Piston Backwards!

[kd460]

Been playing wioth a used Stihl 028, I bought used for $75. It had descent compression of 145ish. It did have minor piston skirt scuffing as viewed from exhaust port.

Played with it by replacing odds and parts, rebuilding carb (leaky welch plug), new buffers, bar chain, etc.

Used the saw for about an hour saturday on and off, and for about an hour sunday cutting some large rounds.

Last night I took muffler off to check scuffing agaian and the marks were about 10 times worse. Pulled cylinder, and TA DA! Piston in backwards.

No biggie, I have a tecomec super cylinder/piston kit that will go in tonight. Everything else checks out, no debris in the crankcase and it looks clean in there.

Just kind of happy to be able to figure oput what caused the scuffing. The saw was purchased from a stihl dealer and scuffing was indicated up front. Someone had did the top end in the past with a Mahle cylinder and I assume piston kit. What a waste of time and money. The letter "E" on the piston was installed at the intake side for sure. Rings scuffed but not broken.

Contary to popular belief, it did not "sieze instantly" and had good compression. Glad I figured it out, KD

 

[HiOctane]

I dont think your scuffing was caused by a backwards piston.When backwards,the ring will catch in the exhaust and break with the piston usually.The scuffing was caused by something else,like damaged chrome above the exhaust port mostly.

 

[kd460]

I would think the damage was caused by the rings catching on the exhaust port opening because the ring ends were so close to it (in the wrong position). This would make them snag the opening of the port, and cause the wear, actually I need to check again (i'm at work), but above the port looked good. The wear on the piston and the cylinder was all below the upper piston ring and below the exhaust port.

 

[Monkeyhanger]

The letter "E" on the piston was installed at the intake side for sure.

Hi,

don't forget that both Stihl and Mahle are German and that E could stand for Einlass (intake) rather than Exhaust! Exhaust in German would probably be an A for Auslass.

Bye

 

[buck futter]

does a piston installed correctly scuff the intake?


the intake and exhaust are close to the same size on a stock saw right?

you may not be wrong but you probably are not right on this one.

 

[kd460]

Well, judging by how the new tecomec piston is made, the fixing pins in the ring grooves helped me confirm (along with the arrow on the piston top) that the Mahle piston was installed incorrectly.

Also the exhaust port is rectangle (wider opening than intake) and the intake is round, so along with the ring end gap being so close to the exhaust opening, I am sure this was the cause of the problem. KD

 

[HiOctane]

I would think the damage was caused by the rings catching on the exhaust port opening because the ring ends were so close to it (in the wrong position). This would make them snag the opening of the port, and cause the wear, actually I need to check again (i'm at work), but above the port looked good. The wear on the piston and the cylinder was all below the upper piston ring and below the exhaust port.

Usually,when the ring catches the ex port,the ring break and make a lot of damage to the cyl ,not only scuffing.

 

[Lakeside53]

... coincidence... I have a tecomec pistoned 028 super in the shop right now with the piston installed backwards (by the owner). It came in for "low compression" after less then 10 hours use and the owner thought we'd sold him a dud piston. The rings were not broken, but badly rounded and damaged by what I assume is getting beaten by the port top/bottom. There is some scuffing from this action.. but it looks like new rings will be all it needs.


After (or before) your new pistons etc.. Go though your carb and make sure it runs about right at standard settings... and fuel lines, filters etc.

 

[timberwolf]

Rings don't do much crossing into the intake, some saws they just enter a bit near BDC, others the ring(s) never get down to intake level.

 

[kd460]

... coincidence... I have a tecomec pistoned 028 super in the shop right now with the piston installed backwards (by the owner). It came in for "low compression" after less then 10 hours use and the owner thought we'd sold him a dud piston. The rings were not broken, but badly rounded and damaged by what I assume is getting beaten by the port top/bottom. There is some scuffing from this action.. but it looks like new rings will be all it needs.


After (or before) your new pistons etc.. Go though your carb and make sure it runs about right at standard settings... and fuel lines, filters etc.

Lakeside, your description sounds similiar to what happened here, but since I had the kit, I'm changing out everything. I even noticed scratches in the rings, but they are intact.

Carb has been rebuilt by me, new impulse line, fuel line and filters, manifold rubber looks good after close inspection. Saw actually ran good after "tuning up" despite the piston problem. Did seem to idle just a tad rough, but held idle fine. Crank big and small end seem fine. Hopefully have it all together tonight. KD



Need to flush (rinse out) crankcase just to be sure.

