Resizing big end??

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Resizing big end??

Post by NXBOY »

How much will have to be taken off rod to decrease .001 from perpendicular distance to the rod.
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Re: Resizing big end??

Post by NXBOY »

I want to get big end into specs for piece of mind and 800 HP. But if rods going to be .015 or so shorter then going another route. Wish i could remember geometry :roll:
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Re: Resizing big end??

Post by BillK »

I rarely have to take more than a couple of thousandths of the rod and cap. Any more than that and I will buy a new rod unless it is something very hard to find or not performance realated at all.
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Re: Resizing big end??

Post by NXBOY »

a couple will get ya one thousanths from crossways not up and down? might need a hair more to work with hone?
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Re: Resizing big end??

Post by rustbucket79 »

Are you asking how much has to be removed from the cap and rod mating surfaces to shrink the housing AT the parting line .001”?
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Re: Resizing big end??

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YES
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Re: Resizing big end??

Post by rustbucket79 »

It takes a bunch to shrink up the sides. Unless this is a critical application, oversize on the parting line is ok so long as 80 % or so of the housing is within specification. IIRC Reher-Morrison used to clamp the rods along the the parting line to intentionally make it big in that area so the bearings don’t pinch the crank at high RPM.
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Re: Resizing big end??

Post by modok »

45 degrees, so x 1.4 (aka divide by the sine of 45)

.0014 removed at parting to get .001 smaller at 45 degrees

In reality... even a very small change the the angle or shape of the parting surfaces can have a dramatic impact on the roundness of the hole.
On a top quality rod, aka, in your imagination.....these parting surfaces are PERFECTLY flat, in the same plane, and exactly 90 degrees to the rod centerline....but
Well they aren't perfectly flat, there is some small degree of grinder push-off, or drop off near the edges of the parting surfaces, often they are not exactly 90 degrees, there is some surface texture on the surface, some tool marks.....
The parting surface will have to be ground in the same WAY and direction as it was originally ground, in order to shrink the rod bore in a totally predictable way.

Just any given rod on any given day, cutting the parting surfaces JUST the right way to produce what you want to happen, is a combination of luck and skill. Re-sizing rods that are just out of shape from normal use, I would expect the center to center length of the rods will end up .002"-.004" shorter.
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Re: Resizing big end??

Post by modok »

I could SWEAR an above post said 45 degrees.....now it says perpendicular?
Well whatever, to shrink it AT the parting, INFINITY....... you can never shrink the parting back in without bending the rod

So, angle cut it, or lets change this question back to 45 degrees, or 60 degrees, not 90

But from what I said above you might get an idea what angle cutting would mean, and that shrinking ht parting surfaces is not required anyway.
Measure your bearing thickness near the parting surface, what is the clearance going to be there?..........
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Re: Resizing big end??

Post by mag2555 »

Where dealing with a round bore so does not the change rate of 3.14 x amount apply here?
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Re: Resizing big end??

Post by modok »

If connecting rods stayed round we would not have to resize them.
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Re: Resizing big end??

Post by novafornow »

This brings up a question that I raised a while back, and would like some input from guys that make a living at this. I check my big ends at freshen with a dial bore gauge. I am a life long industrial machinist, so I am confident in my numbers. Last freshen I had aprox. .0002" out of round, I went ahead and ran with it with no ill effect. But the burning question for me is at what amount of out of round does one resize? .0005"? Was I wrong to use them again at 0.0002"? Hope for a response and thanks in advance.
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Re: Resizing big end??

Post by lance flake »

novafornow wrote: Fri Oct 22, 2021 10:16 am This brings up a question that I raised a while back, and would like some input from guys that make a living at this. I check my big ends at freshen with a dial bore gauge. I am a life long industrial machinist, so I am confident in my numbers. Last freshen I had aprox. .0002" out of round, I went ahead and ran with it with no ill effect. But the burning question for me is at what amount of out of round does one resize? .0005"? Was I wrong to use them again at 0.0002"? Hope for a response and thanks in advance.
Clevite book says .001 out of round as long as horizonal (parting line) is larger than vertical.
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Re: Resizing big end??

Post by PackardV8 »

I had a long discussion with a Clevite engineer on resizing big end bores and measurements thereof; he said, "Don't worry about making the hole round and don't even measure the parting line. Concentrate on getting the verticle dimension exact and the rest will be what it will be."

Interestingly, the powdered-metal-cracked-cap rods all the old-school guys were questioning when they first came along, the bores are perfectly round.
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Re: Resizing big end??

Post by modok »

only .0002" out of round is fantastic
It depends how tight the clearance is.....but if the average size of the housing is in spec, AND if the clearance is reasonable all the way around, then I'd run it. Now, some people only check clearance in one spot so maybe this is a more advanced concept.

.0005 tighter vertical VS horizontal is common, and probably acceptable.
.0005 misalignment is common, soon as you loosened the bolts, the cap just won't go on the how it was before, and that alone is not big deal.

But if you have both....... then you in a "bind".
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