[captainDL]

Hi everyone. I am going to be making a set of connecting rods for my snowmobile engine. I need a shorter center to center distance so off the shelf isn't possible. My question is: If I machine these from 4140 should I have them thru hardened and tempered, or should I make them from a material that can be case hardened? The bores need a hard surface because they act as races for the needle bearings. Any info./ advice would be appreciated.

[RICHARD ZASTROW]

Look up specs on 8620. Good toughness and case hardening characteristics.

I don't know the reasoning for the cc shortening, but I hope you considered the effect on balance and harmonics. If this is a 2-stroke engine, the high engine rpm potential can put you into "shattering mode". Somewhere there is a bunch of clutch parts orbiting the earth. I've never determined where they landed or if they ever did. LOL

Dick Z

[aaron p]

8620 and 8740 are good materials to look at.

[BillTodd]

I need a shorter center to center distance so off the shelf isn't possible.

Not an answer to your material question, but I have heard of con-rods being shortened in a press with a little local heating and some care to keep things straight. (apparently it was common practice when 'stroking' and engine to larger capacity)

[Bill Johns]

Rockwell 38 to 40 C scale, you don't want the whole rod case hardened. I'm not sure what the hardness for the needle bearings should be, you would want to just harden the
bearing surfaces.

[aaron p]

Not an answer to your material question, but I have heard of con-rods being shortened in a press with a little local heating and some care to keep things straight. (apparently it was common practice when 'stroking' and engine to larger capacity)

Ive never ever heard of this. Im around high end race engines all the time and went to school for race engine theory and machining. Putting a rod in a press would make the big end and small end out of round where the wrist pin and rod bearings go.

[BillTodd]

Ive never ever heard of this. Im around high end race engines all the time and went to school for race engine theory and machining. Putting a rod in a press would make the big end and small end out of round where the wrist pin and rod bearings go.

Obviously care has to be taken not to squash the ends

A couple of mandrels in the ends and a bit of local heating to soften the rod in the middle would reduce the load on the ends enough I think.

[captainDL]

Thanks for all the advice so far. So what I'm understanding 4140 isn't a good choice? I only need to harden the races not the entire rod. 8620 may be a better option. Will 4140 case or flame harden in the bearing area? I know most aftermarket rods are made from 4340 and forged, but I can't find 4340 in flat stock and forging isn't really feasible for a single set of rods. Just want to have the best chance of having long lasting rods.