Construction details for a reliable high performance connecting rod

[Andy Brown]

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

"There is also a very distinct advantage to machining a slight taper on the crank pin to prevent the connecting rod from backing off."

Jim, Do I understand correctly, that the crank pin is a larger diameter at the end then it is where it meets the crank wheel?

Thanks for taking the time to post your vast knowledge.

Andy

 

[Jim Allen]

Andy,

That is correct. The total taper amount is .0002" to .0003" over a distance of .256". There is also a .0469" radius machined on the inside corner & the outside edge of the crank pin. Any & all machining burrs are removed prior to the flow ramp being glued in. We have also moved the oil hole in the bottom of the connecting rod to a position where it will not be affected by the rods compression loads.

JA

 

[Timbo Whalen]

Jim-

Very interesting, indeed!... in all due honesty, that's the first I've heard of tapering the crank pin, and it makes perfect sense.

It seems that the majority of these drum rotor marine engines have the pin closed at the end because it engages the rotor via

a small pocket to drive the drum...and then the backside face of the rod seems to thrust against the rotor to a degree. I suppose the float of the rod

on the wrist pin would determine by how much. Like most things in life, finding the happy balance is the key.

A reamer somewhat 'peels', or shaves, the material and it's clear how honing is the much better route.

We just happened to have a pretty good assortment of small diamond hones from the slot racing days that got the 46DD bushing to .0035" TIR-

at least the best we would could measure it. We used a Starrett 3809A indicator to validate and we believe it's well within .0005" max.

The fit was checked before and after... however, the means by which we fixtured everything to hone is MacGyvered... the rod 'rock' on either

side of perpendicular is acceptable, on one plane... hopefully it's similar on the adjacent plane.

If it stays together, we'll know we were close...lol!

A quick question regarding the rod above you mentioned- on the oil hole relocation, did you orient the hole in such a way as to force feed

the lubrication to maximum effect, relative to crank rotation, and center it mid way between perpendicular and along rod beam c/l ?... seems

like the safest place to be to stay away from tension/compression loading, no?...

Thank you, and again, we really enjoy reading your threads.

tim

 

[Jim Allen]

Tim,

"A quick question regarding the rod above you mentioned- on the oil hole relocation, did you orient the hole in such a way as to force feed

the lubrication to maximum effect, relative to crank rotation, and center it mid way between perpendicular and along rod beam c/l ?"

Our intention in moving the oil hole was not what you posted because the hole is still facing upwards. Some customers stated that they were seeing some egg shaping of the bottom side bushing after many hours of use. This could come from ordinary metal fatigue. One thing that seems to increase the lubrication at this point is to place a chamfer on the hole's entrance & exit point. The angled hole's exit point on the inside of the bushing is carefully chamfered with an Exacto knife. There are some front intake engines that have an additional hole drilled through the crank pin, at an angle, into the front intake to provide additional lubrication to the bottom end bushing. All of these things can help the connecting rod to survive, however, none of these things can be substituted for the ability to "MACHINE THE CORRECT NUMBER".

Jim Allen

 

[Andy Brown]

Andy,

That is correct. The total taper amount is .0002" to .0003" over a distance of .256". There is also a .0469" radius machined on the inside corner & the outside edge of the crank pin. Any & all machining burrs are removed prior to the flow ramp being glued in. We have also moved the oil hole in the bottom of the connecting rod to a position where it will not be affected by the rods compression loads.

JA

Great Jim. That is a method of compensation I did not think of. I did think of boring the crankcase to less than 90 degrees between the cylinder and crankshaft. I talked to CMB about doing that in the MAC engines. The rod needles always run on their ends and the rod is forced into the valve/backplate when the piston is near TDC.

Good job on the rod oil hole also. The normal oil hole location that most all manufactures use on aluminum rods causes the rod to split right up the middle. The crank pin acts just like a spitting wedge on a log. Not to mention that the oil hole gets closed off when under compression. A side or bottom location is best. Top or side location for the wrist pin end.

 

[Andy Brown]

Tim,

"A quick question regarding the rod above you mentioned- on the oil hole relocation, did you orient the hole in such a way as to force feed

the lubrication to maximum effect, relative to crank rotation, and center it mid way between perpendicular and along rod beam c/l ?"

