First choice in aluminum alloy has traditionally been either 2024 ( T-4 or T-6 temper), or 6061 T-6. 7075 T-6 is also good with the highest cold strength, but poorer elevated temp strength. The newer 7068 alloy has even better strength numbers but I haven't been able to locate a source for this in usable sizes and reasonable cost. Either small round rod or flat plate/bar can be used.
Hardened steel rods were used by Picco in the fairly recent past on his R/C boat engines, and his personal 10 cc tether car engine that holds the World record of 338+ km/h is using this with a home-made cage, and fairly large (and few) rollers. Others are also contemplating using this system.
OPS has featured a Dooling/ YJ .61 style aluminum rod with a thin hardened steel race pressed in the bottom, and using small dia. loose needles. 6-4 Titanium can also be used for needle bearing rods, and has also been used for bushed rods with some success. (TITAN RODS, Terry Keeley, IMPBA- Boats)
First choice for bushing material is B-544, (B-2) leaded phosphor bronze, @88% Cu, 3.5~4.5% Pb, .10%Fe, 3.5~4.5% Sn, 1.5~4.5% Zn, .10~.50% P. numbers 532, 534, & 545 are similar. This is what Willy [Bill Wisnewski] always used at K&B. Another one you might try is one of the Be-Cu alloys #170, 172, 175. Or the special Brush-Wellman 10% Be alloy. (M-10?) Very hard to get, and the hazard of the high Be content. Most of the model boat props are 1~2% Be-Cu. Willy also experimented with Be-Al alloys (lock-a-loy?) but I don't have any data or source on that. Also used it for rods w/o bushings. Some of the (MANY) other choices are various, bronzes, brasses, and coppers. 390 SI-AL has also been used.
Adequate section thickness need to be used, especially in the bushing wall thickness. Sharp corners should be avoided by using a small radius on the lathe/mill cutting tool tip, and a finer/slower feed rate.
The newer "streamlined" or diamond beam cross-section is worth the extra effort for racing and high performance engines. Oil holes/slits for lubrication have had MANY design variations, but need to be carefully deburred with a countersink. A modern CAD-CAM System will greatly simplify designing and programming these parts for manufacture. A CNC mill or machining center will allow the use of "paper dolling" the parts to the extent of the machine travel, and speed up production, due to the reduced need for tool changes.
A Digital Read-Out (DRO) will save a lot of time here on your milling machine! Aluminum plate needs to be aligned with the grain running in the long direction, end to end, for best strength. Bushing holes should be center-drilled, drilled and reamed with SHARP tools, and COOLANT to produce clean, uniform hole sizes, that are perfectly parallel with each other, end to end. After the plate has all the holes in it, remove it and use a small arbor press to press in the bronze bushings.
Bushing ODs should be kept to within .0002"~.0003" size range on your lathe. IDs should be minus .002"~.004", and CENTERED!!! Loctite is optional here, but some kind of lubrication is necessary to avoid galling. A .002"~.003" LIGHT PRESS FIT is desired. You should feel the aluminum "stretch"’ a little when installing the bushings.
Bushings with thinner than @ .030" wall will collapse and NOT hold the necessary ‘crush’ to keep them from spinning when run. Too tight a press fit will excessively stretch and fatigue the aluminum and lead to premature fracture, usually of the big end, with catastrophic results. After the bushings have been installed; Go back and mill the excess material on both sides of the beam. ( 2 ops.) Ream to ID of both bushings to @ minus .001" (undersize). Then cut apart and profile mill the ends and sides (2 ops.) A lathe can also be used to turn the front and back sides, of the top and bottom ends, with a simple fixture held in a collet or 4 jaw chuck.
After finishing machining the profile, drill the oil holes and then tumble in a small rock tumbler to remove all remaining sharp edges. Then it should be lightly glass-beaded to remove residual stress.
Final honing of the bushing to the desired size and surface finish is done last. Top bushing clearance should be less than @ >.001". Check the wrist-pin for size, taper, and roundness! Bottom bushing clearance runs from @ .002" ~ .005". A fairly fine stone should be used. A Sunnen J-83 (400 grit) stone is suggest as a starting point.
On used engines: Mike the crankpin for size, roundness and taper wear, at 3 places @ 120 degrees apart, starting at the 12 o'clock position BEFORE making the bushings! More than .0005" Out of round, tapered, or more than .001" under-size calls for replacement/ rebuild....
The big O S .82’s & .91’s Have SOFT crankpins, and show wear approaching > .001"/hour in ducted fan and boat applications. Some of the larger Picco engines also show this problem. Possible solutions are to hard chrome or nickel plate the crankpins, or do as Rossi did some time back and use one of the newer TIN of similar cutting-tool coatings.
Chrome-plated steel parts need to be baked at @ 375~400 F for 2 hours immediately after plating, to take out the hydrogen embrittlement.
The pins should then be ground to a fine finish in an offset grinding fixture, although polishing and lapping can achieve the desired results, IF care is taken to measure the roundness/taper, and get it within tolerance. Another alternative is to use a harder, press-in crankpin, like a gauge-pin or hardened drill-rod stock or pre-manufactured rollers. (Webras)