OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs and stratton 12hp keyway sheared

Hi all

Just rebuilt a 12hp Biggs and stratton model 252707 0026-01 7905** it was all going well had spark spun it over fired and then sheared the flywheel key. No worries got two news ones tried again, cleaned the surfaces better nipped it up tighter and sheared another. The last one didn't shear and flogged out the keyway of the flywheel and crank. It ran for 30 seconds or so.

The photo is the result.

What are the main causes?
Have I put it together wrong?
Did I miss something?

Any thoughts on how to fix it?

P.s the engine owes me little, I have a spare flywheel to suit.

Mick

This isn't something at all familiar. From the final appearance of the keyway it seems clear it was not a vibration problem. Briggs engines rotate clockwise viewed from the flywheel end, and as I read it, the side of the keyway that is damaged is the trailing side, not the leading side. It looks as if something stopped the flywheel and the crankshaft inertia caused the damage, not the much more usual situation where the crankshaft stops and the flywheel keeps going. Could something have jammed the flywheel?

Thanks grumpy for a very observant diagnosis.

No nothing could have jammed/caught the flywheel except the starter motor. But by design disengages when the engine has started.

My thoughts are I'll get the crank out and get it fixed, replace the flywheel and have another crack.

there was a touch of damage to the keyway before the project started but once there is play I guess it causes timing issues and h a merging and the damage escalates

Anyone else have experience welding repairing a crank?

I think the starter motor may be the culprit, Mick. It may be staying engaged, seizing its bearings due to overspeed, and shearing the flywheel key. If the starter has a Bendix drive (helix on the armature so the pinion winds into mesh due to inertia when the starter motor operates, then is thrown out of mesh when the flywheel speed exceeds the starter motor's speed) it is likely that all of your problems have been due to a sticky Bendix - a very common phenomenon on old Briggs starters. If you inspect the starter's bearings you should see evidence of severe trauma, if my guess is right. The bearings will probably be so loose that the carbon brushes cannot operate properly.

If the problem has been caused by the starter, I think at this point you will need to replace it. With regard to the keyway, we do have one thread in which a member MIG-welded a damaged crankshaft keyway - the PTO keyway on a Honda GX160. Here is the thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=23509#Post23509

Unless you have access both to a skilled and experienced welder with a good MIG machine, and a milling machine to re-cut the keyway, I suggest you look for a second-hand crankshaft, or a second-hand engine. However it seems essential to fix the cause before you go through a difficult repair process which might come to nothing 30 seconds after you start the engine.

Bugger, I didn't notice it catching

I'll get it off on the weekend and take some photos.

Mick

Thanks Mick - it seems like rather an unusually severe outcome, I'd really like to see the details.

Just to clarify it was genuine alloy flywheel keys being used not a steel one?

I really honestly fail to see how a grabbing starter can cause this, the teeth on the starter are plastic, they shred before anything else breaks.

The flywheel has to be pulled down TIGHT, with the big flat washer under the nut, we're talking the same flywheel key as used in small 2hp engines, except the flywheel is a lot heavier in a 12hp. if the flywheel is not bolteddown tight enough it will break the key with ease.

If you get stuck for a crankshaft or getting it repaired, I will check later I may have a 25cube crank floating around, forsome reason though I think all my bigger engines are 28 cubic inch.

I've had some difficulty seeing the tiny starter motor bearings being able to generate enough resistance to shear the keys Joe, but the key point seems to be that the "wrong" side of the keyway was damaged. It seems to be the side that would get mashed if the flywheel stopped and the crankshaft kept going, or if the crankshaft were suddenly accelerated relative to the flywheel. It would be easily explained if some kind of wedge dropped in between the outside of the flywheel and some stationary part of the engine, or between the flywheel ring gear and the engine, but Mick is sure that kind of thing didn't happen.

Edit: I'm wondering now if maybe the flywheel isn't fitting down onto the taper. It might be the wrong flywheel? If it doesn't sit on the taper, the key could easily be sheared on the side where the keyway damage is. That would be due to the engine always starting on full throttle, due to the way the governor operates. With a full throttle start and a loose flywheel, the crankshaft will rotate faster than the flywheel does and thump the key from the left side. It will keep on thumping the left side of the key every firing stroke, until the zinc key gives out - which might be quite a few thumps, but that would only take a very few seconds. It could even damage the keyway, by the time you'd done it over and over again with new keys.

I'm really looking forward to finding out what has actually been happening.

Joe and grumpy,

I've checked the starter and it is fine no damage no movement nothing, looks fine.

when it did start it wanted to run flat out so definitely put excess pressure on the key.

I'm going nail it down and say that I have not nipped up the flywheel adequately, as I could have tightened it up further. With the repeated shearing play has built up exponentially and every time it has made the tapper worse and more damage, worse fit, blah blah.

Ok, I have a spare crank and matching flywheel but the pto end is incorrect for the mower so will be a good crank to test the engine, get the other crank mended try again.

Early keyway damage may have messed up the fit of the flywheel on the shaft, so it did not seat properly, and slid back and forth around the shaft on each power and compression stroke. The key would not last long if that happened.