Construction details for a reliable high performance connecting rod

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Jim Allen

Well-Known Member
Joined
Jun 7, 2005
Messages
1,622
I will begin a series of post on how to construct a steel "I" beam type, roller connecting rod. The rods can be used in high performance gas & nitro engines.

Jim Allen
 
I always enjoy following your build threads Jim.

It was nice to finally meet you at the Charleston race this year.
 
Thanks Buck, the feelings are mutual. In the near future I'll be moving further East so that I will have much closer access to running sites located on the East Coast. I'll be ending the work done at Aero Precision on the Nelson Q-40 & Q-500 pylon racing engines.

JA
 
Rudy,

I'l be leaving sometime in the Spring. Someone else will have to do what I was doing. LOL!

JA
 
The first thing to consider when building a connecting rod for a minature, high performance 2 cycle engine is; what type of metallurgy is required? There are many alloys that qualify strength wise, however, the alloy needs to have very high strength, very high fatigue resistance & shock resistance, at least a Rockwell hardness of 57, be easily heat treatable if necessary, & have good machineability.

Jim Allen
 
For a high performance large size nitro or gas engine, .15 cc & larger, a steel connecting rod is best. Since this rod will be a roller type, (bottom end only) with no bushing in the top end & will use the bottom's ID as a bearing surface, the correct selection of an alloy is critical! Another important consideration will be the alloy used for the bottom end roller assemblies retainer! It must have a low weight (mass), high strength, high hardness & wear resistance.

Jim Allen
 
For a high performance large size nitro or gas engine, .15 cc & larger, a steel connecting rod is best. Since this rod will be a roller type, (bottom end only) with no bushing in the top end & will use the bottom's ID as a bearing surface, the correct selection of an alloy is critical! Another important consideration will be the alloy used for the bottom end roller assemblies retainer! It must have a low weight (mass), high strength, high hardness & wear resistance.

Jim Allen
could i be 1st on the list??
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The connecting rod's final design & metallurgy was developed over a three period, begining in 1986. Further improvements continued until 1992 to give the steel roller rod assemblies presently used. This work, which was done in my machine shop, involved in depth testing of many different alloys & technologies before arriving at the final product. Connecting rods that were originally developed for very high RPM nitro engines were adapted to gas engines. Both types give the same greatly increased level of performance & reliability that cannot be achieved with any production or modified connecting rod available.

After testing 5 different materials including AISI 1018 steel, 7075-T651 aluminum, 6AL4V titanium & #4340 steel, AISI-S7 proved to be the best material. This is because of S-7's very high tensile & yield strengths (315,000 psi & 210,000 psi at 58 Rc); S-7's high impact resistance (224+ ft lbs at 58 Rc); S-7's ease of machining & its very low distortion during heat treating. The photos show the connecting rod blanks & the simple holding fixture which is mounted to a rotary that allows the complete profiling of the connecting rod. The thick cold rolled steel plates prevent any distortion during heat treating.

Jim Allen

.625 bore barrel carburetor on QD 211.jpg

.625 bore barrel carburetor on QD 212.jpg

.625 bore barrel carburetor on QD 213.jpg
 
A great deal of research went into finding a super strong alloy that would have a very low weight. One of the recurring problems with lighter weight alloys such as aluminum & Titanium is their lack of surface hardness which would always require some type of additional bushing material to be pressed into the lower & upper ends of connecting rods. Bushing materials pressed into the upper end of connecting rods were no problem at all because of the low rotational speeds in this area. Bottom ends were another case because of the high rotational speeds & the resultant forces that bottom ends are subjected to, especially when rotational speeds exceed 26,000 RPM.

JA
 
S-7 great stuff I used to machine it at a Proto type place I worked at in the 70's. Like you said easy to machine and very hard after heat treat. Did not need it for rods back then we did not have the power like today. Great job
 
It became obvious that the connecting rod needed to be a "one piece design", made of a material that would be 58 Rc after hardening & tempering. Since this connecting rod would have no bushing or roller assembly in the top end, the reciprocating weight is greatly reduced. By not having a roller assembly or a bushing also allows the connecting rod to be at a maximum length which reduces the length of the piston boss areas to a minimum height. Lengthening the connecting rod to its maximum length increases its weight slightly, but the reduction in the amount of connecting rod's angularity is much more important in reducing piston rocking.

As an example, the connecting rod's length in my 15 cc nitro engine is 1.741" with a .902" stroke. The connecting rod's length in my 27 cc gas engine is 2.155" with a 1.102" stroke. The upper end of the bush less, roller less, one piece connecting rod has a wall thickness of .094" with no thrust washers needed giving a mechanically reliable design with very few parts.

After profiling the connecting rod's shape, including the staggered double oiling slots in the bottom end & the single slot in the top end, the rod is ready to be heat treated. Notice the groove in the bottom end that channels any crankcase oil into the slots.

Jim Allen

Note: I'll post better photos from my computer when I return to the shop.

normal_connecting%20rod%20010.jpg

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normal_connecting%20rod%20015.jpg

normal_connecting%20rod%20016.jpg

normal_connecting%20rod%20009.jpg
 
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It became obvious that the connecting rod needed to be a "one piece design", made of a material that would be 58 Rc after hardening & tempering. Since this connecting rod would have no bushing or roller assembly in the top end, the reciprocating weight is greatly reduced. By not having a roller assembly or a bushing also allows the connecting rod to be at a maximum length which reduces the length of the piston boss areas to a minimum height. Lengthening the connecting rod to its maximum length increases its weight slightly, but the reduction in the amount of connecting rod's angularity is much more important in reducing piston rocking.

