TS700 investigation

[belgian]

Ok, this is not a chainsaw problem but close anyway. A friend of me brought in a Stihl concrete cut off saw Stihl TS700 earlier this week for repair. It suddenly stopped, he said.
I know a bit about chainsaws but have never worked on these things before, except a single TS400 and a TS760 in the past.

The starter blocked so I took off the muffler to have a look at the piston. The top ring land had blown off and the top ring was reduced to bits. Took the cylinder off and noticed a deep scratch in the compression zone, so a new P/C kit was the ticket. I did not know what caused the piston to let go. Checked the crank and bearings, and they seem OK.

Told him the bad news, so he brings in an aftermarket P/C kit which he bought from a local web dealer. Looked like a decent product with good finish, so I installed it today. Now the problem : it appears that when you turn the crankshaft by hand, it seems that the crank/piston/rings seem to 'hang' sometimes, but can't find the cause. My first suspect is the piston rings, that may hang up in a port somewhere, but the problem is only very occasional.

but I am afraid to start it and possibly ruining the new P/C kit. Maybe there is another problem that ruined the P/C kit in the first place.

What to look for now ....? any help is appreciated.

 

[HarleyT]

I would say that you have a bad bearing, but at any rate, pull it back apart immediately.

 

[belgian]

I would say that you have a bad bearing, but at any rate, pull it back apart immediately.

I was afraid you were going to say that. I checked the case thoroughly and also checked the bearing before installing the new P/C kit. Those bearings appeared very good to me at first sight, but I know these machines eat a lot of dirt.

 

[jackjcc]

Is the flywheel and ignition still on? I did that once, got all worked up and once I tore it back down the hang up was gone.

If it feels like a solid stop and then forces by you could check that the piston is on the right way. Look in the exhaust for a ring locating pin, sometimes the aftermarket kits don't have the arrow.

Other than that there could be debris in the bearings.


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[belgian]

Is the flywheel and ignition still on? I did that once, got all worked up and once I tore it back down the hang up was gone.

If it feels like a solid stop and then forces by you could check that the piston is on the right way. Look in the exhaust for a ring locating pin, sometimes the aftermarket kits don't have the arrow.

Other than that there could be debris in the bearings.


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Ignition coil has been checked and found OK.

When it stops, it is a suddon stop, and mechanical. The piston had the arrow and which is mounted towards the exhaust. The debris theory may fit, but I can't understand I didn't notice it.

 

[jackjcc]

I had a saw with a plug electrode end up in the bottom of the case, very small, but it kept the crank from turning over. I would guess one of those pieces of the ring remained in the case? If it's a pretty solid stop and no advancing after I would look for debris, the clearance is tight so it wouldn't take much and oil tends to build up making something for it to stick to. Flush with clean gas and then flush with mix?


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[jackjcc]

On second thought double check the locating pin. It has happened that the arrow ended up pointing the wrong way on aftermarket kits.


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[alexcagle]

I've seen several AM pistons that didn't clear the crank counterweights at near bottom dead center.
K960, K970 Husqvarna need clearanced on some of those crappy Golf brand pistons. I've seen Dolmar 50mm pistons that have a casting protrusion that needs ground off, because there is an interference like you are describing .
*
The only worthy aftermarket cylinder kit I would recommend on a 4224 series Stihl is a Cross brand kit. They are really close to OEM in quality and power. They come with Caber rings too.
*
The four separate reed plates on the two statified charge intake ports evidently must be difficult to machine or too complex to cast, because I've seen some awful attempts that run terrible. The reeds have to seal, or the backflow causes spitback that upsets the intake charge and soaks the air filters with fuel...No bueno.
*
You said you checked the bearings, but did you visually check the bearing races, and the lower rod bearing for a cracked or broken cage. Radial play; (up and down looseness), isn't all that signifies bad bearings. There could have been a ring fragment that got outboard through the crank bearings, then got thrown back inside the crankcase.
I've also seen a ring piece get trapped between a crank counterweight and a bearing casing. It would grind against the crankcase and there would be a catch during every revolution.
*
As to what caused the broken ring carnage; concrete cutoff saws wear about three times faster than a chainsaw, even with diligent filter maintenance, (98% do not). They usually get used all day every day five days a week, and they are almost always used at max RPM.
What usually happens is, they wear enough to work past the ring locator pins, then rotate around until they hang on a port. (usually the exhaust port). then ka-pow. dead saw.
*
One important thing they don't tell you in school at Stihl is that when you grenade a piston or bearing, the bits get shat out into the muffler. You gotta bang it around and shake it to make sure you get the bits out of there. They'll get back in the combustion chamber through the exhaust port.

 

[alexcagle]

On second thought double check the locating pin. It has happened that the arrow ended up pointing the wrong way .