 

[kd460]

New cylinder/piston kit is installed. Fired right up. Runs nice and smooth. Only ran it for enough time to give it a few basic adjustments. I am running it a little rich and my oil mix is a little rich. It was 9 pm and the garage was filling with smoke, to cold to keep the door open and I'm sure the neighbors would not approve.

I will put the covers and the bar/chain on in the morning. Run it again on and off to give it a few heat cool cycles and pull the muffler to check things out.

Looking forward to seeing how it cuts with the larger cylinder and with one than is assembled correctly.

Lakeside, your trick with the string for the intake manifold work perfectly.

I used a zip tie for a ring compressor and was not all that impressed with how that worked. Probably will pick up the plastic ring compressor set that Bailey's sells, as that part of the job seemed to be the toughest part of it and took the longest mount of time. KD

 

[Finnbear]

New cylinder/piston kit is installed. Fired right up. Runs nice and smooth. Only ran it for enough time to give it a few basic adjustments. I am running it a little rich and my oil mix is a little rich. It was 9 pm and the garage was filling with smoke, to cold to keep the door open and I'm sure the neighbors would not approve.

I will put the covers and the bar/chain on in the morning. Run it again on and off to give it a few heat cool cycles and pull the muffler to check things out.

Looking forward to seeing how it cuts with the larger cylinder and with one than is assembled correctly. KD

Per this and an earlier post I gather you put an 028 Super piston and cylinder (larger bore) on a standard 028? I'm tearing down an 028 right now and would like to do the same if possible. Mine is an older 028 Wood Boss that needs an overhaul. BTW - what size long t-handle allen wrench fits the 4 socket head cap screws holding the cylinder on? It looks like 4mm and I need to pick up e Bondhus tee-handle wrench at work tomorrow.
Finnbear

 

[kd460]

Finnbear, yes on mine it was 4mm allen (socket) or 5/32. I have noticed on some models (I asume newer) that they use a torx head on the bolts. Don't know the size of that.

This is the kit I used: http://www.mfgsupply.com/m/c/44105.html?id=sTqeb5bV

A few things to mention tht make life a little easier. A dab of grease or non hardenening sealer on the base gasket helps to keep it from moving around while trying to get the piston/rings to slip into the cylinder.

The rear (nearest carb) cylinder bolts need to be installed before you set the cylinder. The right rear bolt is a bugger to get to if you don't. Put a little plumbers putty (or similiar) on the bolt about halfway up the threads of the bolt and put the bolt in the cylinder hole, it does a nice job of holding the bolt in the hole. Only have the bolt puttied in so only a small portion of the threads protrude beyond th bottom of the clylinder. The putty holds the bolts in place while fitting the piston into the cylinder.

The rings gave me the most trouble. My zip ties were to big I think. I should have come up with something a little better like a real small engine ring compressor or a cut piece of aluminum sheet metal wraped around the piston and used like the plastic ring compressors.

http://store.baileys-online.com/cgi-bin/baileys/1982?mv_session_id=tgwhpHQ3&product_sku=10429

I used a screwdriver handle (plastic) to "rest" the bottom of the piston on (between block and piston skirt) to keep things steady while lowering the cylinder on to the piston.

Next time I might try installing the piston into the cylinder, then position the cylinder, then install the piston pin to the connecting rod. This is a trick I have used when fitting cylinders on to multiple cylinder motorcycle engines (like a inline 3 cylinder triumph-what a nightmare).

Hope this helps, KD

 

[Finnbear]

