Stihl MS290 problems

[wisecobandit]

290 56cc

390 64cc

 

290 3.7hp

390 4.6hp

 

Same weight.

 

I know what I would do but then im the overkill sort That said that's if it needs the complete topend not just piston.

 

As spud says if you can salvage the cylinder and its original stihl then the meteor piston route is your best option.

 

I would certainly change the bearings and seals whilst its apart. They just come out on the crank when you remove it. Just a case of pulling them off and replacing with new.

[Aledafis]

Ok, I took a pic of the piston through the exhaust port, and decided that the piston had seen better days, so stripped the engine down.

 

Here's the piston in all it's glory.

 

 

A little bit tired??

[spudulike]

Ok, I took a pic of the piston through the exhaust port, and decided that the piston had seen better days, so stripped the engine down.

 

Here's the piston in all it's glory.

 

 

A little bit tired??

 

Well you did well getting it apart without any help - taking the bar stud out usually separates the men from the boys!

 

The piston looks like it may have been changed already and the aluminium transfer left on the cylinder....anyway, it is FUBARed.

 

You will need to ensure ALL aluminium transfer is removed from the bore, some use acid, some use alkali - the choice is yours. You should get a good fizzing going on when your concoction is put on to the bad area.

 

Once the cylinder has stopped fizzing, try going over the area with 400 grit paper and then try the gloop again and when you get no more fizzing, the bore needs to be finally cleaned with 180 grit then finally 400 grit.

 

Make sure the exhaust bevels are smooth and the bore is 100% smooth, small scores or imperfections are OK and then fit a new piston - choose Meteor, Episan or Golf and in that order.

 

Good luck - at least you are learning something!

 

You will really need to know the reason the saw failed - fitting new crank seals is easy and is also worth doing now you have gone this far as any air leak will kill the engine again. Hopefully it failed on old fuel but make sure the H screw is set rich before you start it up and tune it.

[Aledafis]

Thanks, I found the disassembly fairly straightforward to be honest, it was just a case of systematically finding the fixings and taking off parts as required. My background is in engineering, but from a design point of view (bakery) rather than repair and maintenance. I'm definitely enjoying the journey!

 

I'll have a close look at the bore tomorrow, I take it that you think that it will be ok. What acid/alkali to you recommend, bearing in mind that I live in the back of beyond, here in West Wales and getting hold of specialist stuff can be an issue?

 

I already have a bearing/seal kit on order as it must be worth replacing them now that the saw is in pieces.

 

Thanks again

Aled

[spudulike]

Alkali can be caustic soda otherwise known as Sodium Hydroxide, Acid is typically Hydrochloric Acid or Muriatic Acid to give it its traditional name.

 

You need to lay the cylinder flat to the bench and apply the chemical with a cotton bud - the aluminium should fizz when the mix is applied - I usually pre treat it by degreasing and abrading it lightly first.

 

Wear gloves and eye protection and clean the cylinder well after using it. If the aluminium doesn't fizz, you need a more concentrated solution.

 

The cleaned area will need the residue abraded off and make sure that you can't feel any marking with your finger otherwise the ring will be damaged very fast.

 

There are lots of examples of me doing this on "Whats on my bench", the success rate is over 90% and have even had some good success with some borderline cylinders.

 

Just don't get hung up on rubbing small scores out - the abrasive paper should be used in swiping motions around the bore and not up and down, especially in the compression area above the exhaust port!

[Aledafis]

Ahh OK, Hydrochloric Acid I can do, I'll have a word with the science dept. at school tomorrow. The caustic is also an option.

 

Cheers

[peatff]

5 litres of brick acid from Travis Perkins or Screwfix patio cleaner for around £7 should do the job. That piston looks incredibly clean, it looks like it's never fired in anger.

Edited October 2, 2013 by peatff

[spudulike]

My thinking is the original owner seized it and fitted a new piston without cleaning the transfer off the bore!

[Aledafis]

My thinking is the original owner seized it and fitted a new piston without cleaning the transfer off the bore!

 

Possibly, but I'd say that the saw has had quite a bit of work since it's last rebuild going by the ammount of crud around the engine compartment - yuck!

[H-A]

Hydrochloric acid (HCl in chemical terms) is available in small quantities from hardware stores, it is sold as "spirits of salts".

 

If you want more concentrate Hydrochloric acid, pop into your local car valeting suppliers, ask for steam cleaner de-scaler.

 

I bought a 5 gallon drum for £15, it is concentrated enough to smoke when you take the lid off the drum. Don't breathe the fumes or it will make your lungs hurt, don't get it in your eyes, or on your skin.

 

Another benefit, put half a pint down your toilet and leave over night, super strength Vical.(warn every one who may use the loo, that it has acid in it.)

 

It removes all the lime scale, she will be pleased.

 

H-A