Con Rod Machining

[Molochnik]

Hi all,
I'm new here (1st post), and attempted due diligence of searching this out, but couldn't find anything that looked like it was what I needed.

I'm attempting to remove .070" from the outside of the big end on a 2 stroke motorcycle con-rod. I don't have much info on the metalurgy of the rod, but I imagine they're all about the same.......for bike anyway. But it's pretty hard.

I held out some hope that MAYBE HSS would do the trick, but as I suspected, no joy. I attempted to use an indexable end mill w/ carbide inserts, but no luck there either. I figured it would be a miracle if was able able to cut it period, and it looks like some kind of a grinding bit may be in order. I generally only ever machine aluminum, and only ever work low-med hot and cold roll every now and then, so I am admittedly ignorant on the ways of the tough stuff.

Any suggestions? Carbide burrs? Thanks in advance.....I appreciate any input.

[holbieone]

Originally Posted by Molochnik Hi all, I'm new here (1st post), and attempted due diligence of searching this out, but couldn't find anything that looked like it was what I needed. I'm attempting to remove .070" from the outside of the big end on a 2 stroke motorcycle con-rod. I don't have much info on the metalurgy of the rod, but I imagine they're all about the same.......for bike anyway. But it's pretty hard. I held out some hope that MAYBE HSS would do the trick, but as I suspected, no joy. I attempted to use an indexable end mill w/ carbide inserts, but no luck there either. I figured it would be a miracle if was able able to cut it period, and it looks like some kind of a grinding bit may be in order. I generally only ever machine aluminum, and only ever work low-med hot and cold roll every now and then, so I am admittedly ignorant on the ways of the tough stuff. Any suggestions? Carbide burrs? Thanks in advance.....I appreciate any input.

why are you removing .07 from the lower end of the connecting rod ?

try a solid carbide tapered ball end mill .0625 rad. slow rpms to start

[Molochnik]

The crank was stroked A LOT, so the big end pokes out past the crank cheek. If the material isn't removed it'll contact the case. The prior rod was done this way and saw a considerable amount of supermoto, road race, and desert riding with no stretch at all, comparing the new to the old. The old rod just had a lot of time on it so I decided to rebuild the crank, myself, and not send it back out.

Thanks for the advice! Any reason to use a ball?

[holbieone]

a ball is stronger

you could use a bench top belt sander

[Molochnik]

Aha, I understand.......didn't think of that.

I figure I'll have a little more control on the mill, but the sander probably would be more expedient.

Thanks a lot for the help!

[holbieone]

Originally Posted by Molochnik Aha, I understand.......didn't think of that. I figure I'll have a little more control on the mill, but the sander probably would be more expedient. Thanks a lot for the help!

just take a little at a time with smooth even passes and measure in several places on each side ever couple of passes

was the old rod stretched or is it stock length ?

[Molochnik]

Stretched from use? No. The crank is out of a YZ250, but the rod and piston are from the earlier TRX250 quads. The TRXs rod has the right length for the stroke added to the YZ crank (.25"). The piston is the largest oversize for the TRX (can't remember exact size at the moment), which gives a total displacement of a smidge less than 300ccs.

I compared the I.D. of the new and old rod (both aftermarket "Hot Rod" units), and they were within half a thousandth, w/ the new rod being the bigger one. I didn't bother comparing overall stretch center to center.

[Molochnik]

The bottom end was actually no problem at all; like I said, I just thought it was time since there was a good hundred hours on it. I would've felt like a fool if there was a catastrophic failure letting it go that long.

[Molochnik]

Originally Posted by holbieone just take a little at a time with smooth even passes and measure in several places on each side ever couple of passes was the old rod stretched or is it stock length ?

Well, I compromised between using a sander and the mill......I mounted a 2.25" flap wheel in the quill and left the rod mounted to my rotary table. It's worked pretty well. I decided to use the flap wheel since it was a better match for the fillet where the big end meets the shank.

Again, I appreciate the advice, holbieone.