard to get they are though. You could even mix and match, only installing the oversize bearings on big journals and regular bearings on "normal" journals. Or as an extreme, you could regrind all the mains .010" under. A competent crank shop could then accomplish any desired clearance. Problem is, competent crank grinding shops are getting few and far between.
Most crank guys will tell you don't try to polish .001" or less from a crank journal. It's really hard to keep it round over its entire length. I do believe you can get bearings that will fit .001" looser. Don't know how hard to get they are though. You could even mix and match, only installing the oversize bearings on big journals and regular bearings on "normal" journals. Or as an extreme, you could regrind all the mains .010" under. A competent crank shop could then accomplish any desired clearance. Problem is, competent crank grinding shops are getting few and far between.
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I’ve seen (ACL) and (EnginePro) tend to be a thou smaller than (king) or (clevite)in regards to clearances .
I’d go extreme and grind it to get even across the board then you can have exactly what is needed.
Is the crank bent at all?
I’d only miss match if it was last resort and I had a heat that I wanted to complete , we all do crazy things sometimes to get that finish…. make sure it’s 68-70 when taking your measurements is all I can add.
I’d go extreme and grind it to get even across the board then you can have exactly what is needed.
Is the crank bent at all?
I’d only miss match if it was last resort and I had a heat that I wanted to complete , we all do crazy things sometimes to get that finish…. make sure it’s 68-70 when taking your measurements is all I can add.
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Running a half set of standards and half set of .001 under is pretty common. Done it on several motors actually. Good news with that is you end up with a full set of spares for a freshen down the road. It’s been awhile but I think I kept the undersize shells in the caps.
I have been racing, bracket and turning left, for over 40 years. I have never lost a crank with .003 but I have at .0025. I have lost 3 in fact during all those years and all were between .002 and .0025. I like rod clearances .003-.0032.
Getting to your issue. Have you checked all your rod bore diameters? Are they all the same? How close to minimum or maximum size are they? Small block Chevy with 2.1 crankpin the rods bores should the 2.3247 to 2.3252.
If you have some rod bores larger than others could you put those on the tight crank pins? If the rod bores are at the 2.3247 could you open them up to the 2.3252 size to get some extra clearance?
Sorry, I just reread and saw the issue was the mains. I will let the above stay in case that is also a problem.
It is almost impossible to polish a .001 off a main and not dip it in the center. You might polish off .0003-4 and be ok if your machinist is careful. Get the machinist to use a new polishing belt the same size of the journal if possible. If that can be done then use the X bearings for the extra .001 clearance and you should be close to .003. On the mains I would not care if it was.0035. Mix and match the X and std. to get your desired clearances. It doesn't matter which side the X is on. Just try to keep them all on the same side is mixing.
Use a good oil Lucas Racing or Penn Grade in 10-40 or if in the heat of the summer even 20-50 depending on how long the engine is run. You could also put a oil temp gauge in. Oil usually runs about 25 degrees hotter than the coolant. I do not like the oil temp to stay above 240 much. Sometimes the situation arises you just have to keep running and hope nothing gets hurt regardless of the temp.
If the engine clearances are a little lose nobody will know. If they are tight everyone will know when you push the car back on the trailer.
Getting to your issue. Have you checked all your rod bore diameters? Are they all the same? How close to minimum or maximum size are they? Small block Chevy with 2.1 crankpin the rods bores should the 2.3247 to 2.3252.
If you have some rod bores larger than others could you put those on the tight crank pins? If the rod bores are at the 2.3247 could you open them up to the 2.3252 size to get some extra clearance?
Sorry, I just reread and saw the issue was the mains. I will let the above stay in case that is also a problem.
It is almost impossible to polish a .001 off a main and not dip it in the center. You might polish off .0003-4 and be ok if your machinist is careful. Get the machinist to use a new polishing belt the same size of the journal if possible. If that can be done then use the X bearings for the extra .001 clearance and you should be close to .003. On the mains I would not care if it was.0035. Mix and match the X and std. to get your desired clearances. It doesn't matter which side the X is on. Just try to keep them all on the same side is mixing.
Use a good oil Lucas Racing or Penn Grade in 10-40 or if in the heat of the summer even 20-50 depending on how long the engine is run. You could also put a oil temp gauge in. Oil usually runs about 25 degrees hotter than the coolant. I do not like the oil temp to stay above 240 much. Sometimes the situation arises you just have to keep running and hope nothing gets hurt regardless of the temp.
If the engine clearances are a little lose nobody will know. If they are tight everyone will know when you push the car back on the trailer.
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If you have a high quality machine shop that has crank grinding capability the mains could be turned .002 under, bearings are available in that size at least then you would have consistent clearances to work with Ive seen the shop I use do that on more that one occasion.
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