MS390 decompressor failure

[Megatron]

This thread worries me...

 

Have you removed the entire cylinder and pan from the housing? to end up with what is shown in the attached picture.

[spudulike]

This thread worries me...

 

Have you removed the entire cylinder and pan from the housing? to end up with what is shown in the attached picture.

 

Bugger - forgot the 390 was clam type crankcase, see your point - been too long on pro Huskys and Stihls - he can't have been able to tell if the rod was bent or not.

 

Looks like you will have to reassemble the engine as in the pictire and make sure it turns over very freely with the plug removed - a picture tells a thousand stories - give us a look up the bore at the damage so we can give a better assesment. Having seen the type of engine it is, are you sure the rod is straight?

 

Personally I would get the saw running and have a few tanks of fuel through it before tuning it - the clam will need resealling and pressure testing before running it - not quite as simple as it could be.

 

Should have known, got one on the shelf:001_rolleyes:

[windfall]

Why are you worried megatron? No i have not removed the base from the saw was hoping i could just put it alll back easily but it would seem not.

 

I cant see how taking the engine base out is going to make it easier for me to compress the rings and slip the top on!

 

So how do i get the rings compressed and into the cylinder.

[spudulike]

He is probably concerned, he will correct me if I am wrong, that you will need to clean both mating surfaces of the crank case and add a suitable sealent between the two before joining them. Failure to get a good seal will seize the engine.

 

He will also be concerned that you havent truely found out if the con rod is bent, if this is bent, it will wear the piston and bore very quickly. I thought the saw was a vertically split crankcase but know the 390 is a clam type crank case, just missed it this time.

 

It is difficult to know what damage has been done to the bore without seeing the damage. The test for a bent rod is pretty much valid but would suggest taking out the motor as in Megatrons pic and assembling without rings and seeing if the crank can be turned over without much effort, if you can't turn the crankshaft round by twisting the crank shaft NOT the flywheel or clutch, you probably have a bent rod.

 

The lower clam part of the engine should always be removed so it can be cleaned and sealed properly - any split crankcase should be pressure tested on re-assembly.

 

re-assembly is easier if you have the whole engine out of the saw, the bore should be oiled and the rings eased up the bore - I use a small blunt screwdriver and patience, some use ring compressors - just depends on what you find works - Husky single rings are easier than Stihl two but be patient with it.

 

Sometimes the old sealent looks like an O ring gasket, it isn't and should be cleaned off and fresh sealent used - much easier with the bottom clam removed - these engines are much harder to work on when doing top end work as the whole unit needs removing and generally the flywheel and clutch being removed to get it out! The top part of the crankcase is formed out of the lower part of the cylinder!

 

Appologies for missing the fact it is a clam engine - don't do quite so many of this type.....fortunately! Doing a 023 Stihl soon and will show pictures of it's rebuild!

[windfall]

Just been back out to have another look, yeah megatron if thats what you were mystically elluding to your spot on, took the clutch and flywheel off and on removing the crank seal there was loads of old red sealant i had missed and would not have replaced had i not gone further with the strip down!

 

Thanks to you both.

 

So rookie question whats now holding the base in the shell, there seems to be one bolt behind one of the mounts for the bar?! what am i missing?

[windfall]

piston top looks pretty bad in the pic!

[spudulike]

Good pics - The bore/ports look ok from what I can see, the deep impact on the combustion chamber worries me as this will have transferred it's energy directly in to the con rod and big/small end bearings.

 

If you are lucky, all will have survived but reckon the rod may well be bent and is impossible to tell visually.

 

The piston may well have survived, just need to be sure the impact marks havent cracked the crown of the piston - have seen this before with use of piston stops. Personally I would remove, smooth the top and inspect the end bearing for damage and piston for cracks.

 

To remove the lower clam you will need to take the crank shaft out and to do this you normally need to remove both the flywheel, the clutch and any ancillaries like the oil pump etc, all saws are a little different and I haven't had a 390 apart but am suspecting it is similar to the Stihl 180, and Husky 345/ Jonsered 2149....etc that I have had apart. The clam sometimes needs a light tap from underneath with a drift and mallet to shift it.

 

When you come to reassemble (I would certainly make 100% sure the rod is ok) then the seal between the clam and the bottom of the cylinder is critical, a small bead of sealer is required and any technician worth his money will pressure check the engine, I appreciate you probably can't do this but that is your choice - if you were local - I would do it for you.

 

Two strokes use the crankcase to pull fuel in to it and then disperses the fuel in to the combustion area by way of transfer ports, it is critical the only air that gets in to the engine comes in via the carb otherwise you will be dogged with erratic idle, poor pick up and run the risk of seizure - the saw, not you:laugh1:

 

Hope this all helps but guess you are probably understanding why many just send their kit to me and get it back running with the satisfaction I have done the works on their saws!

 

Keep us posted

[Megatron]

Check to see if the big end bearing cage is damaged. Personally I wouldn't reinstall that piston or small end bearing due to the trauma they have gone through. Is that a fracture on your picture, circled in red?

 

Whatever you decide, to get the engine pan out having removed the clutch, flywheel, oil pump etc you have to remove the rear bar stud. Screw two bar nuts against each other and then unscrew the bottom one to unwind the bar stud.

 

IMO it is only possible to reinstall the piston, crank etc into the cylinder with the engine pan removed. This is because you need to clean and de-grease the mating surfaces of the cylinder and engine pan in order to reseal with Dirko HT red sealant. Also trying to reinstall the way it looks/sounded like you were going to risks the oil seals not properly seating.

 

When reinstalling coat the top of the piston and rings with 2-stroke oil and postion the rings such that the rings ends are either side of the locating pins. Invert the cylinder and guide the piston in(with crank attached), the cylinder is tapered to help installation, you can use a cocktail stick etc to ease the rings in past the transfer ports etc.

 

If reusing the same oil seals, ensure they are in the same orientation as before. Next run a small bead of sealant around the groove of the engine pan and lightly smear a thin layer in the crank seal seat in the engine pan. Mate the engine pan and cylinder together ensuring the oil seals to do get clamped diagonally i.e. twisted. Without separating put the engine block back in the housing and tighten down the bolts in a diagonal pattern.

 

When back together like Spud said a Vac and Pressure test should be carried out.

 

Some pictures, I haven't used sealant as I don't want to reassembe. The bearings have spun.

[Megatron]

This is the same cylinder as in my first picture I believe;

[windfall]

I bloody knew that it was that nut! lol

 

Thanks megatron, yes i was just going to stick it back together and see what happened. I am not now though, there is no point going through all this work without rebuilding her.

 

Thanks for you assistance, its greatly apreciated.