Conrod

[David Ashcroft]

Anyone have a k&b 1.0 cube conrod would like to have a few machined

 

[Geraghty]

The connecting rod is the same for all the .67 , .82 , .91 and 1.0CI K&B engines......[there was also a .72 motor in there made for Bob Violett]
There were several rod designs used by K&B over the years....All the big block rods carry the same K&B part number 16-6701....[old P/N 5321]....
There were solid beams , "H" beams and the round beam used on the K&B conn rods.....
I preferred the early solid beam rods.....I ran them for years without problems once I discovered how to install them....
The most important thing to consider is the K&B rod needs to be clearanced when installing the rod......A tight rod will fail....
This is what the good rod looks like.....RPM made a great rod...... clearance it when you installed it and it was bulletproof.....

 

[James E Beasley]

What do you use to clearance it, and how much? Assume only the crank end or also the piston pin end? Sorry, I'm a shade tree machinist. Thnx, James

 

[Geraghty]

What do you use to clearance it, and how much? Assume only the crank end or also the piston pin end? Sorry, I'm a shade tree machinist. Thnx, James

I have tried reamers and that is not the way to go.....Leave the wristpin end alone.....
I just use a hone made of a piece of turned down slotted nylon dowel and some 600 wet and dry sand paper wrapped around it , and a cordless drill....
Remember , when the motor is running the conrod is always in compression.....
You have to reduce that contact patch between the rod and crankpin to reduce the friction causing heat....That is why a lean run will spin rod bushings....
These motors all have tramp oiling systems , meaning no pressure oiling system.....you need some clearances to get oil onto the crankpin and rod bushing...
The oil holes in the rod bottom are to let oil out not force oil in.....
Really the only need for the bottom of the rod is to start the motor and to clean it out....
When the motor is running the crankpin never sees the bottom of the rod bushing......Heat is what kills rod bushings.....
There are no set numbers for clearances....
I hold the rod on the crankpin and when I get the wrist pin end to rock up and down on the crank pin about 1/16" of an inch I call it good....
I polish the hell out of the crank pin with a buffing wheel and red jewelers rouge .....
If you are replacing a rod that has spun a bushing you have to make sure you get all the residual bronze bushing material off the crankpin...
That residual bushing is hard to see.....I check he crankpin with a 10 power eye loupe .........
Harbor freight has a good loupe set for about $5.00.......
If you don't get that residual bushing material off , you will be replacing the rod again in about one minute....
Anybody......I know all this can be confusing so........if you have any questions....
Call me
1-715-926-6096 landline
1-715-495-5025 cell
When a rod gets loose that is when it is good......

 

[Grimracer]

One method that seem to help me (adding to Rods post) is I take a 3/32in wide length of note book paper and place it between the pin and the rod.. slide the rod in place with the paper in there.. if the rod (cuts, breaks the paper) its to tight..

 

[Terry Keeley]

If you have dimensions I'll look through my old titanium rod stock and see if there's something there. Just calipers will do.

I remember we made some rods for Violett back then...

 

[Ricky Neal]

Hi Guys,

I am by no means an expert on this but have spoken to a few that would be regarded as experts in relation to model engine design/development and manufacture and I have followed their advice when it comes to connecting rod clearance and honing as well as measurement.

Everything I mention here will be in relation to the development of 3.5cc engines and of course as materials and dimensions change of things like the crankpin and the gudgeon pin the clearances will also need to change.

In the case of the engine we are developing, we have a crankpin diameter of 5mm with a clearance of 25-30 microns and the gudgeon pin is 4mm in diameter with a clearance of 10-15 microns.

As an example, I have just measured a new Novarossi crankpin and conrod, the big end bore of the conrod is exactly 5mm and the crankpin is 4.980 mm so there is a 20-micron clearance.

If I was trying to figure out what clearance I should run on a particular engine, I think the best thing to do is just measure a new rod and crankpin if you have one available, better yet a few to get an average and then work back from there. Failing that I would start by taking what I know works (25-30 microns on a 5mm crankpin) and scale it up as a percentage from there.

For example, if it was a 6mm crankpin, that is 20% larger than a 5mm crankpin so I would add 20% to the clearance of 25 microns so that would be 30-35 microns, maybe start at 30 and see how it feels.

I think the hardest part is accurately measuring and then honing to such small clearances, for this I took advice on what tools to use and I use the below bore measuring tool





This tool has 4 balls at the end and is better than 3 point micrometer because you can also measure ovality. It does take a little more setup though and I have bought settings rings at specific sizes to use for setup and reference. Incidentally, the brand new Novarossi rod I just measured appears to have a 4-micron ovality.

This tool is made by Carl Zeiss and I have only ever seen it in ex-Soviet countries. I have 2 sets, this one for small bores 2-11mm and I have another set I use to measure liner bores, tapers and ovality that goes from about 100 to 20 is mm I think. Both mu sets come from Ukraine.


Now honing to size...

When honing rod bushes to size I use a Pemamo 220 adjustable honing tool. I don't have mine with me now to sho it in use but see below for a picture of a typical set



This tool can be fitted to a Lathe or a mill or even a drill press, you purchase the mandrels you need in the sizes required and it can hone bores very accurately.

See this link for the only video I could find of one being used -

This is a very nice tool but it's very expensive.

This setup works really well for prototype and low volume production but there are better and more efficient but much more expensive solutions for higher production volumes.


Hope that helps

Ricky

 

[Grimracer]

James, get you shade tree micron reader out. Or I an borrow you mine.. send 400 dollars for 600g sand paper and a plastic dowel and some note book paper to Grimracer..

