Engine Trouble Big end conrod bearing failure.

[tinochids]

I've had continuous big end needle bearing failure on my 66cc engine. Now I have heard of uncaged crowded roller bearings. What are those? The casings fro the ones I have been using, break and send debris into the cylinder, scratching it and damaging both piston and rings and this happens with less than a 100km on the clock for the new bearing.

 

[Nickt919]

I’m quite familiar with this issue myself.
The tolerance between the cage and crank pin is probably .5mm or less when it’s new.
The crank and pin are made of soft Chinese metal. Repeated hard runs causes fretting on the crank pin. This in turn causes the cage to touch the crank pin. Eventually the cage wears through and goes to pieces. The pieces get sucked up the transfers and into the combustion chamber.

I have not had much luck pressing the crank pin out and back in successfully. Once the pin is pushed out and back in the fit is too loose to run it. Keeping the crank true after taking the pin out is impossible…at least for me.

I’ve been just changing out the crank around every 600-700 miles…before it goes to pieces.

I figure it as a consumable item and a cost of running these engines 10k+

A long stroke zl40 is about $40 shipped from CDH Power

 

[Whitewill1412]

I don't think I've heard of this issue before now. What engines are you running? Are you using proper lubrication/oil mix? Have any of you other guys run into this problem?

 

[Nickt919]

I run a Minarelli hybrid. CG Case & crank…Minarelli top end..Athena sport kit 70cc.. vm20..jog pipe.
I never alter port timing…case to cylinder matching only and the engine runs like scalded dog.
I do run an aftermarket cdi and alter the timing a few degrees.

I’m pretty sure I get the type and mix correct.

If you run these engines above 10K rpm’s on a regular basis it’ll happen.

 

[ImpulseRocket]

Yeah, the bottom ends just aren't built to a good enough standard for long term/sustained high power and rpm. The bearing rollers are harder than the crank pin, which doesn't help matters..

I had toyed with the idea of making a crank pin out of tool steel and case hardening and tempering it, but I never got around to it and I no longer have access to a lathe.

Then there is the issue of the "chain" of weaknesses that follows when you modify something like that. Which is the main reason I never bothered.

 

[GW's Motorized]

I wonder how much it would cost to have someone make a pin.

 

[Nickt919]

I tried a hard pin from McMaster unsuccessfully. I couldn’t press it in with a fit that was acceptable.

Even without removing the pin my cranks do not stay true. Over time running the Minarelli conversion, vibes creep back in.
This mean the crank halves are moving on the pin.

This problem is probably the very reason RDM started making cranks.

The Minarelli top ends work on a CG case but are not intended for this purpose.
It holds up for a while but eventually it will fail if I keep on running it.

 

[GW's Motorized]

I was just wondering about someone making one that would fit if you heated the crank halves and cooled the pin

 

[NeedSpeed]

I don't think I've heard of this issue before now. What engines are you running? Are you using proper lubrication/oil mix? Have any of you other guys run into this problem?

I've dusted 2 BBR's at 20:1 ratio. The first one had 194 psi cranking compression though haha

 

[ImpulseRocket]

I was just wondering about someone making one that would fit if you heated the crank halves and cooled the pin

The issue goes back to the chain of weakness that I mentioned, and what Nick was just explaining. Even if the pin can handle it the rest of the chinesium crank can't.