A funky shaft repair..... to deal with

[J Tiers]

I should have taken a picture.....

Anyway, I have a small Briggs engine, a 6B, happens to be a Navy generator set.

The flywheel end of the shaft is OK up past the taper, but the extension that carries the LH nut must have broken off in the past. Shaft extension is 9/16 or so, LH threaded.

Someone drilled it out, and tapped in a RH thread of 5/16-18 to both pieces, and then loc-tited in a piece of bolt to secure the pieces.

All was well when I took the FW off to adjust the points, but putting it back is proving to be interesting. When I went to torque the nut, the torque didn't increase right... then it let loose altogether. I thought I stripped something, but it turned out that the stub had unscrewed.

If it were LH, no hassle, it would tighten with the nut, but being that it is RH, tightening the nut unscrews it.

They thoughtfully put a flat bit on the shaft end, but with the nut down in the rope pulley, its a bit tricky to hold that while tightening the nut, since the torque wrench covers it up 100%.

I have studlocker on it now, but I suspect that ain't gonna hold it.

Any ideas other than replacing the crankshaft? It's a special that carries the generator, so I don't think I can get another.

[Millman]

Drill?

May be visualizing this wrong, but can you cross-drill, pin and weld what you need? Seems that would make a strong enough joint and since it's not a bearing surface.....got a pic?

[japcas]

A picture would definitely be handy. Anywhere you could just tack weld a small spot to keep the stud from spinning.

[J Tiers]

A weld might be good...... if I owned a welder.

It would heve to be a small one to still let the flywheel onto the taper.

Think of a funnel with the spout part broken off it. That's the taper and extension shaft.

[A.K. Boomer]

poor pholk splines

Iv had good luck with making "makeshift keyways" out of hardened dowel pins, run the pin length wise just like you would a key way, bore it with both pieces in place, no need for broaching and take my word for it , its very difficult to shear a round pin at its maximum diameter if you get your bore dimensions right and if you take into account the radius of where your two other parts meet and the size of your dowel and adapt that into your overall calculations (otherwise you will be shallow on your male piece)

This is also the way i do my "poor pholk splines" if you have a bolt hole calculator on your DRO set both pieces up vertically and do the same around the entire diameter, if no DRO then do the math, This is very strong esp. when assembled with locktight stud and bearing mount........... pins should be a slight press fit so test a dull end mill in a test hole or use a reamer.....

make sure you have a temperary way of holding both workpieces while boring or all helll could break loose!!!!!!!!!!

[Mike Burdick]

J. Tiers,

Apparently the crankshaft is too small to redrill to next size and thread with left hand thread. So...if the shaft doesn't have too much of an axial or radial load, can you redrill the old thread out and make a new shaft extension that would be a shrink fit?

[Fasttrack]

If you can't redrill into the bearing journal and taper w/ say a 3/8 left hand thread, maybe you could just wip out the old jb weld... Put a generous dab of jb weld on the stud when you reassemble the two pieces so they dont keep loosening as you tighten the flyhweel nut. Otherwise can you make a new throw for it? I don't know how easy it would be to replicate the taper, but if you feel comfortable you could cut off 3/4 of the throw (leave enough remaining so you can drill a new hole for a new stud) so you could mate two untapered pieces. The new stud to hold them together would have to be big and dead center to keep the crankshaft from bending or breaking and to prevent it from causing excessive wear on the bearing. That seems drastic - if it were me i'd probably wip out the jb and if that didnt do i'd look for a replacement.


or... i made a replacement starter cup for an engine once. i'm not sure that i'm thinking of the same thing as what you called the rope pulley - the engines i'm most familiar with have the shroud and starter rope and rewind mechanism all as one piece. That deal engages a cup on the flywheel -they are pretty simple and easy to make in a pinch. Can you make a replacement rope pulley deal for it? I don't know what yours looks like but the one i made was pretty easy. I used some large diameter schd 40 pipe and turned it down, welded a back to it and faced it. Maybe you can make a one that has a flat/square hole to match the flat section on the shaft so you can hold the pulley to keep the shaft from spinning.

