TS420 Project Cutoff Saw

[Automender]

I picked up a Stihl TS420 saw for $50 and wanted to try to refurbish it. This is my first time to completely rebuild a saw. I have installed new rings in a Husky K970 cutoff saw and rebuilding the carb. I have no history on this saw nor why it failed other than the person said it was a top end issue. I decided to strip the saw down the other day and found two thing I could use some advice on. On the strip down I found play in the crank bearings and what looks like pieces of bearing cages in the crankcase. So those need to be replaced but the rod bearings do not have any play up or down and back and forth. The rod however rocks side to side and I am not talking the normal sliding side to side. I am thinking this needs to be replace due to the rocking motion but wonder why no vertical play.
Second issue is that the piston looks pretty good and likewise the cylinder looks good compared to You tube videos on failed saws. However the piston top looks like it has some erosion on the top. What causes this type of damage?
View attachment IMG_4221 (1).MOV

 

[TLG]

I'm no expert but looks like detonation issues to me. Especially due to the fact it's around the outer perimeter of piston where the flame front meets cylinder wall. The carbon doesn't look heavy enough to cause early ignition issues//pre ignition. Looks as poor fuel/ mix issues looking at bottom end dryness, contaminants, and known bearing failure. To me that rod bearing is toast no matter if the cage is intact or not. Again, more knowledgeable guys are on here.

 

[sawfan123]

I would recommend a different type of saw for a project. The conditions those saw run under, usually there’s not much left. But, if money’s no object, they aren’t that difficult to work on and there’s thousands of them out there.
Just check the cases very carefully for external corrosion from the concrete.
It looks like detonation and that could wipe out the bottom end about also.

 

[frank_]

done a few like that for a couple of local hire shops
they were busted crank bearing cage, those are oddball size bearings with black plastic retaining rings (which can cut into the casings)
that casing wear will determine if its worth fixing

 

[Automender]

I'm no expert but looks like detonation issues to me. Especially due to the fact it's around the outer perimeter of piston where the flame front meets cylinder wall. The carbon doesn't look heavy enough to cause early ignition issues//pre ignition. Looks as poor fuel/ mix issues looking at bottom end dryness, contaminants, and known bearing failure. To me that rod bearing is toast no matter if the cage is intact or not. Again, more knowledgeable guys are on here.

Thanks for the insight. The bottom may be dry because I drain gas and let everything dry out since I was going to disassemble in my basement and didn't want the gas smell.

I would recommend a different type of saw for a project. The conditions those saw run under, usually there’s not much left. But, if money’s no object, they aren’t that difficult to work on and there’s thousands of them out there.
Just check the cases very carefully for external corrosion from the concrete.
It looks like detonation and that could wipe out the bottom end about also.

I agree that the conditions of use is terrible. I had a TS400 I purchase a while ago to do a home job and it was used at a scrap yard to cut steel. It seemed that that service was maybe good for internals but terrible for the case corrosion. This case wasn't too bad. This is a learning experience at age of 62 since I never worked on 2 cycle just 40 years on my own cars.

 

[Automender]

done a few like that for a couple of local hire shops
they were busted crank bearing cage, those are oddball size bearings with black plastic retaining rings (which can cut into the casings)
that casing wear will determine if its worth fixing

View attachment 949441

Thanks for the case tip. I need to look at the case wear today because that will decide whether I buy a crank or not and move forward. I was wondering what those black rings were on the bearing race. From what I have read in the manual the bearing should fall out if you heat the case with a heat gun. I am planning on all aftermarket parts on the rebuild since this is personal use and would not be cost effective if I went with OEM. On your rebuilds did you use OEM or AF?

 

[frank_]

Thanks for the case tip. I need to look at the case wear today because that will decide whether I buy a crank or not and move forward. I was wondering what those black rings were on the bearing race. From what I have read in the manual the bearing should fall out if you heat the case with a heat gun. I am planning on all aftermarket parts on the rebuild since this is personal use and would not be cost effective if I went with OEM. On your rebuilds did you use OEM or AF?

the bearings are oversize 6202,s (36.3mm iirc) so pretty pricey, and the plastic rings are there to hold them snug in the case, rather than the usual interference fit
they stop the bearing from spinning in the case, but it means the metal to metal contact is very small, and that causes the wear
i have only ever used copy p+c,s , but with an oem top ring, because i have had a few top rings fail after 1 yr ish (the hire ones have a hard life eh)

