I have an old husk,chain saw, about 25 years oldand need a sprocket, how can i find whatn model it is, many thanks

I would find a good lawn and garden shop that sells that brand of chain saws. Not a department store.Try your yellow pages under saws.

Here is the manual for the 51 saw. The sprocket should not wobble and I think you are on the right track, probably should replace it!

http://www.manualslib.com/products/Husqvarna-51-240393.html

Hope this helps !
Cheers!

all chain saws have a sprocket that the chain runs on this sprocket is what make the chain rotate on the bar. the bar is the long part that sticks out in front of the motor. you must remove the side cover of the saw to access the sprocket region of the saw. be sure the saw is unplugged and then remove side cover the bar is usually held in place by a couple of bolts. on gas saws these two bolts also hold the side cover in place and must be removed to replace the bar and chain. once you have removed the side cover look for bolts holding the bar in place. if you only want to change the chain then you may be able to only loosen the bolts or screws holding the bar in place.there should be a screw to adjust the bar forwards and backwards. this screw put tension on the chain so when you put a new chain on turning the screw will proberly tension the chain /do not overtighten the chain.
also be sure you put the new chain and blade on the same as the old one that comes off trying to cut with a chain that is on backward does not work. last note if you are replacing the blade because the old one is dull take the saw to a saw shop they can sharpen the exsisting chain and save you money.

You can find the bars, chains, sprockets, etc for your saw here - Partsbuggy.com.

Remove the new chain and compare it against the old one. Do the new drive teeth match the 'pitch' of the old chain? If incorrect, the chain cannot be adjusted properly and will 'climb' over the sprocket due to the chain teeth not matching the teeth on the drive sprocket. Getting the proper chain is the answer in this case. Hope this helps!

I'm assuming your kill switch is not engaged, the front handle should be pulled to the rear. It's normal to have some damage on the sprocket from loose chains catching on the corner so I doubt you need a new one. The motor turns? I'm assuming you're talking about the clutch, and if the clutch spins but the chain does not it sounds like your bar may be damaged or as you said the sprocket may need to be replaced if it's seriously been misused.

The model 028 Stihl Chainsaw was in production for many years and the clutch drum sprocket (or chain drive) came in three (3) versions over the years of production.
For several reasons, it is best to take the saw to an authorized Stihl Dealership (Locate your nearest dealership HERE) for production identification, evaluation of cost-effectivness and availability of parts.

Reason 1) The first-generation 028 saw is approaching 30 years old.

Reason 2) Extreme heat caused by a locked chain-brake can damage the engine case and render the saw unfixable (part support is going away due to age).

Reason 3) The shear cost of parts and amount of time it would take for a DIY repair.

Reason 4) The plastic gears you mentioned are the drive system that make the chain's oil pump work. If the plastic is melted, the oil pump is likely to be damaged as well. In most cases the oil pump cover gets damaged ... the list goes on and on.

Genuine Stihl parts are not sold online and aftermarket parts would be a gamble. It's up to you to decide if the cost, time and effort are worth it.
Hope this helps.

please tell us about the make(manufacturing company) of that machine,

Depending to what model you have, the Sprocket cover tab (located on the round part of the sprocket cover) isn’t inserted into the saw body tab slot. Or your cahin is not tensioned properly. According to the manufacturer... "The adjusting block on the adjusting screw isn’t inserted into the adjusting hole on the guide bar. Reinstall and align the sprocket cover by first removing the sprocket cover from the saw. Turn tensioning screw on the sprocket cover counter-clockwise until it stops. Pull guide bar away from saw body to remove slack from chain. Insert sprocket cover tab into saw body tab slot and lower sprocket cover into guide bar bolt. Finger tighten guide bar nut onto guide bar bolt. Slowly push guide bar toward saw body until adjusting block on the adjusting screw drops into round adjusting hole on the guide bar. Securely tighten guide bar nut after properly tensioning saw chain." If these tips do not work, you may need a new chain. You can order an new chain on-line at www.masterparts.net. I hope this helps you!

The chain must be adjusted so that pulling up on the chain you cannot see more than three guides. If the saw is really old you should invest in a new bar, chain and it its a gas model a new clutch.