Pressed Pin versus machined on crankpin - discussion of virtues

[edholly]

I would be interested in other's opinions of whether to machine the conrod crankpin on or use a pressed in one.

My preference is actually for a pressed in one.

As I see it these are the fore and against reasons.

The fore's for a pressed in one .. to start. (or against machined on)

1. Pressed in gives a super smooth surface on the pin for the conrod to rotate on depending on the material used for the pin itself..
2. No bruising of the metal around the base of the crankpin due to the interrupted cut required to form it in an offset jaw.
3. Can be replaced at some time in the future if required.
4. Can really be custom made with at least 3 opportunities to get the fit right, as adjacent metal can be machined away to help with balance and this machining away will remove any traces of "trial fitting"
5. It is so much simpler and a lot quicker to do.
6. It is deadly accurate when using a DRO to place the centre to centre, something that is not the case with a 4 jaw in my experience, close but not 100% accurate.

Now against a pressed in one ( or fore the machined on one.)
1. No chance of it coming loose in service

Photo is of a the crankshaft for an ML Midge I am building which I changed to a pressed in pin - some beautiful silver steel drill rod with a super fine finish. Have to admit one of the two lightening holes was due to not being happy with the press fit, was just a tad not tight enough, the second time was very nice. Note the metal surrounding the extra holes will be removed for balance purposes.

Would really be interested in others thoughts on this subject please...

regards

Ed

 

[stevehuckss396]

Well. . . Can't argue too much with what you have said. I have not seen a pressed in pin. Most of the pieced cranks i have seen have been pinned or silver soldered.

1. If the crank is 1 piece you can still get a nice finish for the rod to run on.
2. If the crank pin is machined in the mill first the interrupted cut is not a huge deal.
3. Cant be replaced
4. If 1 piece there is no worries about how the pin will fit.
5. That's debatable as to easier. I would argue that it is easier to machine the crank than to screw around with pressing the pin.
6. There are ways to get the pin exactly where you want it.

Personally I would make it one piece and not worry about it coming apart. The extra time would be minimal and well worth the extra time. I guess it comes down to what you have in the way of tooling and what you are comfy doing with the machines you have.

 

[Nerdz]

Im glad this thread came up. I actually couldnt figure out how to machine a crank pin, so I ended up parting a small piece of round stock off, and drilling a offset hole. The pin was then pressed in place and loctited. In all honesty I did figure out how to machine a crank in one go, but the way I was doing it was using a parting off tool bit, and needless to say, it didnt like cutting off-centered stock. Maybe there is a different way..

 

[Walltoddj]

Im glad this thread came up. I actually couldnt figure out how to machine a crank pin, so I ended up parting a small piece of round stock off, and drilling a offset hole. The pin was then pressed in place and loctited. In all honesty I did figure out how to machine a crank in one go, but the way I was doing it was using a parting off tool bit, and needless to say, it didnt like cutting off-centered stock. Maybe there is a different way..

I don't know what you guys have to work with but if you make an offset holder to put in a spinner or a means to rotate the part and use an end mill to cut it or I plan on a circular cutter. The reason I'm looking at a circular cutter is that is the way we made the cranks for GM engines and it works well. If you move the setup to a grinder you grind a very fine finish.

Todd

 

[Swifty]

I don't see anything wrong with a properly done pressed in pin. You would have to be careful that you don't mushroom the head by having too tight a fit. It would also allow you to use a hardened pin for better wear, we used through hardened dowel pins at work, much better than the case hardened kind, they would make great crankpins.

Paul.

 

[Sshire]

I've always done pressed in pins. I use a thou under reamer, drill rod for the pin and Loctite 609. I like Swifty's idea to use hardened dowel pins. Going to try one on the next engine.

 

[deverett]

Think model railway locomotives (and full size ones)... the crankpins on the driving wheels are always pressed in. Been like that for many, many years quite successfully.

Dave
The Emerald Isle

 

[lohring]

As I've posted before, Jim Allen has had great success with pressed in pins. A serious advantage is that you can use a high hardness crank pin (M-2 reamer blanks at Rockwell 64C) for a needle bearing inner race with a shock resistant crankshaft steel (S-7). It's also a lot easier to build a two piece crankshaft accurately when you don't need to do anything but cut off the crankpin and bore or grind an accurate offset hole in the crankshaft.

Lohring Miller

 

[MachineTom]

Many of the high performance motorcycle engines used pressed in pins, mostly single cylinder stuff, but 40HP out of 125cc is quite a load on a 3/4" crankpin.

 

[mcostello]

Hardly Dangerous (Harley Davidson) uses them also.