CNC Machinist: If Machining a 2 Stroke Crank?

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Turd Ferguson

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Their talking about the Crank Wheel and PTO Shaft being one Piece vs Pressing in PTO Shaft and Welding it.
I understand that and the junction where the wheel is attached to the shaft sees every power pulse and all the torque produced by the engine. There are reasons why cranks are not built that way and I would wager reliability is one of them.
 

blane.c

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If you are trying to remove less metal you could if you machined the effected area away later, weld two or more different diameters together to lock concentricity do the machining and cut out the heat affected area. It may prove to be less wasteful.
 

Armilite

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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.
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Each Brand can use different Grades of Steel and Tolerances do vary. Like all Rotax Cranks can be Pressed apart with a cheap 20 Ton HF Hyd Press. But a few other Brand of Cranks might take a 40-50 Ton Press.

With most Rod kits you get a New Crank Pin. If the Crank Pin Hole in the Crank Wheels needs Repair, some Weld-Up and machine, some use a Rod with Over Size Crank Pin so just ream to that Size, some use an Offset Crank Pin the raise the Stroke.

All of these UL 2 Strokes use a 24mm Crank Pin 277UL to 670. Not sure what the 185UL used either 22mm or 24mm.

Luckily, very few Cranks ever Fail themselves, it's the Crank Roller Bearings and Rod Needle Bearing Cages that usually Fails first. Rotax went to the Upper Rod Cageless bearing that Solved that one from Failing. You don't want to use any Crank Bearings with Plastic Retainers, Nylon Retainers is Best, then Steel Retainers. Today you have Hybrid Crank Bearings with Steel Races with Ceramic Balls.

Notice How much Material he removed out of the 277 Crank Wheel. You want to replace that Hole with usually a Plastic like Delrin/Nylon, otherwise you can lower your Crankcase pressure.
 

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blane.c

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Each Brand can use different Grades of Steel and Tolerances do vary. Like all Rotax Cranks can be Pressed apart with a cheap 20 Ton HF Hyd Press. But a few other Brand of Cranks might take a 40-50 Ton Press.

With most Rod kits you get a New Crank Pin. If the Crank Pin Hole in the Crank Wheels needs Repair, some Weld-Up and machine, some use a Rod with Over Size Crank Pin so just ream to that Size, some use an Offset Crank Pin the raise the Stroke.

All of these UL 2 Strokes use a 24mm Crank Pin 277UL to 670. Not sure what the 185UL used either 22mm or 24mm.

Luckily, very few Cranks ever Fail themselves, it's the Crank Roller Bearings and Rod Needle Bearing Cages that usually Fails first. Rotax went to the Upper Rod Cageless bearing that Solved that one from Failing. You don't want to use any Crank Bearings with Plastic Retainers, Nylon Retainers is Best, then Steel Retainers. Today you have Hybrid Crank Bearings with Steel Races with Ceramic Balls.

Notice How much Material he removed out of the 277 Crank Wheel. You want to replace that Hole with usually a Plastic like Delrin/Nylon, otherwise you can lower your Crankcase pressure.
You could just machine a blind hole instead of a through hole, say leave it .030" thick. The material left in place can't fall out or leak.
 

Armilite

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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.
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The Add said it was a Billet Crank. That textured area on Crank Wheels I think is just glass beaded. Standard Twin Cranks go for around $450-$650, Billet Cranks go for $1600+.
 

Turd Ferguson

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Notice How much Material he removed out of the 277 Crank Wheel. You want to replace that Hole with usually a Plastic like Delrin/Nylon, otherwise you can lower your Crankcase pressure.
lol, The first crank lightening hole stuffing I ever saw was around '69 or '70 and they used cork. It was the latest and greatest speed trick of the day. Never proved to have any benefit.
 

Armilite

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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.
All I can tell you is what they told me. I also know Rotax Rick who has rebuilt 600+ Rotax Cranks and over a 1000 Engines, the last I knew, with No Crank Failures, he Trues them to .001" and then Welds them also. If it can make the Bearings last Longer, and make less Viberation, I think it's worth it. That's Why, some guys win all the Races, don't Blow up their Engines as much by paying attention to Details. That 277 Race Engine Hollerand used was a Stock 277 Lower end turned 9200rpm running 14.5cr and was used all Year long. He's Upgraded it to a Big Bore with a Reed Fed Cylinder now.

Here is a Good example of rebuilding a Crank.
http://www.highwaymanbikes.com/rebuild-of-a-single-cylinder-press-pin-crankshaft-how-i-do-it/

There are Prop Strikes and there is Prop Strikes.
 

Armilite

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What kind of heat treating is done on the part, if any? If so, when it it performed? Typically a billet part is roughed out, heat treated, then finished machined (usually ground)
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I don't know of any Heat Treating done to them. I was thinking of using Cryogenic Treatment. An Old Sled Racer did that to his parts. Had a bunch of trophys on the wall. Yes, many Cranks are machined, then finish ground. You can contact these guys and ask what they do.

Northern Crankshaft
https://www.northerncrankshaft.com/

The Crank Shop
http://thecrankshopvt.com/index.php?cPath=21_40

I know some Car Billet Cranks are made from 4340 Steel and some are Heat Treated, but not all.
 

Armilite

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lol, The first crank lightening hole stuffing I ever saw was around '69 or '70 and they used cork. It was the latest and greatest speed trick of the day. Never proved to have any benefit.
Cork would work, but today most use Plastic. By Reducing Crank Wheel Weight you Reduce Loads on the Bearings, and the Engine will Spin up Faster.
 

Armilite

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You could just machine a blind hole instead of a through hole, say leave it .030" thick. The material left in place can't fall out or leak.
Yes, there are many different ways and materials to use. Here is a Yamaha Crank, Welded, and looks like Alumium used to fill Holes. Looks like the drilled the Crank Pins and then Plugged them also.
 

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pictsidhe

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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
That's run-out, not a phase difference. It comes from a misaligned big end fit. A wrinkly old guy swinging a copper hammer is the traditional way to fix it. Using a suitable press fit of a wheel onto a shaft does work. Just make sure that it can take more torque than the output gear etc to avoid problems. Keys do not take engine torque, they are for assembly alignment only. Splines will take engine torque.
 

Turd Ferguson

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As my engineer bud says, the real test of a product is to put it in the hands of Joe Consumer. He will abuse the crap out of it. If there are any weaknesses, he will find them, lol.

Rotax put millions of engines in powersport vehicles without welding the crank pins and I'll wager their failure rate was pretty low.
 

blane.c

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It seams to me that a round hole press fit has been working for decades, but if you want better a locking taper polygon.
 

Armilite

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That's run-out, not a phase difference. It comes from a misaligned big end fit. A wrinkly old guy swinging a copper hammer is the traditional way to fix it. Using a suitable press fit of a wheel onto a shaft does work. Just make sure that it can take more torque than the output gear etc to avoid problems. Keys do not take engine torque, they are for assembly alignment only. Splines will take engine torque.
That's what Phasing the Crank is pictsidhe, your Aligning the Crank Wheels, which also Aligns the PTO and Mag Shafts that the Bearings go on, so your Runout is .001" or less on the Shafts. Reading should be taken off the Shafts, but some People do it off the Bearings. They usually use a Lead Hammer 5+lb. Today they have some nice Fixtures that Help to Align them faster, but cost a $1000+.
 

proppastie

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One of the problems trying to run needle bearings on the crank (at the rod ?) that part needs to be hardened and ground,....not a simple lathe or CNC lathe job. Maybe you can buy hardened/ground steel rods of the correct diameter. Be nice to get them to the proper length too....That press fit fixture sure would come in handy for the assembly.
 

Armilite

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As my engineer bud says, the real test of a product is to put it in the hands of Joe Consumer. He will abuse the crap out of it. If there are any weaknesses, he will find them, lol.

