CNC Machinist: If Machining a 2 Stroke Crank?

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Armilite

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A 1960's AJS motorcycle was repaired with a built up crank with pressed in journals.I read somewhere that Rotax 912 cranks were pressed and could be tig welded (https://www.experimentalaircraft.info/homebuilt-aircraft/edge-performance.php)
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I'm pretty sure Rotax 912/914 Cranks are One Piece, the Rotax 2 Stroke Cranks are Press Fit. Then you need to align the parts. Factory Crank Specs are Sloppy like .003" or more out of phase. Racers try to phase them .001" or better, then either Weld, Pin, or Epoxy Glue them so they don't twist out of phase. I have talked to people who got New Cranks from Rotax that were as much as .007" out of phase, do we blame Rotax or the UPS guy that delivered it.

That Edge Performance looks like it offers some nice Upgrade Options, EFI, Turbo, Ceramic/Moly Coated Pistons, Crank Rebuilding & Welding, Big Bores.
https://www.experimentalaircraft.info/homebuilt-aircraft/edge-performance.php
 

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Armilite

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Do they really need to be one piece? A welded press fit taper may be just fine.
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Well, since about 99% of them that I have ever seen are made one Piece, probably to keep them Concentric, must be the best way. A low hp Engine may use that process, but like I said, about 99% of them are made one piece. It's bad enough to have to Phase just the PTO/Mag ends of a Single Crank, let alone a Twin(180), Tripple(120), Quad. 2 Stroke Crank Rebuilding is fast becoming a lost art.
 

Turd Ferguson

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Almost all small 2 stroke engines use a built up pushed pin crank because they use either ball, needle, or some combination thereof for bearings on connecting rods and crankshaft.
 
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Turd Ferguson

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A 1960's AJS motorcycle was repaired with a built up crank with pressed in journals.I read somewhere that Rotax 912 cranks were pressed and could be tig welded (https://www.experimentalaircraft.info/homebuilt-aircraft/edge-performance.php)
A big two stroke rebuilder called SBT welds the pin after aligning the crank but I don't see any advantage to doing that from an operation perspective. For them it allows use of crank halves with the pin hole slightly out of spec on the large size that would otherwise be scrapped.
 

Turd Ferguson

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What is there to be out of phase on a single? You can adjust ignition timing.
The bigger problem would be that the full torque of the engine and power impulses have to pass through a pressed fit juncture. Absent a mechanical driving device - key, pin, tang, etc. those parts would invariably work loose from the constant hammering. Have seen a one piece crank twisted into at that point. I have never seen a crank wheel pressed on the shaft in a two-stroke.
 

blane.c

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I think you have to machine the rod side separately. You could machine as one piece both outer crank shafts, PTO and Mag with both ends center drilled and driven with a lathe dog and provided that the rod journal(s) are outside the shaft diameter drill them "in situ" then saw the piece in half and face both sawn cuts.
 

blane.c

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If the rod journal holes are not round but polygonal and the rod journal ends are precision machined, when pressed together correct alignment is assured and they cannot turn in their holes without metal deformation.
 

proppastie

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what seems like a simple crank assembly is not so simple....the people that design and build them have lots of experience and I would guess made lots of scrap in their years of business......If you want to go into business selling these items the easiest way would be to find a company that already makes like items (another engine manufacturer) and if they will make the quantities you want give them a sample of what you want to copy. It will not be cheap, but if there is a market for these items then you can mark up the price.
 

Armilite

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What is there to be out of phase on a single? You can adjust ignition timing.
Hold your Hands together, Think of them as the Crank Wheels. Now turn one hand Outward and the other Hand Inward. You have just casued the Imbalance of your Crank. That is what cause's most of the Crank Failures and Viberation. That's WHY, Racers True the Crank Wheels, then either Weld, Pin, or today some use Epoxy Glue. You Still also have the Spining Imbalance of the Crank Wheel Weight vs the Rod, Piston & Rings, and Bearing.

Bigbore stroker 277 part 13 - Crank balance
https://www.youtube.com/watch?v=phrq8NVkWv8&t=140s

Bigbore stroker 277 part 14 - Testing crank balance
https://www.youtube.com/watch?v=XpnS21EzoSc&t=139s
 

Armilite

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Almost all small 2 stroke engines use a built up pushed pin crank because they use either ball, needle, or some combination thereof for bearings on connecting rods and crankshaft.
Their talking about the Crank Wheel and PTO Shaft being one Piece vs Pressing in PTO Shaft and Welding it. They all use a Pressed on Crank Pin that connects the Crank Wheels.
 

Armilite

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A big two stroke rebuilder called SBT welds the pin after aligning the crank but I don't see any advantage to doing that from an operation perspective. For them it allows use of crank halves with the pin hole slightly out of spec on the large size that would otherwise be scrapped.
Racers Weld them to keep them in Phaze usually .001" or better. They can twist out of Phase from Drag Racing going from a Dead Stop to Full Throttle, Prop Strikes, Kick backs, etc. The Crank Bearings will also last longer and you will have less Viberation.
 

Armilite

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I think you have to machine the rod side separately. You could machine as one piece both outer crank shafts, PTO and Mag with both ends center drilled and driven with a lathe dog and provided that the rod journal(s) are outside the shaft diameter drill them "in situ" then saw the piece in half and face both sawn cuts.
Yes, I think you would have to Machine Rod side, a 2nd operation, but 90% could be done in 1st operation. The Time to turn down the Max 4.0 - 5.5" OD to the 30mm Pto & Mag Shaft is what takes alot of Time if doing a Billet Part.

Here is a Billet Polaris Crank.
 

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blane.c

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Those pieces are either cast or forged, then just the shiny spots are machined. Machining reliefs and boss's not so bad, the pockets and irregular portion that stands proud would be difficult without CNC.

You could machine everything and leave extra at the shiny places, use blast media to get the overall texture, and then finish machining it.
 

Turd Ferguson

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Racers Weld them to keep them in Phaze usually .001" or better. They can twist out of Phase from Drag Racing going from a Dead Stop to Full Throttle, Prop Strikes, Kick backs, etc. The Crank Bearings will also last longer and you will have less Viberation.
So they say. I have seen two stroke engines abused as much as humanly possible and I think worrying about a crank pin slipping is a "majoring in the minor" exercise. Race engines have short lives anyway and if one has a prop strike hopefully the engine will be disassembled and inspected.
 
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