Lots of questions-please help

Got a new set of Bondhus metric t-handles. The 4mm fits just right and I took the cyl off last night. This saw ran great until the last time I used it when it just stopped in a cut on an 18" cherry log. Then it was real hard to pull. I was running Stihl oil/93 octane at 40:1 that I mixed the day before so I was sure it wasn't a (lack of) lube problem. Once I got the cyl off I found the piston looked excellent - no scuffs/scores EXCEPT for two gouges down the side of the piston with mating gouges in the cyl wall. I found that one of the wrist pin retaining clips was missing an ear (about 1/8" long) - broken off and obviously it hung on the edge of a port and then got in between the piston/cylinder and stopped the saw. The remaining part of the wire ring is still in its groove and the pin is located exactly where it should be. What would cause this kind of failure? Obviously I need a new piston/cyl and I'll be looking into the one mentioned earlier. My 028 is an older model with a 42mm bore and 43cc displacement. Will the 46mm set from mfg supply fit my crankcase? Since I sell bearings for a living I will probably put new crank bearings in. What do I need to get the flywheel and clutch off the crank? It looks as if a special puller is needed for the flywheel that threads into the flywheel and then uses a center screw to push it off the crank. Is this puller available from Stihl or aftermarket, how much, and will it fit other Stihl saws (026, 038)? The clutch appears to be LH threads. Can I just zap it off with an air wrench or is that a bad idea? Help - this has been my favorite saw for quite a few years and I have to scrap it. Sorry for hijacking this thread but it seemed to be along the same lines as my troubles.
Finnbear

Finnbear

 

[kd460]

Finnebear, I'll try and help you out, but I'm fairly new at this. Now I have rebuilt cr and motorcycle engine, but new at chainsaws.

As far as the tab on the wrist pin keeper, this has been known to happen before. Just a freak thing that was probably caused by the vibrations and harmonics of the saw over time. Could have been slightly over stressed during install, who knows, but it has happened. Allot of th race/hi performance saw builders prefer to use the clips without the tab for this reason.

As far as fit of the new jug, yes, it should fit fine. Mine is the same as yours. To make sure, pick up a STIHL brand cylinder base gasket for the super first from your dealer (the .50 thick one) and place it in the top of the case. You will see if you have clearance or not. The piston skirt barely drops into the case. The case is stepped or starts to get smaller the deeper into the case you go, but the widest part near the top is plenty. I need to mention that in another post I had what I thought was low on compression after the new cylinder, turns out a bad gauge and I will comment about it in that post in a few minutes.

For the flywheel, I think ebay has pullers for around $20, or I have seen people homebrew one out of a piece of angle iron and some bolts. Another option is a flywheel puller for about $10 at the auto parts store. Just make sure it is wide enough for the bolt hole spread in the flywheel. The holes on the flywheel will need to be tapped to accept the bolts, and I'm not sure what size tap it is, however, use a fine thread bottoming tap. Maybe others have had luck with jaw type pullers, but I don't think that will work.

The clutch is righty loosy, and the if the cylinder was on, I would just use abut 5-6 inches of 3/8 inch nylon rope pushed into the spark plug hole, making sure the piston was close to TDC before doing this (so the rope does not get stuck in the transfer or intake/exhaust ports. I suppose you could just put the jug back on (good practice for installing the new one) or, with bearing removed slip a wood dowel in the small end of the conn. rod and lock it that way. Not sure if I would use a metal rod, maybe if it was protected with some tape. Then it's just a matter of using a wrench. I have heard of others use an air impact, but that was with cylinder intact. May get someone to hold the flywheel (wih gloves) whil you turn the wrench on the clutch side.

No worries about hijack, hope this helps, you will get it back together and it will do just fine. You will like how it runs. Did I cover em all? Good luck, keep us posted, KD

 

[Lakeside53]

It looks as if a special puller is needed for the flywheel that threads into the flywheel and then uses a center screw to push it off the crank. Is this puller available from Stihl or aftermarket, how much, and will it fit other Stihl saws (026, 038)? Finnbear

I'd use the correct puller and never modify the flywheel to accept any other. Don't use an automotive three jaw puller to pull from the outer edges of the flywheel... you likely crack or break it.

Yes, the puller is used in many other stihl saws so consider it an investment.

 

[Finnbear]

Any guess as to how much it will cost? I assume I can't get one anywhere else besides the Stihl dealer. I don't have an 028 service manual (yet) but the service manual for my 038 shows part # 1110 890 4500 as the flywheel puller. Is this the same one used for my 028?
Finnbear

 

[Lakeside53]

Any guess as to how much it will cost? I assume I can't get one anywhere else besides the Stihl dealer. I don't have an 028 service manual (yet) but the service manual for my 038 shows part # 1110 890 4500 as the flywheel puller. Is this the same one used for my 028?
Finnbear

It's the same one, and the most common used on Stihl saws. Check with your dealer.. You can get them for $15 to $20 on Ebay, but your dealer might sell you one for a lot less.

 

[Finnbear]

This looks like the correct one.
[http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=140043414115&fromMakeTrack=true]
They don't give the Stihl number but the style is correct.
Finnbear