As an example, the connecting rod's length in my 15 cc nitro engine is 1.741" with a .902" stroke. The connecting rod's length in my 27 cc gas engine is 2.155" with a 1.102" stroke. The upper end of the bush less, roller less, one piece connecting rod has a wall thickness of .094" with no thrust washers needed giving a mechanically reliable design with very few parts.

After profiling the connecting rod's shape, including the staggered double oiling slots in the bottom end & the single slot in the top end, the rod is ready to be heat treated. Notice the groove in the bottom end that channels any crankcase oil into the slots.

Jim Allen

Note: I'll post better photos from my computer when I return to the shop.
Very nice piece.

tw
 
The "I" beam part of the connecting rod is .282" wide X .156" thick. The radius at the junction of the I-beam & the upper or lower holes is .8125" (1.625" dia end mill). This end mill profiles the entire outside of the rod. A .1875" ball nosed end mill forms the center section of the I-beam which is .031" thick in the center. The wrist pin hole is .250" ID X .360" OD. The crank pin hole is .4538" ID X .6250" OD. The undercut on the outside of the big end is .031" deep X .156" wide & it also has a radius on the inside corner. The center distance of the two holes is 1.741". After hardening (1760* F; standard oil quench) the S-7 rods are double tempered at 400* F. This heat treatment gives a tensile strength of 315,000 lbs/ square in; a yield strength of 210,000 lbs/ square in; at 58 Rc with a 1" Charpy "V" test of over 224 ft lbs.

This heat treatment gives the strongest connecting rod possible. It also provides the necessary bearing surfaces required for the bottom end roller assembly & the un-bushed upper end.

Completed connecting rods after machining have no sharp inside corners anywhere! All oil slots are .032" wide & they also have a radius on the inside corners. The two slots in the bottom end are staggered to each side of the visible undercut. The inside corners of the undercut have a radius also. The undercut lightens the bottom end & forces oil into the connecting rod as it rotates in a groove machined into the crankcase. All connecting rods are guided in the upper end, between the piston bosses (.005") clearance, where the rotational speed is low. Connecting rods should NEVER be guided in the bottom end where the rotational speeds of the crankshaft are very high.

Jim Allen

normal_connecting%20rod%20017.jpg

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The "I" beam part of the connecting rod is .282" wide X .156" thick. The radius at the junction of the I-beam & the upper or lower holes is .8125" (1.625" dia end mill). This end mill profiles the entire outside of the rod. A .1875" ball nosed end mill forms the center section of the I-beam which is .031" thick in the center. The wrist pin hole is .250" ID X .360" OD. The crank pin hole is .4538" ID X .6250" OD. The undercut on the outside of the big end is .031" deep X .156" wide & it also has a radius on the inside corner. The center distance of the two hole is 1.741". After hardening (1760* F; standard oil quench) the S-7 rods are double tempered at 400* F. This heat treatment gives a tensile strength of 315,000 lbs/ square in; a yield strength of 210,000 lbs/ square in; at 58 Rc with a 1" Charpy "V" test of over 224 ft lbs.

This heat treatment gives the strongest connecting rod possible. It also provides the necessary bearing surfaces required for the bottom end roller assembly & the un-bushed upper end.

Completed connecting rods after machining have no sharp inside corners anywhere! All oil slots are .032" wide. The two slots in the bottom end are staggered to each side of the visible undercut. The inside corners of the undercut have a radius also. The undercut lightens the bottom end & forces oil into the connecting rod as it rotates in a groove machined into the crankcase. All connecting rods are guided in the upper end, between the piston bosses (.005") clearance, where the rotational speed is low. Connecting rods should NEVER be guided in the bottom end where the rotational speeds of the crankshaft are very high.

Jim Allen
Very well thought out, Jim... I sure wish you had the time to provide these

pieces to us. I have concerns over the con rod in my NR46DD and am already

planning for an alternative.

Thank you-

tim
 
The "I" beam part of the connecting rod is .282" wide X .156" thick. The radius at the junction of the I-beam & the upper or lower holes is .8125" (1.625" dia end mill). This end mill profiles the entire outside of the rod. A .1875" ball nosed end mill forms the center section of the I-beam which is .031" thick in the center. The wrist pin hole is .250" ID X .360" OD. The crank pin hole is .4538" ID X .6250" OD. The undercut on the outside of the big end is .031" deep X .156" wide & it also has a radius on the inside corner. The center distance of the two hole is 1.741". After hardening (1760* F; standard oil quench) the S-7 rods are double tempered at 400* F. This heat treatment gives a tensile strength of 315,000 lbs/ square in; a yield strength of 210,000 lbs/ square in; at 58 Rc with a 1" Charpy "V" test of over 224 ft lbs.

This heat treatment gives the strongest connecting rod possible. It also provides the necessary bearing surfaces required for the bottom end roller assembly & the un-bushed upper end.

Completed connecting rods after machining have no sharp inside corners anywhere! All oil slots are .032" wide. The two slots in the bottom end are staggered to each side of the visible undercut. The inside corners of the undercut have a radius also. The undercut lightens the bottom end & forces oil into the connecting rod as it rotates in a groove machined into the crankcase. All connecting rods are guided in the upper end, between the piston bosses (.005") clearance, where the rotational speed is low. Connecting rods should NEVER be guided in the bottom end where the rotational speeds of the crankshaft are very high.

Jim Allen
Very well thought out, Jim... I sure wish you had the time to provide these

pieces to us. I have concerns over the con rod in my NR46DD and am already

planning for an alternative.

Thank you-

tim
tim,what do you see with that rod that you are not happy with? i have only run the .12/.21 (S7) rods and never had an issuse..
 
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