Rotax put millions of engines in powersport vehicles without welding the crank pins and I'll wager their failure rate was pretty low.
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Well, I agree, they usually put Products on the market today early and then let Joe Consumer be there Test Dummies so they can also make Millions in Repairs today. Most products will last just long enough to make their Short Warranty. Many Rotax's Engines have Failed, but since they aren't Certified for Airplane use, nobody has to keep Records. Rotax Rick will tell you, 3 out of every 5, 582UL's that come into his Shop has had a Crank Bearing Failure, from either Cranks being out of Phase or Oil Pump Gear Failure, that's Why he takes off the Oil Injection. Now this guy has to Fix over 1000 of these Rotax Engineers Failures to properly design an Engine that should last a 1000hrs. Most Sled Engines Fail around 2500 to 4000 Miles. With and Average Trail Speed of 35mph, 2500/35 = 71.4hrs and 4000/35 = 114.2hrs. Rotax's UL TBO 300hrs. 300*35 = 10,500 Miles. The Highest untouched Skidoo/Rotax Engine I have heard of is 7000 miles. 7000/35 = 200hrs!!!

Detonation is the #1 Failure Point of Rotax 2 Strokes. That's straight from the lips of the head Rotax Engine Manager/Engineer I talked to at Oshkosh years ago. With over 6 Million Snowmobile/Jet Ski/UL/etc., Engines Built, how much Money do you think they made doing Repairs and Replacing Engines?

76 Arctic Cat JAG 2000 275 Free Air, I got given to me for Free, guy Siezed it at around 1900 miles.
93 Arctic Cat 580 EXT around 2800 miles, only had an Exhaust Gasket failure.
94 Yamaha V Max 600 around 3300 miles, bought from a guy who messed with the Jetting and burned a hole in one Piston. Replaced both and only had a Lower Cylinder Gasket Fail after about 6-7 years of use.
95 Yamaha V Max 600 around 3300 miles, No failures so far.
98 Skidoo 670, around 3300 miles, I bought it that way, and don't know of any Failures yet. Guy said he hadn't had anything major since he had it.

Rotax 503UL had just been Rebuilt with about 19hrs on the meter. Guy had died and son was Selling it, so he didn't know much about it's History. The guy had worked for the T-Bird Factory when it was East of Des Moines, Ia. No trouble with it so far.

If, you rebuild your own Engines & Cranks, Welding. Pinning, Epoxy Gluing the Cranks aren't a big deal. High-Level Sled Racers do it. Rotax Rick does it with Zero Failures, so that's good enough for me. Ask these few other Crank Rebuilders how many they have had fail.

If Avg Ultralight Flying Hours a Year is as Kitplanes says 50hrs:
Rotaxs 300hrs/50hrs = 6 years!
Rotaxs 300hrs/75hrs = 4 years!
Rotaxs 300hrs/100hrs = 3 years!

Hirths 1000hrs/50hrs = 20 Years!
Hirths 1000hrs/75hrs = 13.3 Years!
Hirths 1000hrs/10hrs = 10 Years!

Simonini 600hrs/50hrs = 12 Years!
Simonini 600hrs/75hrs = 8 Years!
Simonini 600hrs/100hrs = 6 Years!

Now the Highest Hours I have ever heard of on a pair of Rotax 582's and 503's is around 1300hrs and were still Flying with just a DeCarbon every 200-250 hours! They only used Max 5500rpms. Never thought to ask what they Avg a year, and never asked if they ran Oil Injection or premixed. They did use the Penzoil Air Cooled Oil.

1300hrs/50hrs = 26 Years!
1300hrs/75hrs = 17.3 Years!
1300hrs/100hrs = 13 Years!
1300hrs/125hrs = 10.4 Years!