Our intention in moving the oil hole was not what you posted because the hole is still facing upwards. Some customers stated that they were seeing some egg shaping of the bottom side bushing after many hours of use. This could come from ordinary metal fatigue. One thing that seems to increase the lubrication at this point is to place a chamfer on the hole's entrance & exit point. The angled hole's exit point on the inside of the bushing is carefully chamfered with an Exacto knife. There are some front intake engines that have an additional hole drilled through the crank pin, at an angle, into the front intake to provide additional lubrication to the bottom end bushing. All of these things can help the connecting rod to survive, however, none of these things can be substituted for the ability to "MACHINE THE CORRECT NUMBER".

Jim Allen

The Picco 45 rod has had the best oil hole I have seen. It is situated to "scoop" off the bottom of the case and the inside bushing surface at the hole has a taper groove ground into it to create a peristaltic pumping action.

 

[Timbo Whalen]

Tim,

"A quick question regarding the rod above you mentioned- on the oil hole relocation, did you orient the hole in such a way as to force feed

the lubrication to maximum effect, relative to crank rotation, and center it mid way between perpendicular and along rod beam c/l ?"

Our intention in moving the oil hole was not what you posted because the hole is still facing upwards. Some customers stated that they were seeing some egg shaping of the bottom side bushing after many hours of use. This could come from ordinary metal fatigue. One thing that seems to increase the lubrication at this point is to place a chamfer on the hole's entrance & exit point. The angled hole's exit point on the inside of the bushing is carefully chamfered with an Exacto knife. There are some front intake engines that have an additional hole drilled through the crank pin, at an angle, into the front intake to provide additional lubrication to the bottom end bushing. All of these things can help the connecting rod to survive, however, none of these things can be substituted for the ability to "MACHINE THE CORRECT NUMBER".

Jim Allen

The Picco 45 rod has had the best oil hole I have seen. It is situated to "scoop" off the bottom of the case and the inside bushing surface at the hole has a taper groove ground into it to create a peristaltic pumping action.

Very interesting, and ingenious... putting peristalsis to work in a fashion similar to the Dave Brown fuel pump.

Someone had their thinking cap on.

I had thought, initially, that the scooping arrangement is what Jim's intent was, but had no thought of an annular

'V' groove around the inside of the bushing.

 

[Jim Allen]

We tested several types of annular type grooves in connecting rod bushings but found no advantage over the correct radial clearance. A steel connecting rod with its groove & dual oil slots proved to be the best for lubricating the bottom end when the rod passed through a valley in the case. As yet we have not tested this in any type of aluminum connecting rod. However, the annular groove machined in the backplate of the bell valve's housing was found to be very effective in extending the life of the bushing & the ceramic ball bearing that was used to mount the valve.

JA

 

[Timbo Whalen]

We tested several types of annular type grooves in connecting rod bushings but found no advantage over the correct radial clearance. A steel connecting rod with its groove & dual oil slots proved to be the best for lubricating the bottom end when the rod passed through a valley in the case. As yet we have not tested this in any type of aluminum connecting rod. However, the annular groove machined in the backplate of the bell valve's housing was found to be very effective in extending the life of the bushing & the ceramic ball bearing that was used to mount the valve.

JA

Interesting... oh how the KISS principle sometimes evades us.

 

[Jim Allen]

This is absolutely true Tim. I can tell you, without any reservation, that in building any successful, miniature, very high performance engine, finding the "CORRECT NUMBER" (+ or- .0001 or whatever it may be) is as important as knowing what number you have made! Roundness (+ or - .0001") is the standard tolerance we are building our engines to. If you were to check the results of the recent International Pylon Racing contest held in Phoenix, AZ, March 23, 24 & 25, you would see the results of what we have been able to develop over the years!!

Jim Allen

 

[David Bryant]

Since we're on the subject of rod's:

I have quite a collection of new original stock 1980's Picco marine engines, including the popular P67's and P80's. However, like many others have inquired about, replacement rod's are hard to come by, especially RPM rods (pretty much non-existent). So one would have to custom build a replacement rod.

