Designing a connecting rod, I'd love some insights on how stock rods fail etc.

[busted_blocks]

I'm trying to design a connecting rod for a GX200. My goal is to make a cheap'ish connecting rod that will do something like 8000rpm. I don't want to be as fancy as an Arc Racing rod, the cost for small batch production is too high. I need to limit the number of setups for machining but more importantly I'd love some experience with failed rods to know what I'm trying to improve.

My failed connecting rods all failed due to lack of lubrication. What I don't know is if lack of lubrication was due to stretching and deformation of the rods due to extra stress or simply lack of lubrication. My engines had 15W40 oil which may have contributed to thefailures.

I have a few options. Design a completely new rod or modify existing rods.

1. Does a high rpm rod absolutely need removable bearing surfaces?
2. Any idea of why stock rods fail? Mine have been due to friction and the piston wrist pin end never showed damage. Never saw them break along the length either.
3. Do you think adding an extra lubrication hole in the stock rod would help?
4. Do you think it's absolutely necessary to have removable bearing surfaces? It's not the removable surface that's the problem it's stock screws that have a large head and cut into the top of the rod weakening it. I'd like to avoid custom screws, though socket head cap screws are available, but I don't know if I can get the torque wrench with hex head in the block to torque them. Waiting for the adapter to arrive to check.

Features of an Arc rod:
- 7075 aluminum vs cast
- Custome removable bearing surfaces
- Thicker "top" of the connecting rod side
- Splash stick lubrication scoop/hole
- Oil retentions groove in bearing surfaces

Many car rods have no lubrication holes in the crank end of the rod, but I think they have one in the crank shaft (I'm not really a car guy). Removable bearing surfaces are available for very cheap, but the mounting tab is at a different spot from Arc. I'm not opposed to using those assuming I can solve the bolt issue.

Possible new rod:
7075 is no issue, cost is almost the same as 6061
Extra lubrication holes no issue
Removable bearing surfaces?
I don't want scoop hole, though I can plot a path through the rod for a similar hole, the Arc design needs a deep drill, custom shank bolt and a internal groove cut in the cap bolt hole.

Another option is can I modify a stock rod to be good enough. I could simply add an extra lubrication hole, but I think this has limited value. What I want to try is machining the top of the cap flat, add a piece of tool steel 4mm thick on top and bolt it down. This should increase the stiffness of the stock rod. If the weight is an issue I'm sure I could use titanium and even have a different stiffer shape to the stiffener. I'm pretty sure it will allow me to add a hole in the cap for lubrication as well. I've seen some tik tok car guys groove car rod bearings to a add more oil retention. I could add such a groove, but it reduces the rod bearing surface. I'm not opposed to blowing up some more blocks testing

I drew up a conrod, which absolutely will not work as the cap needs to be at 120degrees to clear the cam shaft, but the overall shape of the fillets etc. is OK. I started another one with the 120 degree angle. I won't forget that again. I also machined a cap ready for a stiffening bar and got longer bolts. Very hard to find M7*1.0...

 

[Darren]

Been down the stock rod 'what makes it fail' road.
My research was on the Honda GX390
For some reason golf kart engines use a shell on the cast rod, go figure. The machinist was able to hone out the cast rod to accept big end shells.
Next was the biggest problem, oil starvation, I would run the engines around the track all day for at least 5 sessions, then strip down the engine. The only problem was a little bit of pick up on the crank due to oil starvation, I moved from standard Honda approved oil to Redline kart oil, this was about 4 times the price, but there was a noticeable difference in big end/crank wear.
At no point did the original HONDA rod fail
I played around with the Honda engine around 15 years ago and didn't really know what an ARC rod was, I needed to get oil into the journal, sometime later with close up pictures of an ARC rod I tigged extra material onto the oil pick up and then cross drilled the new hand made rudimental scoop, it worked! I moved onto trying different carbs after that, 34mm flat slide gave the best results.

 

[busted_blocks]

Thanks, and that kart looks like a ton of fun!

With this rod, if I fit the bushings, there is very little material left on the rod itself. I could Tig more material on top of the cap I guess.

I suspected oil starvation, but EVERYONE seem to just use a CNC rod. My issue is that importing a rod is about the same cost as the engine. I don't like the math on that one.

I'll do a couple of tests.

1. Add stiffening bar to the top with an extra oil hole.
2. modify a stock rod for big end bushings

There is also room to sneak a hole between the stock splash stick and the bolt. I really don't know what the oil in the engine looks like when it's running, one would hope some would get in that hole.

 

[Rat]

Basically the dipper hits the oil surface creating an initial splash, and then flicks it up towards the top end passages for the valves. Considering this as a slow motion break down, you end up with an oil mist being sprayed towards the top end and valvetrain.

