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  #1  
Old 24th September 2013
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Default Connecting rod play

What is an acceptable amount of connecting rod play on my 93 XL? Specifically when I grab the top of the rod and tilt it side to side. The rear rod tilts very little if at all in this manner, and the front one tilts more, perhaps 1/8" but I haven't measured it. Neither rod has vertical play and I haven't measured the side-to-side movement of the big end on the crank.

Thanks for your help!
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  #2  
Old 24th September 2013
NRHS Sales NRHS Sales is offline
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My rule of thumb is as long as there is no up and down(vertical) then I am okay with it. The factory service manual does give side play measurements though if it really worries you. I normally only worry about side play if I am changing a crank pin as then it is a critical measurement.
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  #3  
Old 24th September 2013
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aswracing aswracing is offline
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Originally Posted by Justin Aten View Post
What is an acceptable amount of connecting rod play on my 93 XL? Specifically when I grab the top of the rod and tilt it side to side. The rear rod tilts very little if at all in this manner, and the front one tilts more, perhaps 1/8" but I haven't measured it. Neither rod has vertical play and I haven't measured the side-to-side movement of the big end on the crank.

Thanks for your help!
Justin, what you're feeling really doesn't have anything to do with connecting rod side play. Side play is measured at the big end and only specified for the rear rod. If you want to stick a feeler gauge down there, between the rod and the flywheel, the spec is .005 to .0025 with a .030 service wear limit. That's just about the only bottom end measurement you can do when you've just got the top end off.

The small end moves side to side like that due to the running clearance of the big end roller bearings. The more clearance you have on the big end, the more the small end will go side to side like that. The rear rod moves a lot less because it's got two bearings down there. You don't tip side to side nearly so easily when you spread your feet apart.

I suppose if you really got clever, you could calculate out the big end clearance based on how much the small end moves side to side. Dust off your trig tables. But that's not the way they spec it ... instead they spec the actual running clearance in the bearing, as determined by measuring the individual components. Basically, you've got a rod big end that's supposed to measure 1.6250 i.d., two rollers at .1875 o.d. each, and a crankpin at 1.2490 o.d. Do the math on that and it comes out to .0010 of clearance ... the spec is .0004 to .0017 with a .0027 service wear limit. But how much side to side in the small end correlates to how much clearance on the big end, I dunno, I'd have to work it out.
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  #4  
Old 25th September 2013
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Originally Posted by aswracing View Post
The rear rod moves a lot less because it's got two bearings down there. You don't tip side to side nearly so easily when you spread your feet apart.
Great explanation of normal less side to side movement of the rear forked (on the big end) connecting rod vs the front connecting rod which fits between the forked rod with only one bearing on the big end.
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  #5  
Old 27th August 2018
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I will give this a really rough trigonometric estimate. The male rod rollers are 0.480" wide and the service wear limit is 0.0027" clearance. When the rod is rocked to the side one side of the top most roller will be pinched to zero clearance and the other side of that roller will have a gap that approximates the bearing clearance.

Solving for the angle the rod is allowed to rock:
Angle = arcsin(0.0027/0.480) = 0.322 degrees * 2 (rocked right to left)

Finding the corresponding displacement at the small end of the 6.926" long rod.
Rod Shake = 6.926*sin(0.644) = 0.078" or about 5/64"

Tabulated for different clearances

Clearance Shake Nearest Fraction
0.0004" 0.012" 1/64"
0.0010" 0.029" 1/32"
0.0017" 0.049" 3/64"
0.0027" 0.078" 5/64"
0.0033" 0.095" 3/32"

I calculated it a few different ways and got similar answers.
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  #6  
Old 27th August 2018
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one ?
would you not factor in the true axis? a 6.926 is the total length of the rod.
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Old 27th August 2018
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I got my rod length from here: http://victorylibrary.com/tech/rod-c.htm

It says that the length is from the center of the pin bushing to the center of the big end race. I reason that the big end will pivot approximately around the center of its axis.

The calculations assume that the rollers do not shift on the pin. I tried other calculations where the rollers stayed with the race but the results were only different by 0.001" of shake.

I'm not sure that I buy into these calculations wholly. For example a 1/8" shake at the pin would require over 0.004" of clearance in the bearing! That's huge and I would imagine that anyone paying attention would notice vertical slop at that level. Many report to not feel any vertical movement with 1/8" of shake and old Harley service manuals would allow for 11/64" of shake in the front rod.
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Old 27th August 2018
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One thing I did not calculate in the approximation above is the contribution of side play in the bearing assembly to perceived rod shake.

The service manual for an EVO sportster gives the side play range as 0.005"-0.025" with a 0.030" wear limit. We will assume that side play shows itself as additional rod end displacement. The side play will be reduced as the rods are angled and the races move closer to the thrust washers.

An estimate of the reduction of side play at 0.644 degrees of rod shake corresponding to 0.0027" of bearing clearance (the service wear limit):

1.625/2*sin(0.644) = 0.009"

The worst case rod shake for a crank at the service wear limit in both bearing clearance and side play would be:

0.078" (rod shake) + 0.030" (side play) - 0.009" (side play reduction due to angle of rod) = 0.099" or just over 3/32" rod shake.

To throw a huge monkey wrench in things, I've read some posts from ol' timers that when Harley switched to the aluminum roller cages, that they also switched to slightly barrel shaped rollers which better accommodate misalignment of the rods and cylinders. If this is true then my calculations are going to be on the super tight side of things.

The same ol' timer made the recommendation that 1/32" Rear, 3/32" Front of shake with no detectable up/down play is good in his book so that's what I am personally going to use.
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  #9  
Old 3rd September 2018
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basically what the iron fsm says. haven't ran across the barrel rollers though, but a good question.
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