I've heard conflicting numbers on the actual amount of crush typical for rod bearings, I've heard a few tenths to a few thousandths. What is the typical crush in a typical high performance rod bearing? Is there enough to be able to open up bearing clearance by honing the big end, sacrificing some crush or is that a no-no because the crush amount is too little to play with?
BTW, I was looking at my measurements of a long gone engine built with new Eagle Chevy rods and noticed the big ends were typically around 2.2255 which seems a bit big, should I have worried?
How much do bearings crush?
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Re: How much do bearings crush?
"Crush" is what keeps the bearing in place. It depends on the OD of the bearing and the ID of the bore. The ends of the bearing butt up against one another before the cap seats because the bearing halves are not perfectly semicircular. Crush is kind of misnamed, I think of it more as a linear measurement than a force - it is the excess length of the bearing protruding above the saddle before the cap is torqued. Before you torque the cap, the diameter of the bearing ID measured on the parting line will be greater than the diameter measured 90 degees to the parting line. When the cap is torqued, it forces the bearing to become "rounder" - the difference between the parting line ID and the ID at 90 degrees is decreased. This creates a force which presses the OD of the bearing against the ID of the bore and holds it in place.Barbapapa wrote:I've heard conflicting numbers on the actual amount of crush typical for rod bearings, I've heard a few tenths to a few thousandths. What is the typical crush in a typical high performance rod bearing? Is there enough to be able to open up bearing clearance by honing the big end, sacrificing some crush or is that a no-no because the crush amount is too little to play with?
BTW, I was looking at my measurements of a long gone engine built with new Eagle Chevy rods and noticed the big ends were typically around 2.2255 which seems a bit big, should I have worried?
So crush, as measured, is not a force. I am sure some of the engineer types could calculate the amount of outward force generated by torquing the cap on the bearing. As far how much you need, I guess you had enough if you never spin a bearing on that motor. Crush does not determine bearing clearance - that is mostly a consequence of the shaft diameter and the ID of the bearing. But it does determine the force between the bearing and the ID of the bore. Strange as it sounds, the bore diameter, within a reasonable range, does not change bearing clearance much. But you still want to keep it towards the small end of tolerance to prevent spun bearings as it changes crush. You adjust bearing clearance using bearing of different thickness (the bearings have the same OD, it is the ID which changes) or modifying the shaft (last resort).
On re-reading your question, I bet you already know this and were asking about crush measurement, not "what is crush". Typical is .003-.004", AFAIK. Now the real experts will come and clear this up
Richard
Rkrause, thanks. That's good info, I'd suspected something like .003-.004
Cammer, please spare me the do-a-search-noob bit, you shouldn't go there unless you know that the topic has been discussed. I read every thread from a "bearing crush" search and the only reference I saw the actual amount was a surprising tenths amount hence my new topic.
Cammer, please spare me the do-a-search-noob bit, you shouldn't go there unless you know that the topic has been discussed. I read every thread from a "bearing crush" search and the only reference I saw the actual amount was a surprising tenths amount hence my new topic.
Mike S.