Mercury Rods?

[Sonny]

Does anyone have any Idea what material is used to make Mercury connecting rods? Cast Iron? Steel? If they are steel what kind? A2? High carbon? Are they hardened? If so what is the rockwell? Does anyone have any idea how many pounds of force are exerted on the rods under race conditions? Thanks

[Steve77t]

I Can get you close

Rods are mostly a forged part but there are some that are billet. Material is steel and I can tell you what type for some rods used in Pro motors. A rod is somewhat soft in the beam section as a hardend through rod will break. This softer if you will beam section will absorb the shocks and loads better without breaking. The race ways are hardend so the rollers have a raceway. After rough machining, rods are coated sometimes with a copper covering and the places where it needs to be hard is stripped of this copper and the rod is then heat treated. The copper protects the base metal from carbon I think as the rod raceways are heat treated giving the rod beam a flex but a very hard race way for the bearings. Hope this helps. Steve

[Sonny]

Thank you Steve.

[jdalan]

Case hardening

Case hardening is a process that provides a thin section, perhaps .010-.020 deep on the bearing surfaces of the rod. The thickness is in excess of the requirement, so after heat treat, internal grinding may provide precise dimensioning.
This surface hardness is in addition to the core hardness. Inspection requires cross sectioning and microscopic inspection. Core hardness is measured usually on the Rockwell "C" scale. Surface hardness on the "N" scale. The rod will be heat treated to the core hardness requirement. Copper plating is a masking agent so the case hardening will not affect the areas that do not require a hard surface. This is to be followed by the case hardening procedure on the bearing surfaces. This process can also be described as nitriding, carbo nitriding among others. The process is with respect to required specifications. Inspection requirements would be core hardness, case hardness and case depth. You can consult a metallurgist at about any heat treat facility. Perhaps provide them with a sample? Good luck and I hope you do not see any unfriendly rods say hello through the crankcase.

[ZUL8TR]

Force

Does anyone have any Idea what material is used to make Mercury connecting rods? Cast Iron? Steel? If they are steel what kind? A2? High carbon? Are they hardened? If so what is the rockwell? Does anyone have any idea how many pounds of force are exerted on the rods under race conditions? Thanks

To get the force you asked is not an easy task. If you are math inclined here are some thoughts and means. You need to calculate the acceleration of the piston and rod assembly at the desired race rpm and knowing the mass of the assembly calculate F = MA. Then include the compression force on the piston crown for the total force on the rod. The compression pressure force is difficult to determine It depends on the ignition setting and the fuel burn pattern which is a complecated matter.

Here is a link for the varoius equations for piston movement, velocity and acceleration.

http://en.wikipedia.org/wiki/Piston_motion_equations

Note the graph at the bottom of the page for piston distance, velocity and acceleration for various rod and stroke lengths.

Here is another analysis but for a 4 cycle but the idea for a 2 cycle is the same.

http://emweb.unl.edu/Mechanics-Pages...0Rods%20VI.htm

And another, note force plots at end of text

http://www.engdyn.com/images/uploads...aper_(kea).pdf

[Sonny]

Thanks guys, We have some extra titanium at work left over from a run of medical parts. I was considering scanning in a Mercury rod and making a few but without getting all of the info together I would be risking a very expensive explosion. Thank you all for the help.

[53-W]

Titanium work and stress hardens and can fail very quickly. Some Indy car teams that had enough money to use titanium rods for there time runs to get the pole for the race. They knew they where only good for X number of engine revolutions and where used up stress cycle wise in a few time runs and would fail for sure in the race. They would take them out of the engine and replace them with race rods of different material for the race due to the titanium would stress fail and not finish the race. This was made illegal and you have to start the race with the engine as qualified no changing of parts.
53-W

[Yellowjacket]

The highest stress in the rod is actually a compressive stress that occurrs at bottom dead center. The maximum tensile stress occurs at top dead center and the load due to rotation is the same as the bottom dead center load. The overall load at TDC is lower though because of the gas pressure on the piston offsets some of the centrifugal load.

The load due to reciprocaiting forces is pretty simple to calculate. It is the mass of the rod and piston assembly times 1/2 the stroke times the rotational velocity squared (commonly called omega). Omega is in the units of radians per per second (2 pi x rpm x 60) and there is a units conversion of G (386.4 if you are working in inches). Bottom line is that it's pretty easy to calculate the forces due to rotation. You could make an estimate of the gas loads and then figure the tensile load at TDC.

It depends on the titainum, the most common medical alloy is 6-4. That is the same alloy that we used to make fan and compressor blades out of and it would make a good set of rods that are a lot lighter and stronger than the stock rods. Titanium is almost half the weight as steel so it's pretty easy to be a lot lighter than stock rods.

The hard part is getting a hard enough surface to run needle bearings on. We used to flame spray rubbing surfaces of titanium fan blades with a carbide coating that was hard, but I'm not sure you could grind it to be smooth enough to be a good bearing surface. It would be round and hard and it would retain oil because it had a surface that had tiny crazing in it. You might be able to hard chrome the surface, but not sure that would be thick enough to support the needle bearings. Some kind of steel insert might work but if it isn't part of the rod I'd worry that it might spin so you've got to make sure it doesn't rotate as well as be thick and strong enough, and obtw whatever you take out of the rod to make the insert you have to put back in to get the strength back.

It's probably a lot of work, but it would no doubt be a faster motor. The advantage is that the motor would really come off of corners well, it would be like you took the flywheel off of it.