I've only seen one rod actually let go in a Honda engine and it was on a riding mower. It was ran hard on a hot day, low on oil; the conrod smeared all over the crank and I guess seized at just the right moment in cycle because it was snapped off and shot through the side of the engine block leaving the piston slammed against the crankshaft.

That said, I would think a shallow groove (think like the "blood groove" down the middle of dagger) that led to an oiler hole on the leading and trailing ends of the conrod would pick up the most oil.

 

[Darren]

There's a guy that's made a clear side cover on youtube, this shows the oil sloshing around

I also. or should I say the machine shop, added another oil hole to the rod, not sure if it made a difference. The oil scoop did the job.
I can't be sure but I think the engine shop may have machined my crank and rod to fit the shells, they were concerned about the bolt thread area.

Why not try to add a scoop to a clone rod, then try to snap it off, you'll be able to establish if there is a weak point.
We've got gx200 welder/generators at work, I service them, these are over 1000hrs and still run strong, no failures at all. Kart engines are run hard, all conditions, if a rod fails it's usually oil starvation.

 

[Rat]

There's a guy that's made a clear side cover on youtube, this shows the oil sloshing around

I also. or should I say the machine shop, added another oil hole to the rod, not sure if it made a difference. The oil scoop did the job.
I can't be sure but I think the engine shop may have machined my crank and rod to fit the shells, they were concerned about the bolt thread area.

Why not try to add a scoop to a clone rod, then try to snap it off, you'll be able to establish if there is a weak point.
We've got gx200 welder/generators at work, I service them, these are over 1000hrs and still run strong, no failures at all. Kart engines are run hard, all conditions, if a rod fails it's usually oil starvation.

I know the weakest point in an old Briggs flayhead is the oil slinger, the steel ones break off at high rpm, hence the exsistance of nylon ones. They still break but they don't destroy everything they get caught in, but I can't say I've ever seen a Briggs bust a conrod as much as gall it from low oil or the lack of changing it.

They did add one to the conrod cap casting for the Flathead Raptors though.

 

[busted_blocks]

One of my original concerns was that the rod end is deforming under high RPM, going oval a bit from stretching and the oil is starved as a result, or the film is too thin on the elongated sides. This has no base in data and is likely just my over active imagination.

@RLS_Underground has a good idea, just try and add the oil holes and test. 2 options, weld the stick wider and put in holes or try them in the existing stick.

I'm not sure I understand what mechanism the oil is "scooped" with. When I look at the Arc rods, the scoop always faces down, so it's not gravity, but I guess funnel to direct oil to the hole and it's forced up due to pressure generated from the "funnel" pushing into the surface of the oil in the bottom of the crank case. The forward motion of the scoop or funnel whatever also helps to direct and force the oil into the con rod end.

There is about 6mm of space between the edge of the dipper and the bolt hole, breaking into the bolt hole isn't a issue so can hug that side when drilling. Can for sure fit a 3mm hole in there and shape things to direct oil down the hole.

I have 2 lower ends sitting here waiting to be put together. I'll try 1 with a "scoop" and the other with a reinforced cap with a hole right in the middle. normally you wouldn't put a hole in the cap as it would weaken things, but with the reinforcement I suspect it will be OK. I did a quick mock-up of where I mean for the hole.

I have better hopes for the scoop really, but since I already arranged the reinforcing plate, I'll try that too.

 

[Thepartsguy]

 

[busted_blocks]

View attachment 145048

There is a story behind that! That's is very far from what I'm seeing on my connecting rods, but it sure is interesting.

 

[Darren]

And the dipper is still intact!!

 

[Rat]

One of my original concerns was that the rod end is deforming under high RPM, going oval a bit from stretching and the oil is starved as a result, or the film is too thin on the elongated sides. This has no base in data and is likely just my over active imagination.

@RLS_Underground has a good idea, just try and add the oil holes and test. 2 options, weld the stick wider and put in holes or try them in the existing stick.

I'm not sure I understand what mechanism the oil is "scooped" with. When I look at the Arc rods, the scoop always faces down, so it's not gravity, but I guess funnel to direct oil to the hole and it's forced up due to pressure generated from the "funnel" pushing into the surface of the oil in the bottom of the crank case. The forward motion of the scoop or funnel whatever also helps to direct and force the oil into the con rod end.

There is about 6mm of space between the edge of the dipper and the bolt hole, breaking into the bolt hole isn't a issue so can hug that side when drilling. Can for sure fit a 3mm hole in there and shape things to direct oil down the hole.

I have 2 lower ends sitting here waiting to be put together. I'll try 1 with a "scoop" and the other with a reinforced cap with a hole right in the middle. normally you wouldn't put a hole in the cap as it would weaken things, but with the reinforcement I suspect it will be OK. I did a quick mock-up of where I mean for the hole.