*
That wouldn't cause a "catch".
anyway, an easy verification check for that mistake would be looking in the exhaust port.
If you see a ring gap, it is backwards.
*

 

[jackjcc]

*
That wouldn't cause a "catch".
anyway, an easy verification check for that mistake would be looking in the exhaust port.
If you see a ring gap, it is backwards.
*

Yeah that's what I was saying, if you see the locating pin then the ring gap is there. You explain it simpler than I did though.


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[belgian]

I am definately positive that the piston locator pin was located at the intake side, so it should be OK.
Anyway, some very good info, thanks folks. Will have to tear down again the cylinder and have a closer look.....grrrrrrr.

 

[alexcagle]

Yes.
Thank your customer for that pos kit.

Customers are in-grained tight wads sometimes.

 

[HarleyT]

I have seen the same problem with the Golf piston. I had to grind a bevel on the inner lip of the piston bottom so it would clear the counterweights. The customer brought me the piston, and I advised him to send it back, but he wanted me to use it, it seemed to work out OK.

 

[belgian]

Well, while you folks had your last day sleep of the old year, I took the cylinder off for a closer inspection.

Here's the culprits...... debries from the piston aftermath.
After all, my initial inspection was not good enough. These tiny particles were almost glued to the crankshaft couterweights and hung up to the case.
So I was lucky I guess that I noticed the problem while turning the crank by hand before starting the thing again.

Put her back together and seems the problem is gone. Discovered that the fue line was broke, so have to wait for that part to arrive, before firing her up.

I must admit that the aftermarket P/C kit looked very decent, with quality rings and good piston circlips. I have seen a lot of aftermarket stuff for chainsaws, so know when to call it a POS. Credit where credit is due.

Anyway, lessons learned, and thanks for your support.
Have a good 'slip' into the new year, LOL.

 

[alexcagle]

Good deal.
I blow out the crankcase with compressed air, then stuff a wad of paper towels with a squirt of WD-40 down in the crankcase and rotate the flywheel until it comes back out the opossite side with crud on it.
If the saw ingested a bunch of dirt or cement dust, and there is no apparent damage to the lower end, I usually squirt enough WD-40 to fill the crankcase about a third of the way, and then hand rotate it to float the contamination out of the bearings. If it's narley, it may take three or four flushes to get it clean.

 

[belgian]

Good deal.
I blow out the crankcase with compressed air, then stuff a wad of paper towels with a squirt of WD-40 down in the crankcase and rotate the flywheel until it comes back out the opossite side with crud on it.
If the saw ingested a bunch of dirt or cement dust, and there is no apparent damage to the lower end, I usually squirt enough WD-40 to fill the crankcase about a third of the way, and then hand rotate it to float the contamination out of the bearings. If it's narley, it may take three or four flushes to get it clean.

These concrete saws need indeed some more attention. Once can notice you know your stuff !!

 

[rmh3481]

Hey Roland,
Good to see you about! Now drill some holes in that muffler and lets go..

 

[farmhard]

Well, while you folks had your last day sleep of the old year, I took the cylinder off for a closer inspection.

Here's the culprits...... debries from the piston aftermath.
After all, my initial inspection was not good enough. These tiny particles were almost glued to the crankshaft couterweights and hung up to the case.
So I was lucky I guess that I noticed the problem while turning the crank by hand before starting the thing again.

Put her back together and seems the problem is gone. Discovered that the fue line was broke, so have to wait for that part to arrive, before firing her up.

I must admit that the aftermarket P/C kit looked very decent, with quality rings and good piston circlips. I have seen a lot of aftermarket stuff for chainsaws, so know when to call it a POS. Credit where credit is due.

Anyway, lessons learned, and thanks for your support.
Have a good 'slip' into the new year, LOL.


View attachment 547234

 

[farmhard]

I build alot of the older ts saws. There is no point to adding a pistin without splittng the case IMHO. If you don't and the seals are shot it will bite you.
Don't buy bearings on fleabay, most are junk. Use Stihl or SKF. For example a TS350 uses 6202 which SKF cost is about $6 US, $9 CDN. I had people show up with new bearings that were worse than the old ones.
Buy good seals. The cheap ones flex on install.
Some fleabay cylinders loose the cross hatch in the first 3 minutes of run time even after soaked in oil overnight with new rings.
Hutzl TS760 pistons currently don't fit over the rod (same as 510 or 051). I know what you are working on, just threw my 2 cents in on the a/m junk. Don't waste your time.
Also mic the piston to make sure it is round.
Also check the 700/800 cases on fan side for surface cracks. The mounts crack and flywheel hits coil like the 2 I have here re: my post about swaps.
I absolutely DETEST working on 700/800 saws but I am old and cant start a 510 without decomp. valve.

 

[alexcagle]

These 3120K railroad saws are 50lb monsters.
Not bad to work on though.