2 Stroke Lesson

[Grassbandit]

Bandit, Many on the forum might also think that an aluminum key is a strange material selection. Doubt that is aircraft alloy so seems like it is not as good a selection as steel, and much more subject to fatique cracks. I will try to pull mine and report. I would think any used ones require crack checking before using. There was a neat little 2 can test kit available at auto stores a few years back. Use the cleaner, then the penetrant dye and read it under a black light. Who the hell has a black light??? Just sayin...BTW after the youtube photo I looked again at your pic . It kinda looks like "whoever" may have ground off two nubbins adjacent to the key centerline , put a different taper on and then filed a keyway?
If that is so your poor little engine may be trying to start with spark + or - 20 degrees to TDC

you're very right, it's kinda annoying to work on things after someone else would have tampered with it.
I boxed up the parts this evening so my worktable is clear for the next project that comes along. Might even restore another crapsman mower.

 

[Rokon]

I wonder how or why the key would have broken off?
Perhaps an attempt at removal was incorrect....good luck with getting yours off.

A good backfire on start-up can shear the key.

BTW, the key has to the softest metal between the flywheel and the crank to prevent possible engine damage. Had a mower come in with a steel key (not stock) on a steel crankshaft with an aluminum flywheel. Owner hit something solid with the blade. Mower still ran, but "idled high", according to the owner. Once the top cover came off, the carnage unfolded. The steel key rolled half way and locked. The flywheel was completely fractured from the center out. The high idle was from the engine being advanced due to the grenaded flywheel.

 

[motoman]

Rokon, Good point on the protection a softer flywheel key might provide. I remember outboard propellers... But hmmm, responders on this thread (as many) diy'ers and pros, and I suspect most pros would never care to repair a $125 trimmer when repair parts quickly reach $75... Anyway, if the lowly trimmers are throwaways why would a mfgr choose to protect with a soft key? Another thought which occured ...the locking power of the "taper." I once read how much friction is achieved with a properly torqed taper fit (as in Morse taper in machine tools). So the disigners are relying mainly on the taper of the alminum bore to hold and the anemic aluminum key is mainly for alignment? I love the way the forum can count the angels on the head of the pin to keep things going in winter!!

Edit, Oh yeah, Bandit's flywheel taper looks hand dressed. Not a chance that would mate with the shaft , thus putting it entirely on the key to hold??

Edit, Looked briefly at penetant inspection kits. Black light (ultraviolet)is very expensive @ around $750. Magnaflux sells a 4 can kit with no black light required for about $90 retail, but with a 5 year shelf. Curious if you professionals use this for e.g., valve seats, heads etc..

 

[Grassbandit]

Went dumpster diving today and found this Crapsman Trimmer. I know alot of times engines and parts are interchangeable across brand names so I'm hoping that the flywheel on this one will match the Ryobi we've been talking about.
I'll take it apart later and tell you whats up. No plans on restoring! maybe I'll just hang it as an ornament. lol

 

[motoman]

Below is the flywheel removed from my deceased Ryobi 790R. To remove I just held the flywheel by hand and rapped straight down on the shaft with a small brass hammer. It came right off. Please find relief (and humor?) in the warning words cast into the flywheel..."not to modify for fear of burst hazard." Relief that the 7000 rpm engine did not start with the weak fix Grassbandit inherited at the key/bore. The cast-in aluminum key is visible and the same shown on the youtube...FYI. (Grassbandit, detailed PM sent yesterday)

 

[Grassbandit]

Below is the flywheel removed from my deceased Ryobi 790R. To remove I just held the flywheel by hand and rapped straight down on the shaft with a small brass hammer. It came right off. Please find relief (and humor?) in the warning words cast into the flywheel..."not to modify for fear of burst hazard." Relief that the 7000 rpm engine did not start with the weak fix Grassbandit inherited at the key/bore. The cast-in aluminum key is visible and the same shown on the youtube...FYI. (Grassbandit, detailed PM sent yesterday)

Your flywheel has differences, visually speaking. On the left there are two big holes and 1 smaller one. The right side has 6 small holes and 2 large ones around it.
My flywheel only has the larger holes.
Very cool of you to go through taking it off and posting pictures.....and its good to see how the flywheel with built in key is supposed to look.

