Discussion - big ends on single cylinder engines - machined on or pressed in ?

[edholly]

Have made a few engines now both with machined on big-ends and pressed in ones.

Have found in service both to be satisfactory and of course the pressed in variety is a lot easier to make.

I know the machined on ones should be more sturdy - but - I wonder if that is necessarily the case.

Would welcome your ideas on the subject.

Ed

 

[lohring]

Jim Allen makes an extremely durable crankshaft with a pressed in pin. The choice of materials as well as the amount of press fit is key. See his gallery at http://gallery.intlwaters.com/index.php?cat=10141 (log in required). This is his description with some pictures.

"AISI S-7 tool steel that is through hardened & tempered at 600 deg F is used for crankshafts. The crank pin hole is carefully ground to give a .0013 interference fit. --- Crankpins are full hard M-2 steel reamer blanks"

His connecting rod and big end bearing are the most advanced of any I know of, but that's another story.

Lohring Miller

 

[LindsayWood]

Have made a few engines now both with machined on big-ends and pressed in ones.

Have found in service both to be satisfactory and of course the pressed in variety is a lot easier to make.

I know the machined on ones should be more sturdy - but - I wonder if that is necessarily the case.

Would welcome your ideas on the subject.

Ed

Separate crank pins allow you to use really hard materials pre-ground to exacting sizes. Drill-rod, HSS, etc. I use piano wire. It's available in a range of small sizes, is sized precisely, has an excellent surface finish and is HARD. It's some tough stuff- you'd be lucky to get 1mm into a piece of it before you blunted a new hacksaw. Carbide and abrasives are the only effective means of cutting it hard.

You could harden the pin on a one piece, but you risk distorting your crank. You'll also harden the rest of your crank which is often undesirable.

 

[Hopper]

Full sized motorcycle engines have used pressed-in crankpins and mainshafts for many years, including the newer Harley Davidsons and many high-performance two strokes etc.
So should be just fine for model horsepower. Like Lindsay said above, you can then use hardened precision pins.

 

[Niels Abildgaard]

Have tried to make a real two stroke crankshaft that is two discs .a hardened connecting rod pin and two shafts and failed miserably.Wobly.
Am trying to make a radical different configuration with only one single main bearing and a hardened bush epoxyed/pressed on.
The bush is a standard inner ring from a needle bearing 12 mm inside 16 mm outside.The connecting rod is from a Vespa moped 50 ccm.
Inspiration comes from the french Velo solex 50 ccm moped made in millions,and here the hardened bush is 10 inside 14 outside

http://www.vogts.dk/knallert/velo/ref724_1.jpg

 

[Niels Abildgaard]

When that radical crank is no good either I will try to make a real two stroke crankshaft according to Edgar Westbury Bumble Bee.

http://modelenginenews.org/etw/2s/index.html#A13

The cylinder will be an opposed piston design with two times 60 mm stroke and 40 mm bore.
And two cranks,one at each end of cylinder.