52cc Chinese Brushcutter Engine probs

[Archibald Tarquin Blenkinsopp]

Hi I have a chinese Brushcutter, out of whatever warranty was supplied, yet not used much.

The Woodruf Key sot on the drive shaft was badly made, so the key "escaped" damaging the flywheel slot in the process.

The Engine had worked for a few hours, certainly isn't seized and has no running probs apart from the timing when the flywheel goes out of alignment.

I replaced the key thinking that the unit may not have been secured tightly at manufacture, but the new key failed to clamp the flywheel properly.

The woodruf key slot in the shaft has an excess space of around 2mm that extends 75% of the length of the key, The flywheel slot is worse, with the slot being chewed for most of the length.

The shaft and flywheel mounting hole are in excellent condition for the rest of the diameter.

An oversize key is not something that I can fit as I don't have much above basic DIY tools.

I liberally squirtedLocktite on the shaft before assembly, hoping to may the new wooruf keys job easier, but it didnt stop the flywheel rotating with respect to the shaft and chewing more metal away.

Any ideas apart from the bin?

Regards

AB

[Dave Plowman (News)]

IMHO, the purpose of a woodruff key is merely to locate the flywheel accurately during assembly. For ignition timing purposes, etc. When correctly tightened, it shouldn't really do anything.

Is it on a taper or parallel shaft?

[Archibald Tarquin Blenkinsopp]

It's a taper shaft. I assumed the same, that the only force would be a light accelleration/ decelleration force. Hence, I thought that liberal dribblings of Loctite on the actual shaft and thread would prevent rotation. I fastened the locknut as tight as I dared and left the thing for a couple of weeks before use.

The Brushcutter only worked for a few minutes before the flywheel moved and the timing went out.

AB

[Brian Gaff]

I just wondered if some light weight disc containing the timing stuff might work then if the flywheel slipped it would not matter so much. Sounds like the materials its made of are pretty soft so maybe the actual shaft is twisting due to it being such crap material and this is why its timing does out and the thing gets chewed up. In which case its a bin job I'd say. Brian

[misterroy]

I think the term is lap. Take the flywheel off, put some valve grinding paste on the shaft, no keway yet. Put the flywheel back on and twist it round on the shaft, so the shaft gets ground a bit. It does not take much. Take the flywheel off, clean off the paste and re-assemble.

[Archibald Tarquin Blenkinsopp]

You are probably correct.

If there were clearance I could try drilling the shaft and flywheel for a split pin or the like, but the tools are not available to try.

I was hoping that someone might have had a similar problem and found a substance better than Loctite to fix flywheel and shaft.

If someone has "been there" it's less likely that I will have to dissasemble again.

I did buy a replacement, but having started, I hate to admit defeat.

Apart from that the replacement is an el cheapo Chinese device too.

A lot of their electrical stuff seems to be almost as good as the originals they copied, they haven't quite got their act together with strimmers/ brushcutters though.

Rightly or wrongly I bought Chinese because I didn't think of two stroke as being reliable enough to spend the extra for for a quality brand.

On the plus side I have a source of spares that is organised far better than my own attempts at storage go :-)

AB

[Archibald Tarquin Blenkinsopp]

I can certainly try this, but it is not something that I would have thought produced a vast difference in the "binding" force.

If this is a proven technique then I'll give it a go. I may be able to fashion a better fitting woodruf key also.

The item fitted is a half moon shaped device, not at all like the ones fitted to electric motors like I normally lose :-)

AB

[Dave Plowman (News)]

Does the taper seem well made? Are you sure the woodruff key has some clearance at the top so it is not stopping the flywheel going fully on to the taper?

[T i m]

The chances are it uses 'magneto ignition' Brian and so it's the flywheel itself that does the ignition timing.

Cheers, T i m

[Archibald Tarquin Blenkinsopp]

That is correct. I didn't look up how it works, I assume theres a reed switch embedded someplace to break the Voltage rapidly.

Visually it looks like a magnet on the flywheel [fixed], with a coil on the crankcase with a very limited range of movement 0.5 degrees approx [big mounting holes].

AB

[Archibald Tarquin Blenkinsopp]

The taper is fine, its the keyway that wasn't formed well. There was a fault with the casting. I will see if I can dig out the camera.

Instead of being a longish parallel groove on the shaft, it is shaped a bit like a p with the bulbous bit from the center to the base (bearing side) The movement has allowed the key to chew the corresponding slot in the flywheel, so it's similar but larger.

Whatever I fit in place of the key will only have a small portion of the original keyway to locate it.

AB

[tabbypurr]

sounds like the perfect app for a few weld spots. Some welders are cheaper than a new brushcutter, though those wouldn't be the best choice.

NT

[Archibald Tarquin Blenkinsopp]

It crossed my mind, but aluminium to steel would probably be beyond my ability even if my Parkside 80A was up to it.

AB

[T i m]

I think you will find it's simply the close proximity of the magnet inside the magneto as it passes the coil that generates the spark. No points, no switches, just magnetic fields. ;-)

Yup. Fixed timing, just some adjustment to allow you to set the distance between the coil and the magnet (often the thickness of a postcard or somesuch).

Cheers, T i m

[T i m]

If you are looking at just minimising your losses and assuming it runs with the flywheel poisoned with the slot / key etc, then after cleaning all surfaces meticulously, you could try bonding the flywheel to the shaft with the likes of JB Weld?

I've just managed to salvage an old van by using JB Weld to allow me to 'fix' a loose alternator to be fitted back onto it's worn bracket.

Cheers, T i m

[Archibald Tarquin Blenkinsopp]

Thanks, I assumed that for a spark there had to be a sudden break in the current flowing through the coil. You must be correct, I cannot see any kind of switch anywhere.

Saw your post on JB weld. I have not heard of it, I'll locate a source and give it a go.

Many thanks AB

[T i m]

You are welcome.

Sort of ... the 'poles' of the magnet and coil do provide a fairly sharp 'pulse'.

;-)

Brilliant stuff but they do do different versions. I think this might be the best for your needs:

Or the EU 'approved packaging' version: ;-)

(Both available on eBay etc).

Cheers, T i m

[tabbypurr]

It's epoxy, not something I'd use on a hot engine at all. In case you don't know epoxy goes soft when hot.

Re welding steel to ali... if you've nowt to lose I'd give it a go, just in 2 spots so you've got space to try something else later. Or weld a steel plate (giant washer) onto the shaft, then bolts through that into the ali flywheel.

NT

[T i m]

According to their datasheet it degrades at 220 DecC so I think you would have bigger issues than the flywheel moving at that point.

Do you know what temperature JB Weld softens at and is it lower than the running temperature of that engine?

Yes, that sounds much more viable ... (not).

BTW, do you think the temperature of this flywheel / crankshaft is going to be much hotter than an alternator stuck on the front of a

1.9L Pug diesel and behind a radiator?

Cheers, T i m

[Archibald Tarquin Blenkinsopp]

It's advertised as being suitable for hot engines and as the flywheel is used for cooling also, I would doubt that the temperature gets much beyond 70.

Anyway at less than £4-00 from Ebay, it's on order.

If it works it's a bonus and it's probably going to have even more value for other uses if its performance is an addition to the current range of glues and pastes that exist in the personal armoury.

Thanks Tim

AB