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Old 04-17-2003, 06:48 AM   #1 (permalink)
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Connecting rod side clearance... How much is too much?

In regards to connecting rod side clearance, what are the limits beyond the manufacturers specifications that can the be exceeded and if they are, what are the detriments or negatives that can be expected.

I've throwed out the "urban legend" that the bearing runs cooler by virtue of increased side clearance by doing the math of the exposed bearing surface area versus the surface area of the exposed rod sides. The stock rod side clearance already exceeds by a significant margin the amount of the four exposed bearing clearances on each V8 journal. This would mean no increase in oil at the rod sides as the oil flow is already being restricted by the bearing clearances.

Anyone have any thoughts on this subject?

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Old 04-17-2003, 07:20 AM   #2 (permalink)
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Connecting rod side clearance... How much is too much?

On steel rods, my thinking is over .035" is too much aluminum rods over .065" is too much.

I believe you would run into oil pressure problems (low pressure) with too much side clearance. Also think you would get into oil foaming/frothing also.
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Old 04-17-2003, 09:26 AM   #3 (permalink)
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Connecting rod side clearance... How much is too much?

On steel rods, I like .007-.015 and aluminum rods .025-.030. However on piston guided rods (NASCAR style) the side clearance can be over .100.
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Old 04-17-2003, 09:43 AM   #4 (permalink)
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Connecting rod side clearance... How much is too much?

I see what Tedd is asking....

General conception indicates that not enough side clearance restricts the flow of oil out of the rod bearings. Too much side clearance allows the oil to run out of the bearings too quickly... killing oil pressure.

When bearing clearances are only around 0.001" to 0.003", how can rod side clearance affect oil pressure when they are generally around 0.020"???

It's like letting water out of a 2" pipe flow into a 6" unit, and saying the 6" pipe causes a restriction!

He's thinking that unless the side clearance is REALLY tight, it shouldn't restrict the oil flow... and there's pretty much so such thing as too much side clearance. (as far as oil pressure is concerned)

But, there are indicated maximums and minimums. How do 'they' come up with these figures?

Hmmmmm....

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[ This Message was edited by: n2omike on 4/17/03 11:28pm ]
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Old 04-17-2003, 10:10 AM   #5 (permalink)
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Connecting rod side clearance... How much is too much?

I had problems with mine at .030. Clearances between rods are like running nitrous, if you do it by the book, you shouldn't have a problem.
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Old 04-17-2003, 08:47 PM   #6 (permalink)
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Connecting rod side clearance... How much is too much?

qtrmiler,
What problems did you experience at being 0.030" side clearance. The reason I bring this up is I've run into an engine where after multiple times being turned on the cranksshaft and the rods being resized several times, the clearance ended up being 0.045" on all the journals. Against my better judgement, I put it together and it doesn't seem to be giving any problems what so ever. In fact, the engine runs and performs extremely well and is in a street car.

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Old 04-17-2003, 08:50 PM   #7 (permalink)
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Connecting rod side clearance... How much is too much?

Personally, I've never built an engine where the side clearances were in spec. Never been a problem either, even with floating pins.
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Old 04-17-2003, 09:01 PM   #8 (permalink)
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Connecting rod side clearance... How much is too much?

I ran .030 & a loose clearance. Had 20 psi all the time! I've since tightened up the tolerances & run about .025 & my engine looks like it's never been run
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Old 04-18-2003, 08:54 PM   #9 (permalink)
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Connecting rod side clearance... How much is too much?

Does anyone have an idea how much connecting rod side clearance is being run on the NASCAR engines. I've been told it's "excessive" but can't seem to get a real number.

As a side note: That engine I put together with 0.045" side clearance still had good oil pressure but the bearing clearances were where they belonged. I've ran the opposite in the past where the bearing clearances were loose (0.0045) and the side clearances were tight (<0.020") and the oil pressure was pretty low at idle but pressure came back up as engine rpms went up.
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Old 01-01-2010, 12:58 PM   #10 (permalink)
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Re: Connecting rod side clearance... How much is too much?

Quote:
Originally Posted by tedd View Post
In regards to connecting rod side clearance, what are the limits beyond the manufacturers specifications that can the be exceeded and if they are, what are the detriments or negatives that can be expected.

I've throwed out the "urban legend" that the bearing runs cooler by virtue of increased side clearance by doing the math of the exposed bearing surface area versus the surface area of the exposed rod sides. The stock rod side clearance already exceeds by a significant margin the amount of the four exposed bearing clearances on each V8 journal. This would mean no increase in oil at the rod sides as the oil flow is already being restricted by the bearing clearances.