You really don't have to do all of that stress calculation. You don't need to optimize it and make it as light as possible, the rods are going to be plenty light if you just copy the stock rods and put a bit more material around the rod bolt area because the titanium is more notch sensitive and make sure you have a good radius under the head of the rod bolt it will work fine. The rod will be at least as strong as the steel rod (and since the rod is so much lighter it doesn't need to be as strong as the steel rod). I'd have to look at my Mil handbook, but if memory serves, Ti 6-4 is good for about 140ksi (ultimate) and that is about the same as a good rod steel (like 4340). The Ti rod doesn't have to be an exact duplicate of the steel rod, if it is just as strong it will work fine. Like I said, the tougher part is figuring how to get a proper bearing surface on the inside of the rod big and small ends, and getting proper rod bolts properly fitted and the rod set up to work the way it is supposed to.

If you have the material it's probably a good winter project.

[Sonny]

The bearing surface was the other thing that I was worried about. We have had a ceramic coating put on titanium surfaces for wear resistance and it worked great. An insert is good idea also, maybe with a doweling so the insert cant spin? An insert would also be much easier to grind perfectly round. I would think that the lighter assembly might make rpms a little faster and maybe a few more rpms? If I do it I might as well go ahead and make them a little longer as well, the added compression should help acceleration. Should I consider taking material off of the bob weight of the crank to match the weight of the piston and rod assembly?

[Yellowjacket]

The small end of the rod will work well with an insert. You can make one out of M50 tool steel, add some meat to the rod (keeping the rod wall an equal or slightly greater thickness than it is now). I'd make the small end insert at least .080 wall, and make it about .002 big, then heat the rod and chill the insert and press it together, and then grind it to final. For the big end you are going to have to do some research on coatings. The carbide coatings we used on fan blades were very hard, but were also brittle. I would be afraid they would crack and flake off, but they were very hard. There are about a bizzlion flame spray coatings out there, so there is probably something that will work.

I'm not sure an insert will work on the big end of the rod. Whatever you take out to make the insert you have to add to the titanium somewhere else and there isn't a lot of room to skinny things down. If the insert has flanges it might stay round enough, but if it is too thin it isn't going to stay round. You could put flanges on the insert and wrap those around to the rod bolts, but I'd prefer to see a coating or something that was integral with the rod rather than an insert at the big end.

And yes the counterweights need to be lighter too you need to talk to an engine balance guy to figure out what you need to do there.

[G Stillwill]

The early K series Mercs had a shell type press in needle bearing in the wrist pin end of the rod and the wrist pin dia never changed when Mercury went to the loose needles so there is a torrington bearing already that could be used but wont work on the big pin 44 motors, not at home now so dont have the number of it. The new Sidewinder has a insert over the crankpin that the bearings ride on so if your wreak a rod just the rod and insert needs to be replaced. The early Merc 6,8,9.9 and 15 Hp had a thin steel insert that the center main bearings ride on and used a caged needle bearing the same size that the crankpins used. Never use a caged needle bearing on the wrist pin end as they wont take the pressure with out comming apart in short order.

[Dave_E71]

The Porsche GT3 is using titanium rods.
Fortunately, I haven't had to tear into mine so I'm not sure how they've addressed the bearings, if they're using inserts or a full roller setup.
I do know a replacement engine from Porsche is in the $40k range.
GM is also be using ti rods in the high end vette engines.

Dave

[seagull170]

Rather than use Ti to reduce the rotating mass & marginally increase your acceleration, why not use it to increase your crankcase compression.
As I understand it your merc has a 1 piece crank & split bigends & it sounds like you have access to quite modern machinery.
You could use the steel bigend cap from your existing rod & spark cut a 1/8" slice from the bottom of the rod to provide all the bigend bearing surface, it should not be too difficult to remove the threads in this & find longer bigend bolts with an 1/8" longer shoulder & longer threads that screw into the bulk of the Ti conrod. The rest of the rod could be as bulky as the material weight differential & clearances would permit, the little end bearing surface would just be a steel insert as previously suggested.

The rod would not be as light as the full Ti version but saves any questions on big end bearing construction problems, but never having played with one of these motors I don't know if the increased primary compression & transfer gas speed would cause problems with your blow down & scavenging, but it's a good way to stuff the crankcase.

[sam]

Increasing primary compression was tried around 1960 and not found to have significant effect

[Sonny]

Rather than use Ti to reduce the rotating mass & marginally increase your acceleration, why not use it to increase your crankcase compression.
As I understand it your merc has a 1 piece crank & split bigends & it sounds like you have access to quite modern machinery.
You could use the steel bigend cap from your existing rod & spark cut a 1/8" slice from the bottom of the rod to provide all the bigend bearing surface, it should not be too difficult to remove the threads in this & find longer bigend bolts with an 1/8" longer shoulder & longer threads that screw into the bulk of the Ti conrod. The rest of the rod could be as bulky as the material weight differential & clearances would permit, the little end bearing surface would just be a steel insert as previously suggested.

The rod would not be as light as the full Ti version but saves any questions on big end bearing construction problems, but never having played with one of these motors I don't know if the increased primary compression & transfer gas speed would cause problems with your blow down & scavenging, but it's a good way to stuff the crankcase.

I have a plan for that also. The formula boat guys use what the call a stuffed wrist pin to increase crank case compression. I am going to cut a 1/2 inch off of the wrist pin and set it in the piston 1/4 inch back from the "C" clip on both sides. Then I will turn down some high temperature plastic called Alltemp to fill the wrist pin and the 1/4 inch space on the ends by machining a shoulder in it.