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View Full Version : Replacing a cylinder gasket on a Stihl br 420?


HenryB
02-16-2008, 10:02 PM
Blower is leaking oil at the base of the cylinder (gasket area). Any tips on replacing this gasket. Will the cylinder just lift off after removing the 4 screws?
Thanks in advance.

lwcmattlifter
02-16-2008, 10:56 PM
It lifts off. Be careful with the piston rings they are really fragile.

MowerMedic77
02-16-2008, 11:18 PM
Pull off your carb and muffler, it will make handling the cylinder easier. Drop the piston to the lowest point of travel and remove the screws from the cylinder. Pull off the cylinder from the lower case and clean both surfaces making sure to clean anything that drops into the cylinder or the lower case. Install your new gasket and lube the cylinder walls with a little raw 2stoke oil. Make sure that both rings are floating free and the end gap is in-line with the anti-rotation pins. Take you time and slowly reinstall the cylinder down onto the piston one ring at a time being careful not to force it as they will break very easy. Once both rings are inside the cylinder tighten it back down to the lower case. Reinstall muffler and carb, run and test.

HenryB
02-17-2008, 11:11 AM
Pull off your carb and muffler, it will make handling the cylinder easier. Drop the piston to the lowest point of travel and remove the screws from the cylinder. Pull off the cylinder from the lower case and clean both surfaces making sure to clean anything that drops into the cylinder or the lower case. Install your new gasket and lube the cylinder walls with a little raw 2stoke oil. Make sure that both rings are floating free and the end gap is in-line with the anti-rotation pins. Take you time and slowly reinstall the cylinder down onto the piston one ring at a time being careful not to force it as they will break very easy. Once both rings are inside the cylinder tighten it back down to the lower case. Reinstall muffler and carb, run and test.

Thank you ! Any other last minute tips.:waving:

Landrus2
02-17-2008, 11:16 AM
Thank you ! Any other last minute tips.:waving:


It would be a good idea to replace your piston and rings at this time.:drinkup:

ed2hess
02-17-2008, 07:01 PM
Thank you ! Any other last minute tips.:waving:

Not a tip but a warning those cylinders didn't use to have bevel on them so they are a living hell to get back over the piston/rings. A helper person can be beneficial on helping get it over. The echo is very easy compared to these, unless they changed recently.

heybruck34
02-18-2008, 09:40 AM
Take it to a shop. I tried to do it myself and failed, enlisted my father who used to rebuild engines for extra cash on the side and we failed. I broke 2 rings and almost a 3rd before I gave up. 2 days later and a relatively small amount of money, my blower runs like a top.

I like doing my own maintenance and repairs, but this job was a b---h and I felt better left up to the pros.

topsites
02-20-2008, 01:22 PM
Yeah once we get into that TDC compression bit the only way I'll DIY is if I got nothing left to lose.
The dealer can probably do it for a lot less than the headache is worth.

Stillwater
02-20-2008, 01:54 PM
when they say those rings are easy to break they mean the rings are very very brittle if they get flexed more than a 1/8 of a inch in any direction and in any way they will snap. but it is a easy job....

YardBoss Lawncare
02-20-2008, 02:04 PM
What's the "end gap" and "anti-rotation pins"????

MowerMedic77
02-20-2008, 02:34 PM
If you look at a new 2 cycle piston or one with no rings on it, in the ring groves you will see a pin in each one that is offset and keeps the ring end gap(or the opening in a ring) from lining up and letting compression escape. I will try and attach a pic but may be blurry.100627

YardBoss Lawncare
02-20-2008, 06:13 PM
I understand now. Thanks.

HenryB
02-20-2008, 09:47 PM
Thank you very much everyone for the help. I just finished it took me about 40 minutes. It's running great no more leak.:waving: The gasket was shot in three places. I could not have done it without you guys thanks again.

MowerMedic77
02-20-2008, 11:28 PM
Thank you very much everyone for the help. I just finished it took me about 40 minutes. It's running great no more leak.:waving: The gasket was shot in three places. I could not have done it without you guys thanks again.

Congrats, glad to hear you got it going:clapping:

Mcrover
02-22-2008, 12:30 AM
Had to rebuild one recently after a straight fuel brain faid eppisode by one of our ex employees at the golf club I work for and I didnt think it was that bad.

I found that a bit of shim sheeting and a large hose clamp made it really easy to reinstall the piston, you just dont clamp it up too tight, oil it all up and it slides right in and then just undo the hose clamp and reassemble the whole thing.:drinkup:

heybruck34
02-22-2008, 09:02 AM
you guys are better than me and my father. Glad to hear you were able to get it done. I think those rings were $8 each so after $24, I stopped gambling.

"Know when to hold em and when to fold em..."