The other issue is that the stock rod's only had one bushing on the lower end, the upper-end had no bushing, which led or aided in eventual failure of the engine. I actually own a handfull of original stock replacement rod's. I was wondering: is it even worth installing a bushing onto the upper-end to provide the necessary operational reliability? Is there enough peripheral (aluminum) material to allow for the bushing?

 

[Jim Allen]

David,

Some of the advantages of machining your own connecting rod are as follows; the aluminum alloy used is known; the amount of press fit for bushings is known; the alloy of the bushing is known; the radial clearance numbers are known; the wall thickness of the bushings & the wall thickness of the connecting rod are known. Also the size, as well as, the position of any necessary lubricating holes can be easily determined.

At Aero Precision Machine all of the above things have been carefully tested & the numbers we are using have been proven to be valid with consistent reliable results both in National & International Pylon racing contests! We use 7075-T651 aluminum for the connecting rods & C54400-H04 free cutting phosphor bronze for the bushings. Bushings are pressed in with an .0005" interference fit. Typical wall thickness for the aluminum upper end is .055" & for the bottom end .066". The upper end bushing wall thickness is .022" & the bottom end is .036". Both bushings are "precision honed" to their dimensions after they are pressed in. One thing that will contribute to early connecting rod failure, in a smaller size, high RPM engine (29,000 to 35,000), is the failure to press the wrist into the piston bosses. To get the correct fit, finish & a round hole, the wrist pin holes in the piston should never be reamed! This simple procedure is overlooked by many engine builders. It is also impossible to hone a blind type of hole which is used by some engine manufacturers in their piston bosses.

In my opinion, it is not worth the effort to rebush an existing connecting rod unless there is sufficient aluminum alloy to support the bushing. You would also need to know what alloy was used to make the connecting rod in question. Keep in mind that the position & size of the necessary lubricating holes should be carefully considered.

Note: In the photos, notice the liberal chamfer amount at the entrance of both lubricating holes. Also notice that there are no sharp inside or outside corners.

Jim Allen

 

[David Bryant]

Thanks Jim, that's exactly what I was looking for. I'll probably look into having new custom rods fabricated

 

[Jim Allen]

David,

I forgot to mention that it is possible to use a connecting rod without a bushing in the top end. The method that is used to make this work is to use a larger size wrist pin than is necessary. For example, the size of the wrist pin in the MB-40 is .250" & the connecting rod is not bushed. The rod's material is 7075-T651 aluminum. The size of the wrist pin in the Nelson Q-40 is .197", but the connecting rod is bushed. Both wrist pins in these engines are drilled through leaving one end solid. I prefer to drill the wrist pin from both ends, leaving the middle solid, to prevent any flexing of the wrist pin when it is under load.

Notice in the posted photos how the un-bushed upper end of the MB-40 connecting rod is wider than the bottom end. Also notice the two very long throughs leading into the lubricating holes. These connecting rods operate in the 32,000 to 35,000 RPM range.

Jim Allen

 

[David Bryant]

David,

I forgot to mention that it is possible to use a connecting rod without a bushing in the top end. The method that is used to make this work is to use a larger size wrist pin than is necessary. For example, the size of the wrist pin in the MB-40 is .250" & the connecting rod is not bushed. The rod's material is 7075-T651 aluminum. The size of the wrist pin in the Nelson Q-40 is .197", but the connecting rod is bushed. Both wrist pins in these engines are drilled through leaving one end solid. I prefer to drill the wrist pin from both ends, leaving the middle solid, to prevent any flexing of the wrist pin when it is under load.

Notice in the posted photos how the un-bushed upper end of the MB-40 connecting rod is wider than the bottom end. Also notice the two very long throughs leading into the lubricating holes. These connecting rods operate in the 32,000 to 35,000 RPM range.

Jim Allen

interesting, thanks. .250OD is significantly large for a wrist-pin used in a .40ci piston, but I guess more robust. Amazing it can operate safely in that range (35,000RPM)

 

[Jim Allen]

The bigger diameter wrist pin increases the load bearing area, but there may be some increase in the reciprocating weight. Also, the wider connecting rod upper end is done to guide the connecting rod in the upper end where the rotational speeds are very low. Most production engines are guided in the bottom end.