 

[Automender]

the bearings are oversize 6202,s (36.3mm iirc) so pretty pricey, and the plastic rings are there to hold them snug in the case, rather than the usual interference fit
they stop the bearing from spinning in the case, but it means the metal to metal contact is very small, and that causes the wear
i have only ever used copy p+c,s , but with an oem top ring, because i have had a few top rings fail after 1 yr ish (the hire ones have a hard life eh)

Thanks again. I have seen bearing sets in the $16 to $30 range. Maybe I will go OEM and pay the extra for the bearings. I should get the bearings out in the next day or two to check the casing condition.. I may try some Green Loctite to seat the bearings since I don't think I will ever rebuild this saw again.

 

[Automender]

You were spot on on your comment that the bearing cages fail. Pulled the case and the clutch side of the crank's bearing had a disintegrated bearing cage. The flywheel side pulled off easy with a puller but the clutch side flew apart when the outer race cracked. I wafer wheeled the inner race and then chiseled it to split it. It was fretted on the shaft and bearing so original fit must have been a little lose to cause that. Luckily the case bearing seats are good. Flywheel side bearing was fine
.

 

[ericm979]

That's definitely detonation pitting on the piston crown. Either it was run too lean or the timing was too advanced or it overheated (i.e. blocked air flow, dirty fins). Or more than one. It led a hard life.

 

[Automender]

Not sure on these saws what octane that Stihl recommends. Maybe it needed 90+ octane and not the 87 for regular gas. I know on my Stihl blower seems to ping during startup. Maybe the bearing failure lead to seal leakage and lean mixture. Cylinder looks really good but I bought a Cross Performance cylinder and piston overhaul kit to install. Hard to say but the case looks good for a rebuild.
Headed to Florida next week to visit and there is a shop down in Daytona that has good quality aftermarket parts for reasonable prices. Looks like I need a crank and bearings that will run about $80, which was not what I was hoping for. Looks like I will have about $250 total if nothing else pops up. I bought and AF top shroud, flywheel cover and used filter cover.

 

[Automender]

Picked up some aftermarket bearings and crank over the holidays. I spent three+ hours cleaning parts getting all the concrete dust off most of the saw. I know why these saws get tossed because it just too expensive to do the cleaning alone.
Well I thought I would put some pics of the aftermarket and OEM crank. Pictured is what I would consider a major flaw in the machining of the crank. The OEM has a decent stress relieving radius machined into the shaft while the aftermarket is just a stress riser 90 degrees. this could easily lead to a crack developing at the shaft over time. Not sure of the actual stresses at that point but OEM followed good design practices. The other interesting thing is the OEM and aftermarket rod has the exact same movements as I posted earlier. Seems OEM was still good but I nicked the seal area while I was cutting off the bad bearing inner race.

 

[Automender]

Finally had time today after repairing my mailbox after someone ran into it and installing a new control board in my garage door opener. I decided to install both bearings on the new crankshaft before putting the case halves together. I put the crank in the deep freeze for a few hours to get it to -5 degrees and heated the inner bearing races to 300 degrees per service manual. Both bearings just dropped into place with no resistance at all. I then placed the crank and bearings in the deep freeze for an hour.
I heated the bearing area of the case halves with a heat gun and the cold bearing just slid into place. I installed the case gasket that I coated with Motoseal on both sides on the one half and heated the other case halve bearing area. Case halves slide together with out and force and then ran the case bolts in. I was happy to get that done, first time working on a case.
I oiled the bearings with two cycle oil using a syringe and oiled the shafts.
I made a install sleeve from thin plastic from a notebook page protector. Rolled it into a tube and slid it into the seal. Slid the sleeve and seal on the shaft, coated the seal with a dab of Motoseal and positioned it to press in. I used a 19mm deep well socket for the one side and a 17mm on the small seal. A few taps and the installed easy. Next I need to check the cylinder port chamfers on the Cross Performance Cylinder kit. Any tips on my next step on the cylinder install would be greatly welcomed.

 

[Automender]

Looked at the aftermarket cylinder port chamfering and in general it looked decent to an untrained eye. There were a two transfer ports that seemed sharp so I decided to use a diamond file to ease the edge and then 400 grit wet/dry paper to smooth the chamfer even more. All feel smooth to the finger running over them in any direction. Diamond file fit perfectly in the port to ease both top and bottom edges at once. I think my next step is to measure squish in four different locations. Does any one know the minimum squish for Stihl saws. I am not looking for super just normal performance and reliability.

