Con rod material

[Parksy]

Hi all

I want to know what makes a suitable material to use as a con rod. I want to stay away from aluminium and use some sort of steel instead. Will most steels be suitable? I will make bronze bushes to use as big and small end bearings.
I've got some spare material in the shed, mild steel, stainless steel(304, 316, 431), silver steel, 4140 and probably some I've forgotten.
I understand that this engine won't be highly stressed, but hole elongation is something I want to avoid completely.

Thank you all

Andy

 

[stevehuckss396]

Hi all

I want to know what makes a suitable material to use as a con rod. I want to stay away from aluminium and use some sort of steel instead. Will most steels be suitable? I will make bronze bushes to use as big and small end bearings.
I've got some spare material in the shed, mild steel, stainless steel(304, 316, 431), silver steel, 4140 and probably some I've forgotten.
I understand that this engine won't be highly stressed, but hole elongation is something I want to avoid completely.

Thank you all

Andy

Go with aluminum. The weight reduction will be easier on your crankshaft and the bronze bushings will eliminate the elongation.

 

[Jasonb]

If you do go with Aluminium then use 2014 as that will not stretch, its the usual choice for small iC engines and can usually be run without bearings. Don't use 6082.

 

[Parksy]

Thanks guys. With regards to weight, what is the penalty of a heavier rod on the crankshaft?
I have some 2011 aluminium, is this a suitable material?

 

[lohring]

What size engine are you talking about? The 26 to 36 cc engines I deal with all use heat treated steel rods with needle bearings. Jim Allen builds all his high power 15 cc engines with S-7 steel heat treated to Rockwell 58 C rods. We have had the wrist pin bores in aluminum pistons elongate at high rpm in a 36 cc engine. The steel rod was fine. Below are some of Jim's rods with his thoughts.

A few of many failed connecting rods.

Connecting rod materials tested include 2024, 7075, 7079, & 7068 aluminums; 6AL4V titanium; AISI O-1; AISI A-2; CPM-3V tool steel; M-2 tool steel; & AISI-S7 shock resisting tool steel. Connecting rods made of S-7 (oil hardened & tempered) survived many test runs beyond 32,000+ rpm, but completely failed when pushed to 38,000 rpm. The one titanium rod standing on its side used the Perry designed oil pump hole through the middle of a bushed rod to help improve sleeve bearing lubrication. A slot, cut into side of the rod allowed crankcase lubrication to enter on each engine revolution. This was also done to aluminum rods. There was only a slight improvement in performance. In the upper left hand corner is the first steel roller rod (not hardened with pressed in hardened race) which survived 6 runs at 32,000 rpm before failing . The pressed in race turned, closing off the oil lubricating holes. It used a hardened berillium copper retainer. In the middle row, the six rods on the right are made of 6AL4V Titanium. This super alloy does not transfer heat, & requires a separate material for the bearing surface. These two things make it unsuitable for connecting rods in large motors.

 

[stevehuckss396]

Due to the nature of this forum I assumed we were talking about a model display engine that would see limited RPM and no load. In the case of a 6000 RPM engine with less than an inch bore, aluminum should be just fine. I have 6061 rods in mine and it has been running without problems since 2010. Guess I should have asked a few questions before answering.

 

[Alec Ryals]

Hi all

I want to know what makes a suitable material to use as a con rod. I want to stay away from aluminium and use some sort of steel instead. Will most steels be suitable? I will make bronze bushes to use as big and small end bearings.
I've got some spare material in the shed, mild steel, stainless steel(304, 316, 431), silver steel, 4140 and probably some I've forgotten.
I understand that this engine won't be highly stressed, but hole elongation is something I want to avoid completely.

Thank you all

Andy

I have used straight 7075 T-6 up to 10K rpm and heavy loads on a twin 30cc engine with no problems and the beanings look great !
Alec Ryals

 

[Parksy]

Thanks guys. While this engine will be a low stressed engine, I would prefer to use a material that is usually seen as an appropriate material to use as a conrod. Not so much something that is specifically suited to this very application(being low stress).
Am I after materials with high tensile strength characteristics?

 

[petertha]

Not sure if this is helpful but the radial I'm working on is 50 cc total displacement, so 10cc per cylinder. It calls for a euro equivalent aluminum of 7075-T6 (higher tensile stuff which I can source local). The Edwards radial (similar displacement) calls for 2024-T351.

The bottom ends pins of these engines are 6mm & 6.35mm (0.250") respectively. Neither have bronze bushings which kind of goes against everything I've seen on RC rods of comparable or smaller displacement. I'm a little worried that if drill out to accommodate even a skinny wall thickness bronze bushing for wear improvement, I will compromise the bottom end strength of removed aluminum. (I cant accommodate extra diameter in this area dimensionally because of the master rod layout). My 0.40 CI racing engines are meatier rod ends & bushed & look a lot like Lohrings. But they belt out more power and 3-4X the rpm of the radial. So because the radials are established runners, I'm going to assume the designers knew what they were doing both for strength & lubrication so will follow the plan. Here is my work in progress to give you a feel for size.

Another note - my RC rods typically have lube holes that go from the rod body side into the pin hole. This radial design has 2 staggered holes on the bottom end only. Not quite sure what's going on there but I see some of Lohrings pics has different permutations . Any advise on this aspect appreciated.

 

[gbritnell]

In designing a connecting rod for 1:1 or a model engine the priorities are strength and lightness. A lot of stock automotive rods are made from forged steel, what the exact grade is I can't tell you bu they added nickel and vanadium to the mix. The early Ford flathead rods are definitely spindly to look at but hold up quite well with performance upgrades.
The structure of most rods has an I-beam cross section. This is to reduce weight while keeping them strong. The reason behind the minimal weight is because of the reciprocating mass. In the case of a connecting rod the more weight you move up and down the harder it is to decelerate it at the end of the stroke. Along with throwing it around in an arc on the crank.
Drag racing engine and high performance engines use aluminum rods but the life span is short because of the intended use. Nowadays with the use of CNC most NASCAR and other high performance rods are made from billet steel.
I would say that if you wanted to be more of a purist you could make your rods from 1144 or 4140 but as mentioned use an I-beam cross section to reduce the weight, otherwise a quality aluminum like 7075 with insert bearings will be more than adequate. In a couple of my engines I have even used aluminum bronze mainly with the thought of not having to make bearing inserts.
Remember these engines won't be put to work on a daily basis and the small amount of time that they are operated won't require anything but a high quality metal, whether aluminum or steel.
gbritnell

 

[lohring]

It's true that aluminum is a great rod material. The 26 cc engines I was talking about started out as low performance industrial engines. I suspect the choice of steel was based on needing needle bearings for long life. the manufacturing costs would be lower in all steel.

lohring Miller