# CNC Machinist: If Machining a 2 Stroke Crank?

### Help Support HomeBuiltAirplanes.com:

#### Turd Ferguson

##### Well-Known Member
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

##### Well-Known Member
HBA Supporter
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.

#### Turd Ferguson

##### Well-Known Member
Here is a Billet Polaris Crank.
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)

#### Armilite

##### Well-Known Member
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.
=================================================================

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.

#### Attachments

• 37.2 KB Views: 6
• 26.2 KB Views: 5

#### blane.c

##### Well-Known Member
HBA Supporter
=================================================================

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

##### Well-Known Member
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.
=========================================================

#### proppastie

##### Well-Known Member
Log Member
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

##### Well-Known Member
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.
=============================================================================

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.

#### Attachments

• 139.4 KB Views: 3
• 163.9 KB Views: 3
• 48.7 KB Views: 2
Last edited: