Advice on shortening engine piston/wrist/gudgeon pins?

PackardV8 · Mar 18, 2018

Engine wrist pins are the hardest piece of steel we ever see in an automotive engine machine shop and are never cut, just replaced.

For a modified piston in an obsolete engine, we need to shorten eight 1.00" x 3.00" thick wall pins by .250".

Since length to a thou is not critical, a shadetree method would be to hand grind for stock removal and finish to length and square in the tappet grinder.

How would the pros do it?

jack vines
Obsolete Engine Division
of Mager Engine

Larry Dickman · Mar 18, 2018

Wire EDM

1234567890

atex57 · Mar 18, 2018

Ceramic should walk and talk through them. Use lots of RPM!

Ed.

JS · Mar 18, 2018

Yep ,

Wire EDM.

Probably 2-3 hrs of run time.

CarbideBob · Mar 18, 2018

Hard turn or grinder cut off in a cnc spin fixture.
Both with CBN.
Cycle measured in minutes and seconds so faster and nicer than a bench grinder.
Bob

JRIowa · Mar 18, 2018

Well, if I had the very well equipped shop, I'd do like above. WEDM has less of a chance of leaving any chucking marks to polish out. A cut-off wheel on a surface grinder also works good on stuff like that. That's what I'd used 'cause I don't have all of the other stuff.
JR

Limy Sami · Mar 18, 2018

For 8 off? ..even if I'd shop full of every machine you care to mention, for 8 off I'd cut them off on an abrasive chop saw and finish to length on the tappet grinder.

redlee · Mar 18, 2018

I have no idea,but are the pins case hardened and if so will it matter.
Ceramic tooling would work well.

PackardV8 · Mar 18, 2018

Limy Sami said:
For 8 off? ..even if I'd shop full of every machine you care to mention, for 8 off I'd cut them off on an abrasive chop saw and finish to length on the tappet grinder.

Thanks, Sami. So far, that's the winner, as it doesn't require any tooling purchase.

jack vines
Obsolete Engineering Division
of Mager Engine

Danny VanVoorn · Mar 18, 2018

I'm certain those could be cut on a lathe in a collet, the right carbide insert in the proper grade and finish would go right through them.
Dan

adama · Mar 18, 2018

I don't have a abrasive cut off, so i would mark them and then thin disc free hand with a angle grinder, then take them all at once for a ride on the surface grinder. Have tried turing one and it was very hard, far easier to hack it off and then true up all with abrasives.

Biggest thing i don't see anyone mention is edge brakes on the trued up parts, you probably just can chuck them up in a lathe and do that with a hand stone, but you want to smooth that corner out so you don't scratch the bores as you assemble things. Little details matter and if you de-bur them unlike me you won't be sitting here with a realy nastily cut finger (made a 5" puddle of blood on the floor!!) on a burr you decided you did not need to remove just yet and then slipped on a trial fit slicing open a lovely gash of knuckle!!!

Limy Sami · Mar 18, 2018

Thanks Jack, ...I had a feeling it mightn't

Clive603 · Mar 18, 2018

The one time I did something similar I put a thin (1 mm) cut off blade in my Clarkson T&C grinder and set up to roll the part under the disk to make the cut. Want to think that rolling through to give a cut all round stopped it getting a sharp splintery edge. Came out pretty clean so a little touching up with a diamond file sufficed to put a small, clean, chamfer on.

Disk was running too slow really but job got done fine. Luck, judgement or skill? Who knows! Take your pick but probably not door no 3.

Clive

CarbideBob · Mar 18, 2018

If the engine is worth a lot of money or has high output I'd be a bit hesitant to use a dry chop saw.
Likely not a problem with cracks but I've seen too many recalls and market failures on powertrain things that should have worked fine so I'm a bit gun shy.
On a lawnmower engine I'd do the chop saw thing all day long and would deburr on a Scotch-brite wheel ignoring any nice square end finishing.
In a race motor or a high dollar rebuild.....details count even though a hack might work just fine.
Is the cost of a pin seize/failure high?
Bob

Dan from Oakland · Mar 18, 2018

Those who suggested a cutoff wheel in a T&C grinder motorized work head are spot on. Finish off the end to length with a cup wheel. Only really low dollar wrist pins are carburized and soft in the middle.

The cutoff wheel on a rotating part tends to cut pretty darn straight. Adama makes some good points as well- including points referring to deburring the end. Trying to part them off or turn them with an insert has never worked for me.

tdmidget · Mar 18, 2018

Sounds like a perfect application for surfaced hardened material to me. I think Nitriding is pretty easily done to .030" and flame hardening much deeper. Why would you want a wrist pin to be in a brittle range?

Dan from Oakland · Mar 19, 2018

Wrist pins are reciprocating weight and reducing the weight is good for a multitude of reasons. Reduced weight = reduced wall thickness. Reduced wall thickness = higher stresses. Quality wrist pins are not made from mild steel for this reason- even up to the point where you can buy wrist pins made from premium tool steel for performance applications. Regardless of why, wrist pins are mostly thru hardened and that makes them somewhat of a challenge to shorten. Also, I'm not sure what hard has to do with brittle, but. . . just sayin.

MattiJ · Mar 19, 2018

My choice? Far from pro but I would turn the extra quarter inch into orange hot chips with CBN insert on lathe. Won't take long even on my skimpy hobby lathe.

PackardV8 · Mar 19, 2018

Clive603 said:
The one time I did something similar I put a thin (1 mm) cut off blade in my Clarkson T&C grinder and set up to roll the part under the disk to make the cut. Want to think that rolling through to give a cut all round stopped it getting a sharp splintery edge. Disk was running too slow really but job got done fine. Luck, judgement or skill? Who knows! Take your pick but probably not door no 3. Clive

That's also an option, Clive. I have a tool cutter grinder. A bit more description of how you controlled the workpiece to roll under the disc? I can picture laying it in one of the T-slots in the table, with a stop at each end; but I can also picture it jumping out and breaking that 1mm cutoff wheel.

jack vines

converterking · Mar 19, 2018

I would order new pins to the length I specify and charge the customer.

Advice on shortening engine piston/wrist/gudgeon pins?

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