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Old 11-03-2022, 04:13 PM
trooper1954 trooper1954 is offline
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Default Question about Stihl BR 400 blower

Hi all,
I have a 30 year old Stihl BR 400 blower that has suddenly started breaking woodruff keys. I've now replaced 3 of them, and each time the blower is a bear to pull, and breaks the key almost immediately. When the plug is removed and there's no compression the blower turns over like a charm. Does this mean that I have a crankshaft issue, and if so, how do I determine if I need to replace the bearings or the whole crankshaft or??
Any and all help would be appreciated...I finally took it to a shop and after replacing another key (which promptly broke!!) they told me they wouldn't work on it any more!! I don't want to throw it in the trash as previously it has been working fine and it's a good machine.
Thanks.
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Old 11-03-2022, 05:00 PM
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Ironman Ironman is offline
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Does this engine have a compression release button?
Other than that, if this engine originally has a hardened key and you are replacing it with a soft key that may be the problem.
But I tend to think that the main purpose of a woodruff key is to make the spark timing at the right spot. The main thing holding the flywheel to the crank is the taper. Even if the key is sheared off, you should have to use a flywheel puller or wedges and rap the crank end with a brass hammer. Normal compression should never be able to turn the flywheel unless the taper of the crank or the flywheel is boogered.
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Old 11-03-2022, 05:11 PM
trooper1954 trooper1954 is offline
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Thanks Ironman....no, no compression release button. How do I determine if the key should be a hardened one? And how to determine if the tapers boggered?
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Old 11-03-2022, 05:38 PM
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Shade Tree Welder Shade Tree Welder is offline
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One thing it might be is excessive carbon and ash build up in the cylinder and
on the face of the piston, this could lead to excessive compression and some of
the issues you are having. Something not unheard of in 2 strokes. I have seen
it once.
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Old 11-03-2022, 07:03 PM
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Quote:
Originally Posted by trooper1954 View Post
Thanks Ironman....no, no compression release button. How do I determine if the key should be a hardened one? And how to determine if the tapers boggered?
Look for scarring rings in the taper bore of the flywheel. The only way to know it this is a special woodruff key is to contact Stihl.
Without the key in place you can install the flywheel, tighten the flywheel nut, and then attempt to remove. The flywheel should not come free with the nut off.
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Old 11-03-2022, 09:36 PM
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Along with the other folks good recommendations. Make sure the nut is torqued correctly. A loose nut will cause this.

Some of these have a spark arrest screen in the muffler that will become completely plugged and cause this. remove muffler and find it. if it is plugged burn it out with a flame torch and it usually cleans up.

Scott
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Old 11-04-2022, 08:26 AM
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Quote:
Originally Posted by trooper1954 View Post
...And how to determine if the tapers boggered?
Like Gerry says a visual inspection is the first step. Both the bore and the shaft should be smooth and free of any burrs. You can also get some machinists blue and check the fit of the pieces with that--any imperfections in either shaft or bore should stand out...
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Old 11-04-2022, 11:20 AM
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Originally Posted by LKeithR View Post
Like Gerry says a visual inspection is the first step. Both the bore and the shaft should be smooth and free of any burrs. You can also get some machinists blue and check the fit of the pieces with that--any imperfections in either shaft or bore should stand out...
If you haven't got and machinist blue, a felt magic marker will work fine.

Also....if the taper bore and crank are perfect, try a sprinkle of baby powder talcum, if it is not illegal yet. Sprinkle it on the crank taper, and assemble. It makes the assembly seat better and much tighter. I use it to keep my Jacobs taper chucks from attempting a moonshot.
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Last edited by Ironman; 11-04-2022 at 12:09 PM.
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