Anyone have any thoughts on this subject?
if its the hottest part of the engine or is it, i now have just put together the bottom end and come to find out i have not enough side clearance, where it restricts rotation, i guess i should of miced it before i installed it, first i thought it was the free floating pistons, but the first set of torque @ 24lbs i have come up with restriction of rotation.
if the crankjournals are too small then the machine shop needs to fix it?
but if the Scat rods are too wide do i ship them back and wait?
or do i have the machine shop make the adjustments?
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Old 01-01-2010, 01:28 PM   #11 (permalink)
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Re: Connecting rod side clearance... How much is too much?

Rod side clearence has nothing to do with oil restriction/Oil PSI. That's controlled by the bearing clearence. Side clearence is for expandation and oil to get out of the bearing area and oil the rod sides. Also so that the crand/rod rotation can sling oil on the cam/lifters and not having them rub against each other and the crank. It also centers the piston pin and keeps it from moving to much. It has to have some movement so as to get oiled. Full floating piston pins are locked into position within a small tolerence and the side clearence on the rod keeps the rod center in the piston pin.
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Old 01-01-2010, 06:49 PM   #12 (permalink)
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Re: Connecting rod side clearance... How much is too much?

Quote:
Originally Posted by BigLeo69 View Post
if its the hottest part of the engine or is it, i now have just put together the bottom end and come to find out i have not enough side clearance, where it restricts rotation, i guess i should of miced it before i installed it, first i thought it was the free floating pistons, but the first set of torque @ 24lbs i have come up with restriction of rotation.
if the crankjournals are too small then the machine shop needs to fix it?
but if the Scat rods are too wide do i ship them back and wait?
or do i have the machine shop make the adjustments?
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I was told that if I had rod side clearance issues to sand down the sides of the rods by hand... I was trying to hold a .015-.020 tolerance. all came up at around .018 so i didnt have to sand anything. If you dont have any room then i would double check things out.
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Old 01-02-2010, 03:51 AM   #13 (permalink)
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Re: Connecting rod side clearance... How much is too much?

Quote:
Originally Posted by BigLeo69 View Post
if its the hottest part of the engine or is it, i now have just put together the bottom end and come to find out i have not enough side clearance, where it restricts rotation, i guess i should of miced it before i installed it, first i thought it was the free floating pistons, but the first set of torque @ 24lbs i have come up with restriction of rotation.
if the crankjournals are too small then the machine shop needs to fix it?
but if the Scat rods are too wide do i ship them back and wait?
or do i have the machine shop make the adjustments?
HAPPY NEW YEAR>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Examine the large end of the rods. You will find that one side of the bearing surface has a taper (chamfer) to it. This taper should always face the un-finished portion of the throw of the crank. The tapered end should never be butted up against another rod end. On a SBF, the left hand cylinders (1-4) the taper will be aligned with the front of the crank journal (engine front.) For cylinders 5-8, the taper will be aligned with the rear of the crank journal (engine rear.) Do not get this messed up!!!

Larger chamfer that faces the crank:



Small chamfer that faces the adjoining rod on the same throw:

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Old 01-02-2010, 05:18 AM   #14 (permalink)
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Re: Connecting rod side clearance... How much is too much?

i found this old thread thought it was interesting
i do have my rods in sequence with the chamfer at the crankside
i did this orientation and marked them when i installed my wrist pins.and all the rod #'s are aligned.
the rods are chevy 6.2 for this stroker combo, the machine shop offset ground my rod journals for 3.640 stroke and chevy big rod end @ .940
times 2 plus oil clearance, i hope???
a min. of .010 clearance, i will check this today seeing how i was just not into pulling the rod caps apart yesterday, thats the hard part on the scat rods with the collet for alignment. any ideas to make that easier to remove?
i saw a description from Circle Track that they leave the feeler gauge between the 2 rods during the torque sequence. i may try that today,
i have checked oil clearance and its within tolerance, just maybe some debris is all it is, the 2 rears had rotation till the last 2 fronts were torqued.
will get this going early before its BEER:30. any ideas would be and is appreciated Leo
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Old 01-02-2010, 05:35 AM   #15 (permalink)
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Re: Connecting rod side clearance... How much is too much?

Quote:
Originally Posted by BigLeo69 View Post
leave the feeler gauge between the 2 rods during the torque sequence.
this will also help against twisting of the rod when you tighten them up, so you get more accurate tq numbers.
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