Just kidding Ricky..LOL..

Love to see how its done.. but not sure how this helps James.

(hand wave) Move along.. the droid you are looking for is not here.............. move along.. move along!

Grim

 

[Terry Keeley]

Very nice equipment Ricky!

On the bigger engines (45's and up) we went for 0.0015" - 0.002" clearance for the bottom end of the rod and 3-5 tenths for the top.

I'm going to try this helical lap and non imbedding compound to finish some wrist pin holes in pistons:

https://www.helicallap.com/

 

[John Beardslee]

Very nice equipment Ricky!

On the bigger engines (45's and up) we went for 0.0015" - 0.002" clearance for the bottom end of the rod and 3-5 tenths for the top.

I'm going to try this helical lap and non imbedding compound to finish some wrist pin holes in pistons:

https://www.helicallap.com/

Terry, didn’t Paul Richardson use a spherical broach to size his RPM rods? It worked for the K&B series 70 pylon rods I made in the early days.
John

 

[James E Beasley]

Thanks to all for the input. I'll likely make a dowel with 600 - 1000 grit on it and shoot for the 1/32nds clearance.

 

[John Beardslee]

Thanks to all for the input. I'll likely make a dowel with 600 - 1000 grit on it and shoot for the 1/32nds clearance.

James,
I think 1/32” is waaaayyy to much clearance! Better recheck your information.
John

 

[Ricky Neal]

James, get you shade tree micron reader out. Or I an borrow you mine.. send 400 dollars for 600g sand paper and a plastic dowel and some note book paper to Grimracer..

Just kidding Ricky..LOL..

Love to see how its done.. but not sure how this helps James.

(hand wave) Move along.. the droid you are looking for is not here.............. move along.. move along!

Grim

Lol, fair point - i was hoping that showing the logic may help figure out what kind of clearance may be required but i fully understand that without the specialist tools its not a whole lot of help

 

[Ricky Neal]

Very nice equipment Ricky!

On the bigger engines (45's and up) we went for 0.0015" - 0.002" clearance for the bottom end of the rod and 3-5 tenths for the top.

I'm going to try this helical lap and non imbedding compound to finish some wrist pin holes in pistons:

https://www.helicallap.com/

Thanks.

I had to convert to metric to compare but thats 38-50 microns. Thinking about it - within reason i think its always safer to lean towards a bigger clearance. Too tight will always end in disaster

 

[Terry Keeley]

Thanks.

I had to convert to metric to compare but thats 38-50 microns. Thinking about it - within reason i think its always safer to lean towards a bigger clearance. Too tight will always end in disaster

Make it needle bearing and you'll never run a bushed rod again.

 

[Terry Keeley]

Terry, didn’t Paul Richardson use a spherical broach to size his RPM rods? It worked for the K&B series 70 pylon rods I made in the early days.
John

Not sure what Dave did, maybe he'll see this.

We used a broach to force the bushing into splines cut into the rod then reamed to size.

 

[Tom Foley]

I have tried reamers and that is not the way to go.....Leave the wristpin end alone.....
I just use a hone made of a piece of turned down slotted nylon dowel and some 600 wet and dry sand paper wrapped around it , and a cordless drill....
Remember , when the motor is running the conrod is always in compression.....
You have to reduce that contact patch between the rod and crankpin to reduce the friction causing heat....That is why a lean run will spin rod bushings....
These motors all have tramp oiling systems , meaning no pressure oiling system.....you need some clearances to get oil onto the crankpin and rod bushing...
The oil holes in the rod bottom are to let oil out not force oil in.....
Really the only need for the bottom of the rod is to start the motor and to clean it out....
When the motor is running the crankpin never sees the bottom of the rod bushing......Heat is what kills rod bushings.....
There are no set numbers for clearances....
I hold the rod on the crankpin and when I get the wrist pin end to rock up and down on the crank pin about 1/16" of an inch I call it good....
I polish the hell out of the crank pin with a buffing wheel and red jewelers rouge .....
If you are replacing a rod that has spun a bushing you have to make sure you get all the residual bronze bushing material off the crankpin...
That residual bushing is hard to see.....I check he crankpin with a 10 power eye loupe .........
Harbor freight has a good loupe set for about $5.00.......
If you don't get that residual bushing material off , you will be replacing the rod again in about one minute....
Anybody......I know all this can be confusing so........if you have any questions....
Call me
1-715-926-6096 landline
1-715-495-5025 cell
When a rod gets loose that is when it is good......

Rod you don't think that oil gets forced in through the oil holes in the rod as the rod is spinning around at 20 K rpm ?

 

[Grimracer]

Film thickness.. (I suspect) is in part to blame. The hole does ZERO if there is no room for the oil.

Sorry.. did not mean to answer for Rod. He likely has the right answer!

Grim

 

[LohringMiller]

Notebook paper is .003" (.076 mm) thick. That's the clearance I used for nitro big ends. You needed that much room to develop an oil film and not spin the bushing. You also needed 20% oil. K&B usually had this much clearance, but "precision" engines like the OS often had lower clearance.

Lohring Miller

 

[Geraghty]

Rod you don't think that oil gets forced in through the oil holes in the rod as the rod is spinning around at 20 K rpm ?

I believe the crankpin is the eye of the rotating fan resulting in a low pressure area at the crankpin sucking oil in......I believe that the holes let the oil out....
I may be mistaken but I don't believe so......If you find evidence to the contrary I will gladly apologize.........
I do know this .....spin a rod bushing and cover the holes and a tight rod will last about 2 seconds......