Do you remember why you put your 790R out to pasture?

 

[motoman]

Bandit, don't sweat the holes: the 8 hole side is just pinning in the 2 magnet patches, and the 3 on the other side are for the counter balance lump. Nothing functional. You should pop the cowl on your dumpster find and do a visual . If you have a steel rule you could get the ht and dia easily. Maybe they cranked 1000's of these little buggers out. I notice a big read on this thread. Hope those with lots of experience will chime in.:thumbsup:


Edit. The 790 lasted 10 years of abuse . One day it lost a little top end rpm so I looked at it and saw light contact marks on the 2 magnet patches. That made me think the shaft was wobbly. I dug in and the clutch removal was a no starter as the factory had repair-proofed the retaining screw. I ruined the (good) clutch and with the replacement cost of the clutch, gas tank, and ignition module I was at something like $100. So I bought a new half-cousin weedwacker at sears. The Ryobi was not putting out spark at the end. I tested the Sears in parallel and it would jump 1/4" easily. BTW today since I was finally down to the bearing with the removal of the flywheel, I looked for looseness , but none evident. The flywheel bore is perfect so I still don't know why contact.

 

[Grassbandit]

NFIG]24794[/ATTACH]

The Craftsman trimmer had a flywheel with the exact same numbers and measurements so I got happy! But upon further review....the Inner Diameter where the crankshaft goes through is actually smaller. Totally a let down.
The trimmer was in bad shape from being left outside in salt air and water, but anyways enjoy the pics. If you see something you want let me know.

 

[bertsmobile1]

Rokon, Good point on the protection a softer flywheel key might provide. I remember outboard propellers... But hmmm, responders on this thread (as many) diy'ers and pros, and I suspect most pros would never care to repair a $125 trimmer when repair parts quickly reach $75... Anyway, if the lowly trimmers are throwaways why would a mfgr choose to protect with a soft key? Another thought which occured ...the locking power of the "taper." I once read how much friction is achieved with a properly torqed taper fit (as in Morse taper in machine tools). So the disigners are relying mainly on the taper of the alminum bore to hold and the anemic aluminum key is mainly for alignment? I love the way the forum can count the angels on the head of the pin to keep things going in winter!!

Edit, Oh yeah, Bandit's flywheel taper looks hand dressed. Not a chance that would mate with the shaft , thus putting it entirely on the key to hold??

Edit, Looked briefly at penetant inspection kits. Black light (ultraviolet)is very expensive @ around $750. Magnaflux sells a 4 can kit with no black light required for about $90 retail, but with a 5 year shelf. Curious if you professionals use this for e.g., valve seats, heads etc..

last bit first.
You can get fluro dye penetrant that are died with flouroscene ( the stuff you use to trace drains ) so you cen see it with the naked eye.
Once the surface has dried out spray it with foot powder or spray on talcum powder this will wick up the die from the cracks & make them easy to see
Next those special plant growing tubes have enough UV to activate the glowing dies in the Magna flux kit.
Third a std UV tube ( you know the ones that mke your undies show through your cloths ) in a std fluro light fitting will also work.
A tanning lamp works really well and you can usually find them in second hand shops.
Only down side of these three is they have a short duration so you either need to turn them on & off a lot or do the inspection while the lights are on.

As for break away keys, even on the cheapest piece of equipment the makers both have to offer a warrantee and take out product liability insurance both of which require the fitting of a safety device like a shear key.

 

[motoman]

Bandit, Tough it's not a match, but not a surprise . I looked at some online flywheels and there are lots of them which kind of look alike but.. $18-$80 yikes. So check carefully the dimensions I sent. You say go and I will send you a USPS track no. to the address given, no charge, no big deal.

Berts, Good tips. So a good US plumbing store will have the drain tracer? You did not mention if you do crack checking on customer machines??