Construction details for a reliable high performance connecting rod

[Jim Allen]

Tim,

I do not think that it is always necessary to use a steel, roller type, connecting rod in a .46 size engine. Consider the fact that the connecting rod used in the MB-40 engine is aluminum (7075-T 651) & it is only bushed in the bottom end. This engine operates in the 33,000 to 35,000 RPM range. I'll post some detailed photos of the connecting rod & the inside of this unique engine when I return to the shop.

JA

 

[Jim Allen]

To keep the connecting rods as straight as possible during the hardening process, they are fastened between two heavy #1018 cold rolled steel blocks. The blocks & rods are wrapped in an airtight stainless steel bag (.002" thick, #309 stainless steel) to prevent decarburization, while holding the rods at the hardening temperature. S-7 requires a pre-heat of 1375* F & then an austenitizing temperature of 1760* F for 30 minutes. The rods are quenched in pre- heated (90* F) standard quenching oil. Use a minimum of 5 gallons.

When the correct time & temperature is reached (1775* F for 30 minutes), place the entire stainless steel bag in the hardening oil. Holes are immediately punched in the stainless bag to allow the hardening oil to touch the rod. "NO HARDENING WILL TAKE PLACE IF THE OIL DOES NOT TOUCH THE ROD!!" After cooling the rod to the temperature of the oil (90* F), temper at 400* F for 1 hour. Cool to room temperature & temper again at 400* F for 1 hour.

Close attention to the hardening, tempering process will produce a rod that has the necessary hardness of 58 Rc, with a 315,000 psi tensile strength & a 210,000 psi. yield strength.

Jim Allen

 

[Jim Allen]

One very important design feature of any connecting rod is the accurate alignment of the upper wrist pin hole to the lower crank pin hole, in two axes at the same time. One set of axes can be seen when looking at the connecting rod from a side view. The other set of axes can be seen when looking at the connecting rod from a top or bottom view. Only with the precise alignment of the two hole axes, to each other, can tight tolerances be used in the upper & lower ends! This type of precision becomes necessary when using a roller assembly in the bottom end that has a total radial clearance of .0002" to .0005" & an upper end radial clearance of .0001" to .0002".

A final internal grinding process, with a specially designed fixture, ensures the precise alignment in two axes, the precise size & the precise center distance of both holes at the same time with the internal grinding process. Notice in the first photo that the alignment pins for the location of the connecting rod's upper & lower holes can pass through the bottom of the grinding fixture's two .625" ID mounting holes after the connecting rod has been clamped in place with the small toe clamps shown. Once the connecting rod is mounted to the fixture, the connecting rod remains in place until both holes are ground to size. Since the two .625" mounting holes are bored in the fixture at the same time, these holes must be square to each other in two axes at the same time. The .625" stepped mounting pin mandrel is hardened & ground between centers. It is zeroed (+ or - .0001" total indicator run out) when mounted in a 5-c collet.

Jim Allen

 

[RaceMechaniX]

Jim do you need to fill the stainless bag with an inert gas before sealing it up?

 

[Jim Allen]

No inert gas needs to be used, but the stainless bag needs to be carefully sealed by folding it several times & clamping the folded part in a good milling machine vise. I have made these bags out of .002" thick stainless sheet by folding & sealing three sides as described above.

JA

 

[Jim Allen]

As promised, I have posted some photos of the connecting rod & piston used in the MB-40 pylon racing engine. I have seen this engine operate in the 33,000 to 38,000 RPM range without connecting rod problems, even though the top end is "NOT BUSHED". Notice the interesting profile of the connecting rod & it's very large size wrist pin. The size of wrist pins used in .45 cu in racing engines is 5 mm (.1969"). The wrist pin in the MB-40 is 6 mm (.2362"). Notice the extra machining of the piston bosses that helps to decrease the piston's total weight. Also notice the domed piston crown that helps direct the incoming gas charge in the correct direction & helps to cool the pistons crown from the heat of combustion.

Jim Allen

 

[brad nichols]

Excellent photo's Jim and beautiful work!!

Brad

 

[Jim Allen]

Two of the most important parts of any roller rod assembly is the retainer & its rollers. Metallurgy of the retainer & the rollers chosen for the bottom end must be carefully chosen because of the very high rotational speeds that are possible. High radial clearance amounts should be avoided because they encourage roller "skewing". Retainers which have high weights or mass should also be avoided because they encourage roller "skidding". Retainers & their rollers should be made of very hard wear resistant alloys.

Jim Allen

 

[Jim Allen]

A machined, hardened steel retainer has advantages that cannot be found in a standard "A" or "W" type punched steel retainer. The alloy selected for the retainer is VascoMax C-350 maraging steel. Typical applications for the C-350 maraging steel are missile & rocket motor cases, wind tunnel models, recoil springs, landing gear components, high performance shafting gears & fasteners. This steel achieves full properties (350,000 psi ultimate tensile strength. 340,000 psi yield strength, 388,000 psi compressive strength at 60 Rockwell "C") through a simple, low temperature heat treatment of 9 hours at 925* F for 6 hours. One important mechanical advantage found in a machined retainer is in its ability to guide rollers from end to end, ALONG THEIR CENTER LINE, from end to end, as compared to the above & below their center line in typical punched type cages. This feature prevents roller skewing! The very low weight of a machined retainer helps prevent roller skidding! These two advantages allow a precision machined retainer to operate at very high RPM's without typical punched retainer failures.

The photo shows part of the machining operation, done in a dividing head, to accurately cut the windows in a retainer. A later development of the single window retainer (one roller per window) to a double window retainer (two rollers per window) will be shown latter. This modification increased the load carrying capability of the roller assembly because it increased the number of rollers & it decreased the weight of the retainer itself. It did not decrease the maximum operating RPM of the assembly!

Jim Allen

 

[Terry Keeley]

Incredible!

 

[PaulHail]

Jim,

Have you ever tested a knife edged rod shape vs. an H beam? I know many kart engines used a knife edged shape and with a 100cc engine turning 20,000 rpm they can certainly hold up if properly designed.

It may have been more of a benefit given the reed induction and the path of the incoming charge being directed at the the face of the rod than our engines where the incoming charge may not be as affected due to it's path.

 

[Jim Allen]

Paul,

Your post made me think more about what I had initially posted about the connecting rod's material & design. My connecting rod is neither a knife edge or "H" beam type. It is an "I" beam type that is made of a hardened & tempered, high strength, shock resisting steel (AISI-S7). In the middle of the "I" beam the rod is .030" thick. When comparing rods of the same material & the same measurements, the "I" beam connecting rod will always be lighter in weight than a knife edge or "H" beam type connecting rod. This makes the "I" beam connecting rod more desirable for very high RPM applications because there will be less reciprocating weight. Knife edge or "H" beam connecting rods are more rigid & stiffer, however they are also heavier. Connecting rods operating at very high RPM's fail because of bending & high tensile loads, not because of bending & compressive loads.

The photo shows a steel connecting rod that failed at 38,000+ RPM. This is above the operating rpm of the engine! This engine used a .750" bore carburetor; an exhaust timing of 198*; a transfer timing of 130* & boost timing of 125*. The inverted drum valve opened at 30* ABDC & closed 70* ATDC. The tuned pipes tuned length was 8.75" & the fuel used was 65% nitro. The crankcase was cut to allow the engine to be disassembled!

Jim allen

 

[BobHorowitz]

Hi Jim,

Is the shop you are associated with located in Mineola, N.Y.? I believe it's called Aero Precision or something like that.

Regards,

Bob

 

[Jim Allen]

No Bob,

Aero Precision Machine, who are the builders of the Nelson 45 long stroke Q-40 & Q-500 pylon racing engines, are located in Liberty, NC.

Jim

 

[BobHorowitz]

Jim,

Nice shop and web site.

Regards,

Bob

 

[Jim Allen]

Bob,

The web site could not begin to show the amount of quality equipment & tools available in this facility. Two climate controlled buildings totalling 17,000 Sq ft & more than 13 modern CNC machine tools plus numerous pieces of support equipment make this shop an ideal facility for the manufacture of, precision built, high performance, miniature racing engines!!

Jim Allen

 

[BobHorowitz]

Jim,

I was kind of wondering about the presence of the old school equipment we've talked about in the past. Do you have a vacuum furnace for heat treating as well?

Bob

 

[Jim Allen]

No vacuum furnace but a Lucifer over, under oven with digital control. The oven can maintain + or - 5* F from 0 to 2200 * F. Both quenching oils (11 second & standard) are available.

JA

 

[Timbo Whalen]

Tim,

I do not think that it is always necessary to use a steel, roller type, connecting rod in a .46 size engine. Consider the fact that the connecting rod used in the MB-40 engine is aluminum (7075-T 651) & it is only bushed in the bottom end. This engine operates in the 33,000 to 35,000 RPM range. I'll post some detailed photos of the connecting rod & the inside of this unique engine when I return to the shop.

JA

Thanks, Jim... a bit of a late chime here, sorry... this addresses Mike's query, as well... the issue I see with the NR rod isn't composition/material processing related,

but rather clearance on the big end- we added .0015" clearance to the pin/bushing interface. Now, earlier production NR46DD's were kicking the rod out of them

on a fairly regular basis. I purchased my engine last June and it was latest revision, 'fresh' stock. What we like about NR is that their parts explosion drawings are

always updated and the latest revision dates are highlighted in bold.

Jim, as a retired developmental engineer, I always enjoy your posts...thanks a lot for that.

Regards-

tim

 

[Jim Allen]

Tim,

Many small, overlooked mechanical things, can contribute to connecting rod failure in smaller size, .21 cu in to .45 cu in, high performance racing engines operating in the 30,000 to 35,000 RPM range. Consider the following; some engine manufacturers are reaming the wrist pin holes in their piston bosses & connecting rod bushings. These holes should never be reamed!! The wrist pin holes should be carefully honed to size & their wrist pins pressed in, "AT LEAST" on one side! This means that the wrist pin holes in the piston must pass through with the wrist pin being held in place with two "C" clips. Precise machining of the "C" clip groove depth & the amount of end play (.003" to .005") is also important. The upper & lower connecting rod bushing holes should be honed to size, never reamed! In the upper end .0001" or .0002" total clearance is best with .003" to .004" total clearance in the bottom end. There is also a very distinct advantage to machining a slight taper on the crank pin to prevent the connecting rod from backing off. At Aero Precision Machine we have the necessary tooling & fixtures to precisely "HARD TURN" the crank pin, after it is hardened, instead of grinding it. All of the above things will contribute to an aluminum (7075-T651) connecting rod surviving the tension & thrust loads placed upon it at high RPM's.

Jim Allen