Rebuilt 10hp Briggs Won’t Start

[bertsmobile1]

The whole point of a key, apart from a Scotch key is to shear before the shaft / pulley / whatever breaks.
So the key used needs to be weaker than whatever it is protecting
Even drive keys like in the axel will shear before the axel breaks.
The flywheel key was designed as a safety feature when flywheels were smaller & lighter.
So on a walk behind, the cast zinc keys are needed.
On a belt driven ride on they are not needed from a purely engineering point of view because belts will slip or break

 

[Go-Rebels]

Today’s quick update:

I had 30 minutes spare time to get the motor started but had a little trouble getting my 3/16" key to properly fit the key way. I needed to grind and file ~1/64" from the height of the key to get a good fit and 20 minutes later I was ready to give it a start.

I pulled once to lube things up with the ignition switch off and then proceeded to turn the ignition on and choke the carb. One pull later the motor fired, stumbling a little, and I took the choke off. The engine surged slightly in speed but only reached about half normal RPM while stumbling somewhat. Half a minute later I slowly released the clutch on the chipper shredder drive and the engine stalled and dies. I tried to restart but felt no tension in the pull cord. Either I sheared the flywheel key (again) or broke another crank (again). Times-up for today.

I’ll tear it down again this weekend to ascertain the current problem.

 

[Tinkerer200]

"On a belt driven ride on they are not because belts will slip or break" (soft key needed)

I disagree slightly they are needed to protect against shade tree mechanics who do not use a torque wrench to tighten flywheel. Running with improper tightening will eventually destroy crank and flywheel with steel key.

Walt Conner

 

[Tinkerer200]

In your original post you just say you replaced a bad connecting rod. You do not say anything about why rod was bad and what you did about checking, repairing the crankshaft throw. Generally just replacing a bad con. rod doesn't do it. Sounds like rod off again.

Walt Conner

 

[MowLife]

I removed the flywheel key in my racer mower 2 years ago for timing purposes and it hasn’t slipped yet. I’m leaning towards the rod.

 

[Go-Rebels]

In your original post you just say you replaced a bad connecting rod. You do not say anything about why rod was bad and what you did about checking, repairing the crankshaft throw. Generally just replacing a bad con. rod doesn't do it. Sounds like rod off again.

Walt Conner

The rod broke into half a dozen pieces when it was fast idling under no load to the engine. A piece punched a hole in the thin wall cast aluminum block and it took me a few years to get around to welding that up (that was a bigger project than the rebuild itself!) Initially I was a little surprised that the rod failed as I regularly changed the oil over its 20 year life and checked the oil level prior to every time I ran the chipper. But I've since learned that the con rod is a weak point in the overall design.

Sorry, I don't understand what you mean by 'crankshaft throw.' The crank appears undamaged and the con rod bearing surface is clean and OD measures in-spec. Upon reassembly I could move the crank and piston by hand and everything felt normal through the compression stroke.

Tonight I'll pull the plug and drop a long weld rod into the cylinder to see if the piston moves.

 

[Go-Rebels]

Regarding the steel/stainless/zinc key issue...

Stainless 304 bar has a tensile strength of 73 ksi.
Low carbon steel has a tensile strength of 64 ksi.
Cast zinc has a tensile strength ranging from 40-60 ksi.
Forged crankshafts generally have a tensile strength of 100-110 ksi.

 

[MowLife]

Good info on the keys. Hard to believe the rod would break if it did in such a short time and no load. I’ve only broke one rod on a rebuild at idle and it was because I forgot to put the oil pump gear in.

 

[bertsmobile1]

"On a belt driven ride on they are not because belts will slip or break" (soft key needed)

I disagree slightly they are needed to protect against shade tree mechanics who do not use a torque wrench to tighten flywheel. Running with improper tightening will eventually destroy crank and flywheel with steel key.

Walt Conner

Point taken I still think like an engineer so the "offending post" has been modified.

 

[bertsmobile1]

Regarding the steel/stainless/zinc key issue...

Stainless 304 bar has a tensile strength of 73 ksi.
Low carbon steel has a tensile strength of 64 ksi.
Cast zinc has a tensile strength ranging from 40-60 ksi.
Forged crankshafts generally have a tensile strength of 100-110 ksi.

Thems very rough figures and we are talking shear strength for keys and torsional shear strength for the crank not tensile strength.
Next a lot of mower cranks are cast malliable iron not forged as the latter are about 6 timed the price of the former.
That was the real problem with the Kohler Courage engines.
At some point the forged cranks ( found on the Commands ) got replaced with the cast crank with corresponding heavier counterbalance and a whole different set of vibrations