Rotax Rick told me once, he had a Pair of Rotax 582's that came in once to be Inspected that had burned nothing but 100LL, that had 400hrs a piece on them. The only thing he noticed differently was the Piston Tops were a more Yellowish Color vs Normal Tan/Brown Color. Other 2 Stroke Companies say 100LL is ok to use, you just have to Jet the Engine for it to be fully Optmized for a good GPH. Other wise it will run a little Rich if Jetted for 91. Basically your GPH will go down a little if you don't Re-Jet for it. Example, if your engine is say burning 3.0gph with 91 Octane, with 100L with same Jetting it might be 3.2gph or 3.3gph. Around me, 50 mile Radius, the Airports baiscally only carry 100LL. Only (1) out of 20 Airports even carry mogas.

So you choose, doing a Crank Rebuild to do the extra Work, or having someone else doing it for you, maybe Spend $80 to have it Welded, Pinned, or Glued, some do include it in their Cost, for maybe your a Crank/Bearings to live longer, or Pay to have it maybe rebuilt sooner than the 300hrs. If he gives a 450hr TBO on his 670(93hp), apply the same Tech to your other Rotaxs if you want them to last Longer. He is also using Ceramic Top Coated Pistons.
 

Armilite

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One of the problems trying to run needle bearings on the crank (at the rod ?) that part needs to be hardened and ground,....not a simple lathe or CNC lathe job. Maybe you can buy hardened/ground steel rods of the correct diameter. Be nice to get them to the proper length too....That press fit fixture sure would come in handy for the assembly.
========================================

The Rod kits usually come with New Crank Pins and the Crank bearings have Races. So no Grinding Needed. The 277 came with the Odd Ball 66mm Stroke, not used on Newer Engines. Rods over the Years changed Width 2mm and Lenghts used, 120mm, 125mm 132mm, on some Engines. Any Crank Pin can be shortened in a lathe. As Engines came along, the Rods usually got Wider with Bigger Bearings.

I would suggest if you plan to rebuild your own Cranks Download this pdf that gives the Rod Spec's and Bearings of many different Engines.
http://www.transcanimports.com/downloads/Snowmobile Catalogue/Engine - Connecting Rods.pdf

You can see the 277 use's a:
Bombardier Model Year Part # OEM Part # OEM
Rotax 277 85-89 Mag 565-2410 420-9947-64 PTO 565-2410 420-9947-64
Rotax 277 Single 1990 & up Mag 565-2517 420-9953-09 PTO 565-2517 420-9953-09

565-2410 120 22 30 15 15 24 57 18 x 22 x 22 24 x 30 x 14.7 1
565-2517 120 22 30 17 17 24 57 18 x 22 x 22 24 x 30 x 16.9 1

Difference was 2mm in Widths of the Rod. You have some End Play to Play with. Crank Pins were same 24mm x 57mm. They Solved the Top Bearing Failures with the Cageless Bearings. By using a Ceramic Top Coated Piston you also take away some Heat going to them Top Rod Bearings. I personally would consider Ceramic Coating the Piston Top & Bottom of the Pistons and down to first Ring Groove. They still use a Caged Bearing on the bottom of the Rod. Rotax Rick was expermenting with some Cageless Lower Bearings he made up from Caged bearings, but I haven't talked to him in many months now to see what, if any results he had with them. Some Boat Engines use a Lower Cageless Rod Bearing. The 2nd part to Fail after the Cages on the Rod Bearings is the Shim's made from Copper I think. So far as I know, nobody has tried Ceramic Coating the Rod Shims or just Switch to a Full Ceramic Shim. There are a few better Rod Designs out that also Direct more Oil to the Shims, but none are made for Rotax UL Engines. There are some Special made Bearings I once found in Austrailia, that a Big Motorcycle Racer used, that had the Bearing Rollers made that were Ball Shaped on the Ends vs Flat. He also had the Cages Coated.

Bad thing was, you had to Order Large lot's to get them in your Sizes. If I remember right 500+ at $25ea. You could Mix Sizes. For guys like Rotax Rick who probably builds a 100+ Engines a year, that's nothing. Bad for the Home Guy doing just a few.
 

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