rod bearing clearance

[70mc]

I am working on a smallblock chevy and getting rod bearing clearances of .0045. I am looking for a clearance of .0025. I have a new crank that is measuring 2.099, in spec. but on small side, and new rods that are measuring 2.2252, in spec. but on the big side. I am using a micrometer and dial bore gauge to get these specs. I have tried 2 differant bearing brands,that came out to .0045. I have also tried a bearing that will give you .001 less and it came out to .0035. I am racing and need a chamferd bearing and can't find one that will give me .002 less clearance. I can't return the crank or the rods since both manufacturers say they are in spec. My question is if i have the rods reconditioned on the small side 2.2247 what will that do for my clearances. I dont think the answer is .0005 less because i think the bearing will exagerate the half thousandts.

 

[strokersix]

Why couldn't you use .002 bearings and alter them to clear the crankpin fillet?

 

[70mc]

I was very happy with the wear on the hiperformance line of bearings that came out of my old engine and would like to stick with that type of bearing, they are a little tougher. The engine was together for 3 years and looked like the bearings were new.

 

[enginebob]

Are you torqueing the rod caps? How are you setting the bore gauge? If still the same. You could go with a coating that will shrink them.

 

[patprimmer]

Can you cut and shut the rods to absolute minimum so as to get maximum bearing crush, or even a bit tighter than minimum dia spec, then polish the parting line of the bearing to reduce crush back to tight end of spec.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[70mc]

Yes, the rods are torqued to spec., and the mic.was checked against a standard. The crank then measured with mic. and the dial bore gauge zeroed out to that spec.(2.099), then the rod measured 90 degrees to parting line.

I built an engine once that had new rods and a turned crank, when the rod bearing clearance was checked it was .003 a little more than i like but i ran it. When it came time for a rebuild i asked for the rods to be reconditioned to the small side of spec. and then the clearances came out to .002, but i never checked the id. of the rods before or after,when i built that engine, just the bearing clearance. I know it makes a differance,but i would like to know how much smaller .0005 in the id. of the rod will do to my bearing clearance. Would like to hear from some shops that have done this and know what differance it will make. Thanks everyone.

 

[patprimmer]

I have never done back to back tests on bearing crush, as I never had a crank that was 0.001" under min spec.

I have always found that with crank spot on middle of spec, and rods on min spec (maximum crush, and 0.001" OD bearings, I have always ended up with between 0.002 and 0.003" clearance in the axis of the rod direction.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strokersix]

70mc,

I have not personally done this but I expect a decent approximate answer to your question can be estimated by comparing the relative structural stiffness (thickness) of the rod compared to the bearing shell. If the rod is four times thicker than the bearing shell then I would expect a .0005 inch rod bore change to effect a .0004 journal clearance change.

If you need to know closer than this you'll either have to test it or perhaps a bearing manufacturer could answer the question.

 

[enginebob]

Lately it is a quality issue with bearings. Probably from out sourcing. And metric english conversion etc.
Check the bore that the bearings fit in if they are on spec. I'd take the bearings back. Try another manufacture.

 

[SMOKEY44211]

70mc I've run into the problem you described. I've used the method Patprimmer suggested with good results, however it is very time consuming. It seems all the traditional bearing manufacturers have merged under one umbrella so there's not a whole lot of choices left any longer. For what its worth if you reduce the rod housing .0005 you'll reduce the bearing clearance the same amount till you get to the low side of the tolerance. Furthur reduction either buckles the bearing or prevents the parting sufaces from contacting one another. The only other alternative is go to the next undersize, gage the bearings and have the crank ground to provide the desired clearance.-----------Phil