Help with a ms460 Piston & cylinder problem

[Johnleddo]

Hi Folks,

I have just joined the fourm and have am hoping I might be able to get some advice about my sick MS460.

 

The brief history of the problem is as follows. The saw stopped working about a month ago whilst i was using it, I didn't have time to look into it then but I suspected the piston had siezed in the barrel (pull cord was tight etc).

 

I have since stripped barrel and piston out of it and found that the piston has two deep sctatches on the intake side (the back of the piston when its in the saw I think!)

 

Pic 1 shows the scratches, and the piston had quite a lot of black residue on the exhaust side shown in pic 2

 

The condition of the barrel is shown in the following 3 pictures, other than what can be seen it has two rough lumps that would match with the scratches in the piston.

 

As to the cause I' a bit lost, the only thing I had done differently with the saw was at a job when I ran out of two stroke so went to B&Q and bought a one shot 50:1 two stoke oil mix. it wasn't a stihl, oregon etc brand of oil but i figured it would be ok, I will have ran about 5litres of the mix through this saw, so I'm wondering if that might have done the damage.

 

Other than that whilst stripping it down I noticed one of the circlips that holds the piston in place on the crank had one of the turned in ends missing, so i wondered if this might have found its way in to the barrel?

 

I'm wondering if

(1. Do I need to replace the piston and barrel, or could get away with a new piston and cleaning up the barrel?

(2. Is it likely to have been caused by the oil i used or do I need to investigate for other problems?

 

If anyone has any thoughts or advice I'd be really grateful to hear them as its my first real exploration into the internal workings of a chain saw! Also its a great saw and a shame to have it out of action.

 

Many thanks

John

[spudulike]

Firstly the "black residue" is where the piston has melted and deposited aluminium on the exhaust side of the clinder - this can be removed chemically and can be lightly abraded to sort out part of the problem.

The end of the circlip is almost definitely NOT the problem, if both circlips are different, it makes me wonder if the saw has already been apart - if you look at the bottom of the piston does it say "Mahle" on the underside of the piston?

 

The only time I have seen damage like this on the inlet side of the piston, the big end of the saw had exploded sending metal debris up in to the upper cylinder.

 

I would inspect the big end as much as possible, inspect the crankcase and muffler for debris like rollers from the big end and bits of bearing cage, plus pull the rod up and down and look for excessive play!

 

If the scores are deep, have broken through the Nikasil plating and more importantly - are in the upper combustion part of the cylinder then it is scrap. The cylinder is plated and can't be rebored and oversize pistons fitted.

 

Looks buggered to me and would think the oil wasn't up to much or went off if the saw was left unused for a few weeks, the saw has heat seized and this may have done the big end in!

 

If th episton is non OEM - the ring location pins may have come out of the piston - worth checking!

 

Spud

[Johnleddo]

Hi Spud, thanks very much for your input.

I have checked for any markings on the underside of the piston and there don't seem to be any so if that makes it an origional part or not I'm not sure.

 

Both of the piston ring pins are still there and the big end seems to be intact and working correctly. When I pull the rod vertically there is no play at all, however if I move the rod left to right there is play and I'm not sure if this ok or if it means the big end is goosed?

 

I also can't find any other debris in the crank case or exhaust.

 

What you have mentioned about the oil going off in the saw could be a realistic possibility, this is my biggest saw and its not in use every day so that fuel mix could have sat in it for a month I guess. Could be a valuable lesson learned there!

 

Thanks again for your reply

[spudulike]

If the underside of the piston doesn't state "Mahle" then it is probably non OEM and if the big end has no play and no debris then it looks like that is ok and that os great news. Sideways play on the big end is fine and no problem, 100% normal.

 

You will need to inspect the cylinder like I mentioned earlier, I reckon it is probably toast and the damage has been caused by the circlip coming out - a problem on some non OEM pistons!

 

If you want a cheap way out then look at this - STIHL MS460 046 CYLINDER AND PISTON ASSEMBLY NON GENUINE

 

Andy, who runs the company is a good guy - I haven't used his kits yet but it will get you running again.

 

Just ensure that you check the bottom end for the damaging circlip and would pressure check to ensure no air leaks are present.

 

Spud

[TimberCutterDartmoor]

Looks buggered to me

 

Ditto; scoring on the exhaust side is a definate. Inlet side damage is bad too; can't tell from the photo whether there is damage to the middle land between the two piston rings.

[Johnleddo]

Yep I think after a close inspection you are both absolutely right the damage to the cylinder is too bad to re use, like timber cutter has mentioned its particularly bad damage on the exhaust side and there are a couple of deeper marks on the intake side.

 

I have ordered a non genuine stihl replacement cylinder and piston, to buy origional Stihl parts just seems un affordable. So I'll get them fitted and hopefully be back up an running.

 

I have checked the big end for debris and cant see anything I've also had the saw upside down and given it a good shake with the big end in all positions, If I can I'll get it to a compressor and give it a blow out before reassembling it.

 

When you mentioned pressure testing Spud what exactly does that involve?

Cheers for the advice

John

[spudulike]

:thumbup:Pressure checking - A two stroke engine relies on the crankcase being completely sealed as the fuel/oil mix is drawn in to this chamber and then drawn in to the cylinder via transfer ports. Any airleak in the crank seals, carb boot or mating surfaces causes more air to be drawn in to the crank case than is set by the carb causing a weak mixture which messes up the idle speed and can seize the engine....BANG:thumbdown:

 

If an engine has seized once, it has done it for a reason and that reason should be known before sticking on another £200 worth of spare parts IMO:thumbup1:

 

To pressure check, you have to seal up the exhaust port, the inlet port, the impulse hole/line and then attach a pressure gauge to the spark plug hole (you can plug this hole and use the impulse line) and pressurize the crankcase to around 9psi, the engine should be able to hold this with a slight drop of say 1psi over 15-20 mins. If you have real problems then you will not be able to get to anywhere near this.

 

I have adapted a gauge used for car induction measurement and a bicycle tyre valve, cost me around £15 and works brilliantly - I can pressure check but not got round to vacuum testing yet- the crankcase on two strokes is pressurized and under vacumm at different parts of the stroke!

 

IMO, if the case holds pressure, then you will not experience vacuum problems big enough to damage an engine.

 

You did ask - not an easy task for the DIYer but if you do this sort of work, a compression gauge, pressure check and tach are a must for a 100% job:thumbup:

Spud