connecting rod wear & side play

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connecting rod wear & side play

Postby xlh59 » Sun Dec 26, 2021 7:00 am

Dear All,

while waiting for warmer weather to continue the finetuning of my 59 engine, I am preparing parts for the next projects -- currently I want to assemble two crank sets and I came up with a very general question.

I know that the connecting rod play is measured in different ways and that the main point of interest is the radial play, hence the wear of the rollers and races. Still, there are values for "pure" side play of the rods, measured with a feeler gauge. So here is my question:
The tolerances specified is for the side play, not the connecting rod width -- the flywheel washer are available in different variants (with/without recess) and thicknesses. Is there a minimum width of the lower female rod or is only the side play relevant and should be set with thicker washers? (Of course the width must be wide enough, that the rollers run fully in the races). That beeing asked, what happens if the width of the female rod is not equal (e.g. one side 0.02" smaller than the other side)?

I know, that a connecting rod is not that expensive and that I shouldn´t mess around with the bottom end. But having an almost perfect (still in its original assembly) set of KH flywheels, besides the reduced width of 1.46" of the female rod and the option to have the side faces grinded perfectly parallel and rectangular to the rollers, I am considering to re-use these rods with thicker flywheel washers ...

A friend of mine, a quite experianced engine builder (Indian, Flatheads, Knuckleheads) mentioned, he wouldn´t worry to use the rods with thicker washers -- still, not an expert with Ks and Ironheads, I hope for the well educated feedback from this group! :D

Have a nice "rest-christmas", and thanks in advance

Ralf
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Re: connecting rod wear & side play

Postby dr dick » Sat Jan 01, 2022 7:50 am

one side of rear rod being -.020" isnt a big deal.
in a perfect world the column axis of the i beam of the rod would be symmetrical to the bearing centers of both big end bearings.
the small end would be also symmetrical to the i beam.
if (a big if) we assume that the wristpin and its rod bush always is also perfectly normal to the ibeam, and the bigend bearing races are also perfectly normal there will be no side thrust.

in the real operation everything is elastic from the piston to the crankcases that support the main crank bearings. so no "real" symmetry can exist. that leads to side thrust at bigend of rod. this side thrust is 99% of the time to one side or the other. the big end of rod doesnt show equal wear nor does the thrust areas of the flywheels. best you can do is make sure the length of the roller cages in less than the width of the rear rod, and that the rollers cant run off the race edges in the extreme case.

live within the flex.
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Re: connecting rod wear & side play

Postby xlh59 » Mon Jan 03, 2022 2:20 pm

Thanks a lot for your reply Dr.

I assumed that there are no specific requirements in regards to offset from the middle axis as long as the rollers do not run over the edge of the race ... after the general "go" I wanted to figure out the details of the position of the rollers.

First I found five different designs:

* The KH 54-56 seems to have steel roller cages with the identical design of later aluminum cages -- the flywheel washers are flat on both sides
* The early Ironheads (up to 71) seem to have longer steel roller cages in a different design (especially the middle one) -- flywheel washer have a recess on one side to compensate the longer cages
* The late Ironheads (72 and later) have the same design as the KHs, just the roller cages are now aluminum
* early S&S flywheels do have, again, identical design with aluminum cages and flat flywheel washers
* later S&S flywheels have aluminum cages but no flywheel washers

I think that a read sometime, that the 57-71 steel roller cages are the prefered choice for performance engines, but the fact that even the late S&S flywheels use aluminum cages seems to be a indicator, that all three designs are fine.

Looking at my initial request, I made a drawing to check the position of the rollers under "normal" operation and with worn rods. My drawings have a few flaws, though. The width are exact values, but the overall rod design is of course rather rough guess. A relevant aspect is, that the races of the female rod are a tad wider than the width of the rod sides itself, hence the "edge" of the races is a bit shifted to the middle of the rods. Additionally the rod cages should be smaller in diameter ...

Rod Theory.PNG
Rod Theory.PNG (44.84 KiB) Viewed 1026 times


On the left side you see the version without recess in the flywheel washers. The position is a "worst case scenario": The rod has moved to one side whilst the rollers have moved to the other side - I think this is rather not likely to happen. But it gives a good impression on whats happening. Left side has 0.03" rod side play with a perfect rod. Look at the yellow circle, it shows you where the rod cages change.
The middle one is with 0.03" wear on the rod and identical side play (set by a thicker flywheel washer) -- interestingly, the yellow circle is now in an even better position. On the other hand, the gap at the red arrow became smaller, hence, as long the side play of the roller cages is not to small, the worn rod has no impact on functionality of the flywheel setup.

Does this make sense or did I miss something?

Best, Ralf
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Re: connecting rod wear & side play

Postby EKHKHK56 » Sun Jan 09, 2022 10:59 pm

On the KRs my friend Jay Lawless found out from the Factory that they run double big end side clearance to allow more “fresh” oil. Racing his KRTT in the 50s the oil would cook in the bearing and failure followed at long tracks. It took awhile but he finally found out what the Factory was doing. Willie G purchased one of Jay’s restored KRs for his personal collection. Jay revamped 4 beautiful bikes, 2 KR, KRTT and an XLR. 90s. H-D sent a truck up here to bring it home.
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