[HWooldridge]

Don't know the spec number off the top of my head but Loc-Tite makes some permanent thread lockers that will not loosen unless torch heat is applied. J-B might be OK but the hi-strength lockers are designed for this and work well.

[darryl]

Hmm. A 5/16 bolt won't have much meat on it to cross drill and pin through. Not knowing the diameter of the LH threaded piece, I can't say whether it has enough meat on it to pin from the end, but maybe that could work. Here's my .02- take a 5/16 bolt that would be right to secure the broken off piece to the crankshaft if the broken off piece has the internal threads removed. Drill three small holes through the head if this bolt such that pieces of music wire would pass through and slide past the threads on the bolt. Now bolt the broken piece back to the crankshaft using some jb weld as gap filler. Drill through those holes and insert the music wire pins also using jb weld. I'm thinking you might be able to get 3/32 diameter music wire in there without running out of metal. Before inserting the pins, poke some jb into the bottom of the pin holes with a toothpick or wire, then press the pins in, keeping pressure on until jb is seen oozing out around the holes in the bolt head. This should ensure that jb has filled the gap surrounding the pins and all air pockets have been expelled.

The bolt will be able to hold the broken part in place with significant force because it has not been cross drilled and thus weakened to the point where it has little strength left, while the pins will prevent the part from turning while the LH nut is tightened on. The pins also keep the bolt from turning out. The jb weld in this case serves mostly as filler and to keep the pins from working out. It doesn't have to give any restraining force against the bolt or the broken stub from turning.

[A.K. Boomer]

I would seriously try to stay away from gooping on a bunch of epoxy (JB weld) when attempting to keep a flywheel in its place, to many vibes and to many harmonics not to mention brute forces going on, I dont know if JB still advertises that you can repair burnt exhaust seats in an internal combustion engine, Yeah right, know the limitations of the material at hand...

[Fasttrack]

Jb weld is crap for anything serious, sure - but all he needs is something to keep the stud from twisting while he torques the nut. Once he's got that torqued with out the stud coming loose he should be good to go. The jb weld doesnt actually have to hold the pieces together; that is what the stud is for. I absolutely agree with akb, though. Its no good for anything that has seroius loads on them. Buy a welder for those!

Loctite's toughest thread locker is thier large diameter threat locker - it's break torque is 250 in-lbs. The permanent stuff isn't all that permanent - its break away torque is considerably less than 250 in-lbs. As long as you have a grade eight bolt you can break it pretty easy on almost any diameter bolt w/ a half inch drive ratchet. Most flywheel nuts are torqued to upwards of 400 in-lbs. Briggs specs for series 100200, 100900, 130000 are 65 ft-lbs and the rest are supposedly 55 ft-lbs. I'm not sure how jb weld will hold up, but i've used it as thread locker before and, upon dissassemble i broke the head off the bolt - couldnt get the threads loose. Helps to have a little slop in the threads so it can make a good adherence, thats the nice thing about loctite is it works even when the threads are very tight already.

[winchman]

The only good thing about repairing this is that the engine will stop as soon as the flywheel comes off.

Roger

[J. R. Williams]

Crankshaft Repair

J T
The small engines cranks are usually cast material in the ductile iron family and not readily welded. Shrinking and pinning on a stub may be the best starting point as you do not want a flywheel coming loose.
JRW

[Fasttrack]

"J T
The small engines cranks are usually cast material in the ductile iron family and not readily welded. Shrinking and pinning on a stub may be the best starting point as you do not want a flywheel coming loose.
JRW"

No doubt there - imagine the damage an old cast iron (dunno if yours is but the i/c and older briggs used cast iron flywheels) could do if it came off at say 3600 rpm. ouch...! I guess if it was real well balanced it wouldn't be too horrible.

[CCWKen]

The older briggs used forged steel cranks. We used to buy up every old 5hp we could get our hands on back in the racing days. Bought old 130-xxx engines for $5 just to get the $130 crank.

I think the threaded section with a cross pin is the best idea. The flywheel will keep the pin from comming out. There's not much axial force at the crank. Just don't make the cross pin too large or near the bottom of the taper.