 

[Automender]

I decided to measure the depth of the squish bad on my OEM cylinder and then the Cross Performance one. I was hoping they were the same or maybe because the word performance is in the brand name that aftermarket one maybe a little shorter. The distance form the gasket surface to the squish ring on the OEM is 2.827 inches and the aftermarket is 2.847 inches. So I start out with .020 inches more squish. Using the old cylinder gasket I get squish in three locations of .0600, .0605 and .0595 inches. It looks like the original OEM squish was in the area of .038 inches by using data gathered.
I measured the aftermarket gasket to find that it is .0345 inches so if I used the new AF gasket with the AF cylinder I will have .0725 inch squish which I would give an uneducated guess is way too much. If I do a full gasket delete I will end up with .040, .044, .040. Not exactly to happy with the Cross Performance performance at this point but without a gasket at least I can get back to OEM squish.

Is it a good idea just to use Motoseal and no gasket at all or do I go with a thin gasket and have .040 squish? Or maybe just go with .060 to .070 squish?

 

[Automender]

I guess I just couldn't do a total base gasket delete or maybe just wanted to try to make a base gasket. I used an aluminum soda can and sanded all the coatings off both sides with a random orbit sander. Used a base gasket as template and scored it with an razor blade but used an Exactor blade to puncture outline of the screw holes. I bet a paper hole punch would have work good. I flexed along the score lines to break the waste away. Reamed holes to perfect size with a roll of sand paper I finally place 220 grit wet/dry paper on a flat surface and sandwiched it between another one and lapped flat. It ended up being approximately .004 plus a thin layer of Motoseal on both sides. I guess final squish will be .043 vs OEM of .038.

 

[Automender]

Finally got around to pressure/vac testing the crankcase today. I didn't have a plug to block off the decompression valve so I dabbed a little 5 minute epoxy on a closed decompression valve and let it harden. After an hour it hardened. I figure all I have to do is to wire brush the epoxy off it later. I need to put it on my list of stuff to pick up.
I can understand why people don't pressure test because it is a pain to completely seal the case off. The rubber inner tube worked well on the muffler but the intake was a pain. Rubber didn't work but the plastic covered cardboard that the gasket kit came in worked fine with a little silicone vacuum grease and clamps. I used the impulse nipple for the test connection. I used a Mity Vac to pump it up to 7 psi and it held without dropping any over ten+ minutes. Pulled vacuum to 7 in hg and it held the same. I am thinking the pressure test could also be done easier with 7 psi air from my compressor and just spray to find any bubbles. That way sealing up the inlet and muffler is not as critical. Next step is to tackle the carb which is caked with concrete dust. I need to also measure the finish squish since I eliminated the OEM gasket and installed a .004 soda can gasket plus Motoseal to correct the Cross Performance jug with an additional .020 squish.

 

[Automender]

Assembled the stuff on the crankcase today and measured the squish. I ended up at .o40 inches vs. the OEM .038. Had to reduce the base gasket because of the extra .020 inches of squish in the aftermarket jug. Moved on to the carb cleaning. These saws have a hard life. Total new meaning of carb cleaning. I guess duct tape over the openings and hoses on the nipples should keep the concrete out of the internals.

 

[Automender]

Working on a TS420 Stihl cutoff saw and came across an issue in the carb. On a K970 Husky I did the same test and the pop off pressure was about 15 psi. On this Stihl carb I did the same and I pumped it up to 25 psi and it didn't pop the needle valve. The needle valve is not stuck since I can press the metering lever and it pops. Is this normal. Also what is the sealant they use on the oblong welch plug in the picture?

 

[sawfan123]

Working on a TS420 Stihl cutoff saw and came across an issue in the carb. On a K970 Husky I did the same test and the pop off pressure was about 15 psi. On this Stihl carb I did the same and I pumped it up to 25 psi and it didn't pop the needle valve. The needle valve is not stuck since I can press the metering lever and it pops. Is this normal. Also what is the sealant they use on the oblong welch plug in the picture?View attachment 962525

What does a pop off test accomplish? The sealant is a heat activated epoxy. Haven’t seen anything yet that affects it.