Box Stock

Question about flywheel slipping

Some crank slots are shallower than others. Some keys are taller than others. The slots in the flywheels can be very shallow on the back, letting the key keep the wheel from sitting properly.
 
If the woodruff key interferes with the flywheel it would be wobbling and vibrate. Seems like to me. I know one thing, Its really frustrating when one slips on a plate engine. Until we move to unrestricted engines I will keep using keys.
 
We are having the same issue, not sure what is the cause. Some help would be great.

problem started on an old crank with key installed, modified clone, flywheel slipped and sheared the key, ruined the crank and adjustable flywheel center section.

new motor, same issue, sheared key, saved the crank. Replaced the center section of the flywheel, rebuilt and lapped, no key, 75 ft lbs, still slips minutes after start up on the kart stand.

I have checked the taper on both the crank and flywheel, lapping looks good, nut looks good, tried a different nut, trued the mating surfaces of the nut and flywheel, we have repeated the process 3 more times and it still slips when revving the motor.

Starting to lean to the Loctite direction with some on the taper and the threads of the crank, what's the recommended compound for both?

Should we change the crank? Its a stroker and slipped the first time out, have another that's the same, but wanted to make sure the crank was the issue.

This is the first time we have had this issue, although we think these are the best motors we have built, surely that's not the reason, or is it, can the stroke of the motor be forceful enough to loosen the nut/flywheel as its running?

The one thing that's different is we are using a steel nut, instead of aluminum, would that have anything to do with it? Does the steel not torque properly, that doesn't make sense as the tension is on the crank, not the nut.

Another thing I noticed is the thread on the cranks are slightly looser than others, but the first one to slip has tight threads.

I realize I've asked a few questions here, just trying to figure this out. Thank in advance.
 
The nut should notbe an issue .
The thread could though its highly unlikely . There are different thread tolerances or fits . Do the threads appear damaged or deformed ?
The nut is tight or loose after the flywheel slips ?
A heavy style nut if available . Or hardened .
Anything odd about the flywheel , over or under weight design ?
Possibly a balance issue .
75 ft lbs seems enough , still maybe add 5-10#s of torque .
 
Threads look fine. I'm a machinist by trade, so the they are loose to me, maybe .008-.01 to small on the crank.

The nut can be loosened with wrench after slipping.

Billet flywheel, new one, the old one slipped also.

We have checked torque wrenches to make sure thats not it.
 
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You might not be lapping the crank right. The taper has to grip harder on the big end and less on the small end. If you just slop a bunch of heavy grit on then twist you will cut more on the big end and ruin the taper.

You can check this by cleaning the shaft and flywheel then marking the taper with a sharpy. Put the flywheel on with firm hand pressure and twist. If it smears the ink on the big end you're looking good. Sometimes you have to do it a few times to get the real picture as an uneven taper will tilt around and mess up the results.

Use less compound so the flywheel doesn't wobble. It doesn't take much. Should be about 800 grit aluminum oxide. If you need to change the taper, only put compound on the spot you want to cut. Grit will break down and spread out as you grind so replace grit often if you need to cut allot.

You can test the taper by torquing the nut lightly and see if you can pull the flywheel off by hand. You can get a feel for how good the taper is by how tight you have to make the nut to get it to stick.

You can use Loctite Sleeve Retainer 640 but it won't fix a bad taper.

Sundog
 
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I've also seen cranks where the threads stop prior to the flywheel mating surface -- thus giving the user a false torque feel as the nut bottoms out before it pulls up completely tight on the flywheel. A thicker thrust washer behind the nut is a simple solution.
 
Flatheads always did .
I mean it won't hurt anything .
I think Sundog has a point .
The taper needs checked .
My clones stress the puller when i pull the flywheel .
 
Set the timing . The key only locates the flywheel . It has very little holding ability compared to the taper .
The key gets sheard even on stockers . Its a relatively common situation .
 
Thanks Guys. We will give her heck tonight and hopefully we can figure this out. Worst comes to worst ill cut the taper on a lathe if I have to..
 
Well, it's a no go. Had some help lapping and lapping, and checking and lapping. Still slipped.

Could to much timing be an issue? 36?
 
Cam is from precision and I dont think thats it. My buddy said he has always ran 35-36. Precision didnt send a card with the cam so we dont have all the specs.

We did try a washer and more torque. We took a good 30- 40 minutes to lap the fit. Spun after 2-3 mins, revved the motor and the rpms dropped, revved again and it died...nut spun right off with a wrench.

Can someone give us some pointers on lapping?

Could the compression be an issue? If its hitting hard enough to spin the crank inside the flywheel, and the nut loosens eventually.

Probably gonna swap cranks next...
 
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