JA

 

[Jim Allen]

Several different alloys were tested for rollers. High quality dowel pins would sometimes crack with no resulting engine damage. AISI #5200 & M-2 full hard steel worked the best. M-2 steel rollers running on a M-2 steel crank pin gave the best wear resistance. Cut crank pins & rollers are shown before finish grinding. The crank pin driver for the bell valve is ground in a Suburban Tool Master Grind (.00005" total indicator runout). The driving stub can be seen in other photos. Wrist pins are made of full hard M-2 steel "drill blanks", which are .0002" undersize. "Reamer blanks" are .0002" oversize. The wrist pins are drilled from both ends for lightness with a four faceted carbide drill. Retaining clips made of spring steel are wound on a mandrel, cut into, then tumbled to remove any burrs. Rollers & crank pins are made from full hard (63 Rockwell C) M-2 steel "reamer banks". The rollers are .0627" dia X .2500" long with a .0313" radius on each end. The radius is ground on each roller's end after cutting the roller to length. Therefore, the total contact length for each roller is .1875". The crank pins are .3283" dia with the driving pin for the bell valve ground on one end. The crank pins are a .0013" interference fit in a ground hole in the through hardened AISI S-7 steel crankshafts. These crankpins never come out or break & have very high wear resistance because of their large diameter.

Notice the difference between a typical stamped cage & the machined (C-350, 60 Rockwell C), full hard, cage, which guides the rollers from end to end along their center line. The hardened machined cage allows the engine to operate in the 32,000 to 35,000 RPM range without any of the typical connecting rod bottom end failures that are usually found. The machined cage also allows more rollers per crank pin diameter than is possible with any stamped type cage. When a double roller window is used in a machined cage, the number of rollers & the wear resistance is increased to the maximum! This cannot be done with any type of stamped cage!

Jim Allen

 

[Jim Allen]

Recently, there have been more & more bottom end connecting rod failures in miniature gas engines (26 cc to 35 cc) as they approach the 23,000+ RPM mark. Most of the reasons given for these failures are nothing but verbal nonsense & will do nothing to solve the apparent mechanical problems. Many of the things suggested will increase bottom end connecting rod failures! First thing, in any high RPM, high HP, high performance engine the connecting rod should only be guided in the upper end between the piston bosses. The obvious reason for this is because the rotational speeds in the bottom end of the connecting rod are much greater than those found in the upper end of the connecting rod. The fact that a "suitable" bushing bearing material can easily replace the roller assembly in the upper end of the connecting rod & give a greater load carrying capability further proves this fact. Needle roller type bearings will have rollers whose lengths are at least 4 X their diameters. Needle roller type bearings can accommodate axial forces greater than 5 % of their radial roads. The surface area of the rollers & the number of rolling elements can give roller bearings exceptional load carrying capacities & stiffness. Small axial clearance amounts will not increase the life of a bottom end roller type bearing! Large radial clearance amounts will decrease the life of a bottom end roller type bearing!

All needle roller type bottom end bearings require some type of continuous lubrication to dissipate heat & reduce friction. The amount of friction results from the load carried, the metallurgy & size of the rollers in the bearing, the RPM the bearing rotates at & the type of lubricant used. There is no mention of the great importance of the cage type or the cage metallurgy in this discussion. You can be assured this part is probably the most important part of the bottom end roller assembly!!

Jim Allen

 

[LohringMiller]

I wrote an article on big end failures That featured a failed rod from one of my engines. As some experienced two stroke engine builders pointed out, this was actually a cage failure. The engine had an M&D cylinder on a stock Zenoah crankcase and crankshaft. See below.

Lohring Miller

 

[Jim Allen]

Very good article Lorhing. One of the problems with many "engine builders or engine modifiers" is in their "INABILITY" to provide the necessary metallurgy, precision tolerances & correct engineering design that is required to solve bottom end connecting rod problems. The technology has been available for some time! It just needs to be applied by capable engine builders without the wasted time given to verbal nonsense, which has not solved this problem! As our engines are developed to higher levels of RPM & HP, only sound engineering will make it possible to have the reliability that is needed & wanted.

Jim Allen