I have better hopes for the scoop really, but since I already arranged the reinforcing plate, I'll try that too.

IMHO, You're massively over thinking the matter.
With the proper oil for the given season there shouldn't be any issues as long as it gets changed on schedule, leaks are addressed promptly, and it's never ran low on oil.

First consider the oil level of a properly filled case (typically half qt) and the cap submersion at BDC...all a hole in the cap will do is drain the oil off the crank faster and possibly accelerate galling.
Your best bet is to leverage gravity which returns all the flicked, splashed, and flung oil righ back to the bottom... a pair of countersunk holes at the top front and rear of the rod would be more than sufficient.
If you're worried about breaching the threads or potential weakness the move them 90° and tilt them towards center with one more forward and one more rearward to offset them.

I've never seen issues from a property serviced engine that wasn't neglected and abused

 

[Denny]

I’ve seen a few rod bolt failures. Good rod bolts go a loooong way towards reliability!

 

[busted_blocks]

IMHO, You're massively over thinking the matter.
With the proper oil for the given season there shouldn't be any issues as long as it gets changed on schedule, leaks are addressed promptly, and it's never ran low on oil.

First consider the oil level of a properly filled case (typically half qt) and the cap submersion at BDC...all a hole in the cap will do is drain the oil off the crank faster and possibly accelerate galling.
Your best bet is to leverage gravity which returns all the flicked, splashed, and flung oil righ back to the bottom... a pair of countersunk holes at the top front and rear of the rod would be more than sufficient.
If you're worried about breaching the threads or potential weakness the move them 90° and tilt them towards center with one more forward and one more rearward to offset them.

I've never seen issues from a property serviced engine that wasn't neglected and abused

I have been known to overthink things. Having had rod failure over and over, I've certainly had time to think.

I would like to keep the engine serviced, but I'd very like to abuse it at 8000rpm in a go kart.

I can accept that a hole in the cap is a bad idea. I can skip that. The scoop idea has an almost more downwards facing hole, but it works on the Arc rod, though there are places to potentially trap the oil in that design. There is an otherwise unexplained hole in the Arc rod. I say Arc rod but I have a copy, so I don't actually know if it's 100% exact. This detail I can't confirm without help. I attached a pic of what I mean.

I like the idea of an extra hole on the back side of the rod, giving gravity a chance to keep things oiled.

I've got a rod with the dipper welded up, I'll see if I can carve a scoop with a hole. There is room to squeeze a 3mm hole inbetween the edge and the bolt.

 

[Thepartsguy]

There is a story behind that! That's is very far from what I'm seeing on my connecting rods, but it sure is interesting.

And the dipper is still intact!!

Heard a slight knocking noise and pushed the pedal down farther.. There was a earth shattering kaboom and I pushed the kart home. I‘m 100% thinking it was not oil starvation but just high rpm failure.

 

[Thepartsguy]

Just start with making a few specialty rods that have your name and flare on them. I never new wiseco made rods for Briggs 5hp engines until I saw a wiseco branded rod. Certain Horstman Rods are hard to break.. A one off made BUSTED BLOCKS ROD because the rod doesn’t break.

 

[Denny]

That is one way to remove a piston and rod I guess.

 

[busted_blocks]

I made up a spoon a one connecting rod. After drilling it I realized I should have angled the drill towards the center. Can't angle it too much or it will break through the bolt hole, but a little anyway.

I'll give this a shot.

 

[busted_blocks]

Another idea, what about putting a hole in the crank shaft? Pretty easy to do and it's hollow. There is no oil pump like in a car, but since oil is sprayed everywhere, I'd think this would be a reasonable path for oil to get to the conrod journal?

 

[Rat]

Another idea, what about putting a hole in the crank shaft? Pretty easy to do and it's hollow. There is no oil pump like in a car, but since oil is sprayed everywhere, I'd think this would be a reasonable path for oil to get to the conrod journal?

In theory sure, but in theory you could also be bigger trouble.

A shallow spiral groove around the crank would be sufficient, but the key is buffing it down so there is absolutely no sharp edge leading or trailing. In combination with the inlet hole at the top of the rod, it would behave as crude a pump on its own pulling oil through the journal.

It's actually an old school method that works quite well

 

[busted_blocks]

In theory sure, but in theory you could also be bigger trouble.

A shallow spiral groove around the crank would be sufficient, but the key is buffing it down so there is absolutely no sharp edge leading or trailing. In combination with the inlet hole at the top of the rod, it would behave as crude a pump on its own pulling oil through the journal.

It's actually an old school method that works quite well

Is there any benefit to a groove in the crank rather than a groove in the conrod journal?